                             In the
 United States Court of Appeals
               For the Seventh Circuit
                          ____________

No. 01-3885
HELEN MIHAILOVICH,
                                               Plaintiff-Appellant,
                                 v.

GARY LAATSCH and LAW OFFICE OF PAVALON,
GIFFORD, LAATSCH & MARINO,
                                Defendants-Appellees.

                          ____________
            Appeal from the United States District Court
       for the Northern District of Illinois, Eastern Division.
             No. 99 C 4780—John W. Darrah, Judge.
                          ____________
  ARGUED SEPTEMBER 18, 2002—DECIDED MARCH 5, 2004
                   ____________



 Before BAUER, MANION, and ROVNER, Circuit Judges.
  ROVNER, Circuit Judge. A one-car automobile accident on
a curved roadway maintained by Cook County, Illinois (the
“County”) caused Helen Mihailovich to suffer a spinal cord
injury. Mihailovich filed suit against the County in state
court, alleging that it had been negligent in its care and
upkeep of the roadway and that the condition of the
roadway was a contributing cause of the accident. Attorney
Gary Laatsch and the law firm now known as Pavalon,
Gifford, Laatsch & Marino (collectively “Laatsch”) filed
the suit on Mihailovich’s behalf but withdrew from the case
2                                                No. 01-3885

approximately six months prior to the scheduled trial date.
The attorney who succeeded Laatsch was unable to con-
vince the court to continue the trial date, and after
Mihailovich’s new counsel was unable to retain an expert
and to identify other individuals who might testify on
her behalf, the court entered successive orders barring
Mihailovich from presenting witnesses and then granting
summary judgment in favor of the County. Mihailovich,
now a citizen of Florida, subsequently filed suit in diversity
against Laatsch, charging him with legal malpractice. The
malpractice suit proceeded to trial, and the jury held in
Laatsch’s favor. Mihailovich appeals, contending that the
district judge erred in certain evidentiary rulings that
constrained her ability to show that the roadway on which
her accident occurred was dangerous and that she would
have prevailed in her suit against the County but for
Laatsch’s alleged malpractice. Because we conclude that the
district court abused its discretion in excluding evidence of
other accidents that had occurred at the location of the
Mihailovich accident, we vacate and remand for a new trial.


                              I.
  Early in the evening of July 1, 1986, Mihailovich’s
husband Mike picked her up after work at the Olympia Way
train station in Olympia Fields, Illinois, a suburb southwest
of Chicago. On leaving the train station, they proceeded
south on Kedzie Avenue with Mr. Mihailovich at the wheel
of their 1978 Chevrolet Nomad van. Between 205th and
207th Streets, they encountered what is known as the
Kedzie curve, a reverse S-curve that connects Kedzie
Avenue with Olympian Way. As the “S-curve” appellation
suggests, the Kedzie curve comprises two successive curves:
a northern curve that turns to the west, and a southern
curve turning back to the east. Mihailovich did not know
exactly how fast her husband was driving, but later testi-
No. 01-3885                                                       3

fied that “[h]e was going with the traffic and the speed was
regular.” Tr. 207. She noticed nothing unusual in his
manner of driving. It was raining or drizzling as it had been
throughout the day, and the pavement was wet. Somewhere
along the S-curve (Mihailovich could not recall where), their
van slid off the roadway, tumbled into a ditch, and came to
rest upside down. Mihailovich could not recall whether or
not her husband had braked prior to the accident. The event
was “just like a bullet,” she later testified (see Tr. 209,
quoting her deposition testimony); it happened “in a spare
moment, in a spare second” (Tr. 182). As a result of the
accident, Mihailovich suffered a spinal cord injury that
required months of hospitalization and rehabilitation and
permanently limited her mobility.1
  Police officer Mark Fazzini was summoned to the scene of
the accident at approximately 6:40 p.m. Fazzini served on
the Olympia Fields police force for twenty-five years and
retired in 2000 as its chief. Over the course of his work for
the Village, he traveled the Kedzie curve many times each
day. When Fazzini reached the scene, he discovered the
overturned Mihailovich van on the west side of a ditch in
front of a residence located at 20540 Kedzie Avenue, a
position near the south end of the northern curve. Fazzini
described the Kedzie roadway as a two-lane road that was
long and straight on either end of the S-curve, with only a
broken yellow line separating the two lanes of traffic, each
moving in an opposite direction (north and south). So far as
Fazzini knew, no maintenance work had been done on the
curve from 1980, by which time the County had assumed
responsibility for the curve from the State, through the date
of the Mihailovich accident in 1986. Fazzini’s written report
of the accident indicated that there were “no defects” in the
road.


1
  Mihailovich’s husband was killed in a head-on automobile
collision that occurred after this accident. The record is therefore
confined to Mihailovich’s own limited recollection of the accident.
4                                                No. 01-3885

  By way of background, we pause at this point to note that
the Kedzie curve had something of a notorious reputation
among local officials as a traffic hazard. Accidents occurred
regularly along the S-curve, particularly when the pave-
ment was wet, and there was a perception among officials
that the frequency of accidents had increased since the
County had resurfaced and widened the roadway to the
immediate north and south of the Kedzie curve (but not the
bulk of the S-curve itself) in 1981-1982. Concern about the
accident rate had moved the board of trustees of Olympia
Fields in 1984 to enact a resolution calling upon the County
to address the problem. Partly in response to that resolu-
tion, the County in late 1984 and early 1985 had installed
additional signage alerting motorists to the winding
character of the road, reducing the advised speed through
the curve, and warning drivers that the pavement was
slippery when wet. These signs were in place at the time of
the Mihailovich accident in 1986. However, the County had
not resurfaced or otherwise addressed the condition of the
roadway itself, and notwithstanding the additional warning
signs, accidents continued to occur with regularity until the
County reconstructed the S-curve in 1988-1989. At that
time, the County increased the radius of the north curve,
added a four-foot center rumble strip, added a
superelevation of four percent, and resurfaced the roadway.
In the wake of the reconstruction, the accident rate dropped
from an average of approximately twenty per year to two or
three per year. The failure to take such steps prior to the
Mihailovich accident is what gave rise to Mihailovich’s
negligence claim against the County.
  The jury in this case was aware that the Kedzie curve was
perceived as dangerous by local officials, but heard no
evidence about the other accidents that had occurred at the
S-curve in the years immediately before and after the
Mihailovich accident. Because Mihailovich has made the
district court’s limitation her lead issue on appeal, we shall
No. 01-3885                                                5

summarize both the evidence that the court allowed and the
evidence that the court excluded on this point. Following
those summaries we shall describe the other evidence
presented in support of Mihailovich’s malpractice claim
before turning to the merits of her appeal.


              Admitted Evidence Regarding the
                 Curve’s Potential Hazard
  Arthur Kaindl worked as a highway engineer for Cook
County for thirty-eight years, rising to the post of Chief
Engineer in the Bureau of Maintenance, which he held at
the time of his retirement in August 1986. In September of
1983, Kaindl was the Chief Engineer of Transportation and
Planning. In that capacity, he received a memorandum from
Joseph Marsik, Chief Engineer of Maintenance, forwarding
a second memorandum that Marsik had received from
Frank Reno, the District Engineer of Maintenance for the
district that included Olympia Fields and the Kedzie curve.
Although Kaindl was not permitted to read either of the two
memoranda to the jury, he indicated that Reno’s memoran-
dum expressed a concern that the Kedzie curve was in a
dangerous condition. Tr. 555. Upon receipt of these memo-
randa, Kaindl ordered personnel under his supervision to
undertake a review of the curve, including field surveys to
assess, among other things, whether the existing signage
and posted speed limit at the curve were appropriate to the
condition of the curve. The propriety of the speed limit was
evaluated using a ball-bank study, which by measuring the
degree of sideways sway of a vehicle driving through the
curve ascertains the maximum speed at which a motorist
will feel comfortable. That study indicated that thirty miles
per hour was an appropriate, comfortable speed through the
curve and that was the speed recommended on advisory
signs later posted at the curve. Kaindl acknowledged that
a ball-bank study is conducted on dry pavement, that it
6                                                     No. 01-3885

does not measure the coefficient of friction,2 and that it does
not determine a safe speed for the roadway when the
pavement is wet. After completing its review of the curve
(apparently in 1984), the County did not resurface or
reconstruct the curve at any time through August of 1986,
when Kaindl retired.
   Fred Unger served as the police chief of Olympia Fields
from 1971 to 1990. He traveled through the Kedzie curve
and observed its condition hundreds of times over the
course of his tenure. According to Unger, the pavement
at the curve tended to become slick when dampened by
precipitation. “During inclement weather, accompanied by
precipitation, the roadway would take on a slick surface,
and the oil from the pores of the macadam surface would be
floated to the top by the amount of water that was hitting
the pavement.” Tr. 429. Unger indicated that a light rain
was sufficient to bring about this condition.
  Robert E. Field has lived in Olympia Fields since 1974.
He served as a trustee of the Village from 1981 through
1989 and as its president from 1991 through 1997. His
home is located one block south and west of the Kedzie
curve, and he traveled it at least twice daily through the
date of the Mihailovich accident, although both Field and
his wife avoided the S-curve during inclement weather.
Field described the S-curve as turning to the west and then
back to the east and going downhill as it did so; according
to Field, a driver could not see from one end of the curve to
the other as he entered it. The roadway through the curve
comprised two lanes without a center divider, the edges of
the pavement were crumbling, the shoulders were gravel
rather than paved and were a couple of inches lower than


2
  Simply put, the coefficient of friction is the degree of slip
resistance. The higher the coefficient of friction, the less slippery
the pavement would be.
No. 01-3885                                                     7

the road itself, the surface of the roadway was very smooth,
and over time traffic had worn grooves into the pavement.
  Field testified that his primary reason for seeking a seat
on the Village board of trustees in 1980 was to do some-
thing about the condition of the Kedzie curve, which he had
grown to believe was a hazard. He was particularly con-
cerned that the curved roadway became dangerously slick
when wet. Following his election and at his urging, the
board first enacted a resolution directing the police and
public works departments of the Village to assess the
condition of the roadway and prepare a report. Upon receipt
of that report, Field and another trustee drafted a resolu-
tion calling on the County to address the condition of the
curve. The Village board adopted that resolution by a
unanimous vote on October 10, 1984.
  Anita Healy served on the Olympia Fields board of
trustees beginning in 1979 and as its president from 1981
through 1989. Healy read to the jury the following redacted
version of the Village’s 1984 resolution regarding the
Kedzie curve:
    WHEREAS, it is a fact that a portion of Kedzie Avenue
    lying between 207th Street and 206th Street in the
    Village of Olympia Fields is a county maintained road-
    way;3 and
    WHEREAS, . . . it appears to be an eminently danger-
    ous roadway particularly under certain hazardous
    weather conditions; and


3
  [Footnote by the court] The evidence indicates that as one
moves from north to south, the Kedzie curve actually begins prior
to 206th Street, but the record is silent as to why the resolution
referred only to the roadway between 207th Street and 206th
Street. Field testified that it was his belief that the resolution
covered the entire S-curve; he also indicated that the road begins
to curve in earnest near 206th Street.
8                                                 No. 01-3885

    WHEREAS, the Village Board of the Village of Olympia
    Fields as the governing body of the said Village wishes
    to inform the County Board of Commissioners of the
    County of Cook, State of Illinois[,] of its concern and
    alarm over the state of repair, condition, design and
    road surface material of the aforesaid section of Kedzie
    Avenue; and
    WHEREAS, it has been proposed that the Village by
    official action of this resolution inform the Board of
    Commissioners of the County of Cook of the State of
    Illinois officially and request that immediate action and
    remedies be instituted to correct the condition of the
    roadway herein set out:
    NOW, THEREFORE, BE IT RESOLVED by the Board
    of Trustees of the Village of Olympia Fields, Illinois,
    that they hereby petition on behalf of the Village of
    Olympia Fields, Illinois, a municipal corporation, to the
    Board of Commissioners of the County of Cook of the
    State of Illinois that they cause to have undertaken
    immediate remedial actions to correct the dangerous,
    unsafe and ill-constructed condition of county road
    commonly known as Kedzie Avenue between its inter-
    section of 207th Street and 206th Street that lies within
    the Village of Olympia Fields, Illinois.
    This resolution shall be in full force and effect from and
    after its passage and approval as provided by law.
R. 1 Ex. G; Tr. 235-37.4 Healy sent a copy of the resolution



4
   The resolution that Healy read to the jury was redacted by
agreement between the parties so as to comply with the court’s
order excluding evidence of other accidents that had occurred at
the Kedzie curve. Healy was not permitted to read the following
italicized clause from the resolution:
                                                  (continued...)
No. 01-3885                                                        9

to the County Superintendent of Highways on October 25,
1984.
  In December of 1984, the County installed three types
of signs that were intended to improve the safety of the
curve: a sign advising motorists to reduce their speed to
thirty miles per hour, a reverse curve sign, and a “Slippery
When Wet” sign.5 On January 7, 1985, the County’s Super-
intendent of Highways, Richard Golterman, wrote a letter
to Village president Healy noting that the County had
investigated the “dangerous condition” alluded to in Healy’s
letter of October 25, 1984, forwarding the Village’s resolu-
tion to the County. Golterman’s letter described the signs
that the County had installed and concluded, “We trust that
these corrected measures will serve to alleviate this ‘danger-
ous condition’, but respectfully suggest that the Village
continue to monitor this portion of Kedzie Avenue and keep
us advised.” Tr. 240; R. 1 Ex. I.


      Excluded Evidence Regarding Other Accidents
                    at the Kedzie Curve
  Plaintiff’s retained expert, civil engineer William Berg,
reviewed data maintained by Olympia Fields regarding
accidents that had taken place at the Kedzie curve from



(...continued)
    Whereas, . . . accident records reflect that due to the curvature
    of this road and the construction thereof, it appears to be an
    eminently dangerous roadway particularly under certain
    hazardous weather conditions[.]
R. 1 Ex. G. (emphasis ours).
5
  The County’s decision to install these signs presumably was
based on the studies that it had commenced in response to the
concerns raised by the Reno memorandum in 1983 as well as the
subsequent Olympia Fields resolution in October 1984.
10                                                   No. 01-3885

1983 through July 15, 1987, and prepared a report which,
in relevant part, summarized that data.6 Berg’s report
indicated that a total of ninety-four accidents occurred at
the Kedzie curve during this period of four and one-half
years. In 1983, there were twenty-five accidents, seventeen
of which, or sixty-eight percent, occurred when the pave-
ment was wet. In 1984, fifteen accidents occurred, fourteen
of them, or ninety-three percent, on wet pavement. In the
eighteen months immediately preceding the Mihailovich
accident (January of 1985 through June of 1986), a total
of twenty-nine accidents took place, twenty-two of them,
or seventy-six percent, on wet pavement. In the (approxi-
mately) one-year period following the Mihailovich accident
(July 1, 1986 through July 15, 1987), another twenty-five



6
   It is unclear from the briefs whether Mihailovich, if permitted
to introduce evidence regarding the other accidents, would have
introduced that evidence primarily through Berg or through an-
other witness. As the ensuing summary of the excluded evidence
indicates, a number of witnesses had varying degrees of knowl-
edge about the accident rate at the Kedzie curve. Berg’s written
report appears to contain the most complete analysis of those
accidents. But engineer Paul Box also looked at the accident data
and, like Berg, analyzed and summarized that information. See R.
105 (attachment). Former Olympia Fields police chief Fred Unger
also had compiled a report about the curve for the Village trustees
which included accident data. R. 62 Ex. 24. We assume that any
of these witnesses, who appear to have relied on the same basic
data, could have testified about the other accidents that occurred
at the Kedzie curve in the years immediately preceding and
following the Mihailovich accident. We also note that at least
some of the underlying accident data itself is in the record. See R.
62 Ex. 24 (accident data from Olympia Fields); R. 1 Ex. J (accident
reports). As the parties’ appellate arguments are focused solely
upon the admissibility of other-accident evidence, rather than
upon how that evidence might have been introduced, we need not
explore the latter issue.
No. 01-3885                                                   11

accidents occurred, twenty of them, or eighty percent, when
the pavement was wet. Berg also reviewed police reports
that were available for twenty-four of the twenty-nine
accidents that occurred in the eighteen months prior to the
Mihailovich accident. According to Berg, in twenty-one of
those twenty-four accidents, a vehicle had been proceeding
southbound on Kedzie Avenue into the curve and had either
left the roadway altogether inside or outside of the curve or
had crossed the center line and collided with a northbound
vehicle. R. 62 Ex. 6.7
  Village president Anita Healy would have testified that
the occurrence of serious accidents at the curve, particularly
during inclement weather, had been a topic of discussion
among Village officials for a number of years. She knew of
one fatality and multiple injuries that had resulted from
such accidents. To Healy’s knowledge, no other curved
roadway in the Village had a comparable rate of accidents,
injuries, and fatalities. When Healy sent the Village board
of trustees’ 1984 resolution regarding the Kedzie curve to
the County Superintendent of Highways on October 25 of
that year, she wrote in her transmittal letter that “[w]e
have been receiving increased numbers of complaints about
the roadway in that area and requests to remedy the
situation.” R. 1 Ex. H. Her letter concluded: “It is our hope
that the Highway Department will look into this problem
and will determine a way to remedy this dangerous situa-
tion.” Id. According to Healy, after the curve was recon-
structed between 1988 and 1989, the accident rate dropped
to an “[a]lmost negligible” rate, and there had been no
fatalities since then. Tr. 250-51.


7
  Berg’s report notes that in the aftermath of the Mihailovich ac-
cident, the County reduced the advisory speed through the curve
to twenty-five miles per hour. R. 62 Ex. 6 at 4 ¶ 12. Based on the
accident data, Berg observed that “the wet pavement safety prob-
lem . . . continued unabated.” Id.
12                                               No. 01-3885

  Retired Olympia Fields police officer Mark Fazzini would
have testified that between 1980 and 1986, “dozens and
dozens and dozens of automobile accidents [had occurred]
on the Kedzie curve,” far more than at any other curve in
the Village. Tr. 252. According to Mihailovich’s offer of
proof:
     [I]t was [Fazzini’s] opinion from investigating dozens of
     collisions that when it rained, a film developed on the
     surface of the highway, that it became slick, it looked
     like an oil slick, and that the automobile[ ] collisions
     that he investigated in the wet inclement conditions
     always lost traction in the roadway and slid up, either
     straight ahead, either going into the woods at the far
     end of the curve, or after sliding would then go off the
     roadway to the west.
Tr. 252. Fazzini also would have testified that the Village
police department had made efforts to slow southbound
traffic entering the curve in order to reduce the accident
rate:
     [B]ecause of the Village’s concern about the cars con-
     tinuing to pile up and injure and kill citizens out
     there, . . . the police department literally set up speed
     traps at the highway to try to get drivers to slow down
     because they knew that cars coming from the north
     down into this curve had been traveling through a
     semirural area and that, as we know in this case,
     Kedzie Avenue just north of the curve, was resurfaced
     in 1981, +82 at Biden and this resurfacing went right up
     to the beginning of this north curve and the cars would
     come into the curve at a high rate of speed because of
     their assurance they were on a good, sound highway.
Tr. 253. According to Fazzini, the Olympia Fields police
department had notified the County of the situation with
respect to the Kedzie curve “on more than one occasion,” but
No. 01-3885                                                13

nothing was done in response. Id. Once the curve was
resurfaced in 1988-1989, accidents were virtually elimi-
nated.
  Village trustee Robert Field would have additionally
testified, based on his own observations as a driver who
traveled the curve regularly and as a Village trustee, that
a large number of accidents occurred at the curve, that he
knew of no other location in the Village with such a high
accident rate, and that most or all of the accidents occurred
in inclement weather. Tr. 678. Recall that following Field’s
election to the board of trustees in 1980, he had successfully
urged the board to commission a study of the curve. Field
would have testified that in the course of that study, the
Olympia Fields police department had compiled a multi-
year inventory of traffic accidents that had occurred at the
curve, and that the police chief at that time, Fred Unger,
had presented a report including that compilation to the
Village board. That report, according to Field, indicated
that “more traffic accidents had occurred on the Kedzie
curve than in the rest of the Village combined,” (Tr. 680),
and that the resurfacing of the roadway to the north and
south of the Kedzie curve in 1981-1982 appeared to have
resulted in an increase in the number of accidents. Tr. 681-
82. Field also was familiar with the additional warning
signs that the County had installed at the curve in response
to the board’s October 1984 resolution. Field recalled that
according to the Village’s police department, that signage
had not had any discernible impact on the accidents
occurring at the curve. Tr. 681. However, after the curve
was reconstructed and resurfaced in 1988-1989, the acci-
dent rate dropped to near-zero. Tr. 683-84.
  Frank Reno, who served in the County’s employ for thirty-
eight years, was the District Engineer of Maintenance in
the early 1980s for the district that included Olympia Fields
and the Kedzie curve. In August or September 1983, more
than one year before the Olympia Fields board of trustees
14                                               No. 01-3885

enacted and transmitted its resolution about the curve to
the County, Reno became aware that the rate of accidents
with fatalities had increased in the area of the curve in the
preceding years. Reno thought that the County should look
into the situation at the curve, and for that reason wrote
the September 19, 1983 memorandum to Joseph Marsik,
Chief Engineer of Maintenance, that Arthur Kaindl had
referred to in his testimony. Reno stated in his memoran-
dum:
     In the last two years it has been observed that the
     accident rate in the above subject area has increased
     considerably, with fatalities involved. There is a change
     of alignment with a curve in the pavement alignment
     between Kedzie Ave. and Olympia[n] Way, and this
     curve does not seem that sharp and dangerous. The
     pavement on Olympia[n] Way is very smooth and could
     contribute to the cause of these accidents. The accident
     rate has increased since Kedzie Ave. was improved and
     repaved. It is recommended that this matter be relayed
     to our design and traffic division.
Tr. 434-35; R. 62 Ex. 10. Neither Marsik nor Reno was
permitted to relay the contents of Reno’s memorandum to
the jury.
  Marsik had received Reno’s memorandum and agreed
with his recommendation that the curve be investigated.
Marsik had forwarded Reno’s memorandum to Kaindl. In
his cover memorandum to Kaindl, Marsik had written:
     Attached herewith is a memorandum from District
     Engineer Frank Reno pertaining to an alarming in-
     crease in traffic accidents at subject location (reversed
     curve section) between Olympia[n] [Way] and Kedzie
     Avenue.
     It is requested that this section be reviewed.
No. 01-3885                                                      15

    It is our belief that the traffic is going too fast on the
    straight roadway, entering the curve and then has a
    problem negotiating the curve.
Tr. 500; R. 62 Ex. 9.


               Evidence Relating to Malpractice
  Following her accident in 1986, Mihailovich decided to
sue the County. She eventually engaged attorney Eugene
Pavalon, who in turn assigned the case to his associate
(later his partner) Gary Laatsch. Laatsch filed suit on
Mihailovich’s behalf in June 1987, alleging that the curve
was dangerous, that the County had negligently permitted
the curve to remain in an unsafe condition, and that the
County had failed to alert motorists to the danger. R. 1 Ex.
C. Laatsch believed there was sufficient merit to
Mihailovich’s claim to warrant discovery. Laatsch repre-
sented Mihailovich from 1987 through early 1998, when he
formally withdrew from the case.
  During his representation of Mihailovich, Laatsch took
the following steps (among others) in an effort to build a
case on her behalf: (1) He met with Mihailovich at her home
on multiple occasions and prepared her for her deposition
by the County. (2) According to his trial testimony, Laatsch
obtained a copy of the Village’s 1984 resolution calling on
the County to investigate the Kedzie curve. Tr. 371, 373-74.8


8
  We note that other evidence in the record indicates that Laatsch
may not, in fact, have obtained a copy of the resolution. Laatsch
himself testified at his pre-trial deposition that he did not believe
he had ever received a copy of the resolution. See Tr. 373, 703.
Two letters from Laatsch to the Assistant State’s Attorney
handling the litigation for the County arguably suggest that
Laatsch had not received the resolution. Tr. 393-94. Moreover, the
                                                     (continued...)
16                                                   No. 01-3885

(3) In 1990, Laatsch obtained copies of the field studies that
the County had completed in 1984 in order to ascertain the
appropriate speed limit on the Kedzie curve and appropri-
ate traffic advisory signs. (4) He spoke with structural
engineer Herbert Miller, who had inspected the scene of the
accident two months after it occurred together with
Mihailovich’s first attorney. Miller had taken pictures of the
accident site and had prepared a report for the prior
attorney; Laatsch had copies of those materials. But
according to Laatsch, Miller told him that “he could never
testify” that the pictures he had taken depicted the area
where the Mihailovich vehicle had left the roadway and
that Miller, in fact, had not been able to determine where
precisely the accident had occurred. Tr. 390-92. (5) Laatsch
also issued one set of written interrogatories to the County,
and as a result of those interrogatories he obtained a list of
potential witnesses from the County. He deposed one of
these witnesses—Daniel Waters, a sign technician, whose
knowledge was limited to the signage at the Kedzie curve
and who had no responsibility for the surface of the road-
way. (6) Laatsch contacted William Berg, a civil engineer,
for purposes of obtaining an expert opinion about the
condition of the roadway at the time of the Mihailovich
accident. Laatsch sent Berg, among other things, a copy of
Waters’ deposition, along with certain correspondence
between Olympia Fields and the County Highway Depart-
ment. Based on these materials, Berg informed Laatsch
that the signs posted at or near the Kedzie curve were
sufficient to inform drivers of the condition of the curve.



(...continued)
case file that Laatsch later sent to the attorney who replaced him
did not include a copy of the resolution. See Tr. 256, 373-74, 702-
03. Nonetheless, because we are obligated at this juncture to
construe the evidence in the light most favorable to the defen-
dants, we credit Laatsch’s trial testimony.
No. 01-3885                                              17

Berg could not otherwise determine whether the roadway
was defective at the time of the Mihailovich accident. R. 62
Ex. 5 at 44. (7) In March 1987, Laatsch visited the scene of
the accident. He walked a half block in each direction from
the point where the Mihailovich van had come to rest, and
he also drove through the curve in both directions. He
observed no significant potholes or other defects in the
pavement, nor did he recall the shoulders being gravel
and/or lower than the roadway itself. Based on the informa-
tion available to him (including the accident report),
however, he could not determine where the Mihailovich’s
van had begun to slide or where it left the roadway. He also
spoke to an eyewitness to the accident (Harold Moore, the
police chief of nearby Crestwood), but did not take a formal
statement from him. (8) Finally, he obtained records from
the County indicating that it inspected the roadway twice
per month and periodically maintained it.
  However, Laatsch did not undertake the following steps:
(1) He never spoke with Mark Fazzini, the Olympia Fields
police officer who investigated the scene of the Mihailovich
accident, choosing instead to rely on Fazzini’s accident
report. (2) He neither contacted nor deposed any Olympia
Fields official, including Healy or Field, to discuss the
Village’s 1984 resolution regarding the Kedzie curve or
the basis for that resolution. (3) Other than Waters, who
disclosed in his deposition that he was not responsible
for the road surface of the Kedzie curve, Laatsch did not
depose any of the witnesses disclosed in the County’s
response to his interrogatories or that Waters named in his
deposition—including County Highway Superintendent
Golterman, Chief Engineer Kaindl, or Village President
Healy. In fact, Waters was the sole individual that Laatsch
deposed. (4) Laatsch did not attempt to engage an expert
until 1992, when he contacted Berg. (5) Laatsch did not ob-
tain data regarding the other accidents that had occurred
at the Kedzie curve. (6) He did not attempt to ascertain
18                                              No. 01-3885

whether the signs that the County had installed at the
curve in 1984-1985, partly in response to Olympia Fields’
resolution, had affected the accident rate and/or addressed
the concerns that had prompted the Village to pass the
resolution. (7) He failed to respond to the interrogatories
that the County had issued, and he did not comply with the
state court’s order to identify the witnesses who might
testify on Mihailovich’s behalf at trial.
  Mihailovich’s case against the County proceeded lan-
guorously toward a trial date. The case was initially set for
trial in September 1992. Having contacted Berg in June of
that year and having been unable to obtain from him an
opinion regarding the curve that would have been helpful
to Mihailovich’s case, Laatsch opted to voluntarily dismiss
the suit when it was called for trial. He refiled it the
following year. However, Laatsch engaged in no further
discovery after the case was restored to the court’s docket.
Neither did he inform Mihailovich that he had dismissed
and then refiled the suit. On March 11, 1997, Laatsch
appeared in court for a case management conference and
informed the judge that “extensive discovery” had been
completed and that Mihailovich would probably have
an expert witness testify at trial. Tr. 419. When the judge
suggested a trial date of January 13, 1998, Laatsch replied,
“That will be fine.” Tr. 420. The court entered an order
giving Laatsch until June 9, 1997, to disclose his witnesses.
Laatsch never filed a list of witnesses, however, and in
December 1997, the County moved to bar Mihailovich from
calling any witnesses to testify. In response, Laatsch moved
to continue the trial date. On December 16, 1997, the court
granted the latter motion and struck the trial date.
  Approximately two weeks after the court cancelled
the January 1998 trial date, Laatsch moved to withdraw
from the case. For nearly a year prior to Laatsch’s motion,
Mihailovich and her son had been dissatisfied with the
No. 01-3885                                               19

progress of her suit and had been searching for someone
to replace Laatsch. In February 1997, Mihailovich’s son
had written Laatsch directing him to send her case file to
the Law Offices of Edward Vrdolyak for review. Vrdolyak’s
firm had declined to take the case, however. Two other
firms—Ross & Hardies and Corboy & Demetrio—had
likewise turned the case down. On January 27, 1998, the
court granted Laatsch’s motion to withdraw and gave
Mihailovich forty-five days in which to retain new counsel.
The trial date was re-set to July 22, 1998.
  In April or May of 1998, attorney Marc McKenna agreed
to assume representation of Mihailovich provided that the
court would agree to postpone the trial date further (beyond
July). When McKenna received the Mihailovich case file
from Laatsch, it included only the police report describing
the accident, a videotape of the roadway, some photographs
of the accident scene, some printouts of information from
the County (the contents of which he couldn’t recall), and
two depositions—those of Mihailovich and a County
employee (presumably Daniel Waters). McKenna twice
sought a continuance from the court and was twice denied
one. The court did grant McKenna additional time to
disclose Mihailovich’s witnesses, including any experts who
might testify on her behalf. McKenna contacted two experts
hoping to obtain an opinion as to the condition of the Kedzie
curve and whether it contributed to the Mihailovich
accident, but neither individual was able to form an opinion
based on the limited materials McKenna could provide from
the case file. McKenna thus found himself with no experts
to name, and as for other witnesses, he would later testify
that he did not know whom to disclose. He conceded on
cross-examination, however, that he could at least have
named Mihailovich, her physician, and County employee
Waters. In any event, when McKenna failed to name
anyone, the County renewed its motion to bar Mihailovich
from presenting any witnesses at trial. The court granted
20                                             No. 01-3885

that motion on July 8, 1998. That order in essence left
Mihailovich with no evidence to present in support of her
claim, and, accordingly, on July 24, 1998, the court entered
summary judgment against Mihailovich on the County’s
motion.
  The following year, Mihailovich filed the instant suit
against Laatsch and the law firm of Pavalon, Gifford,
Laatsch & Marino for legal malpractice. Mihailovich con-
tended, in essence, that Laatsch had conducted so little
discovery and had so poorly prepared the case for trial that
by the time he withdrew, there was little that his successor
reasonably could have done to avoid dismissal. At the same
time, according to Mihailovich, there was ample evidence
available that would have supported her negligence claim
against the County. For example, nearly 100 accidents had
occurred on the Kedzie curve between 1983 and 1987; and
discovery conducted in this case revealed that Olympia
Fields officials and County employees, among others, were
aware of the high accident rate at the curve and expressed
concern well in advance of the Mihailovich accident that the
curve was (or might be) unsafe. Moreover, when supplied
with additional information about the Kedzie curve,
including data regarding the other accidents that had
occurred between 1983 and 1987, William Berg—the same
expert who had been unable to tell Laatsch whether the
curve was defective—opined that the roadway was hazard-
ous and that the condition of the road had contributed to
the Mihailovich accident. After reviewing the evidence that
Mihailovich had marshaled in support of her malpractice
claim, the district court denied Laatsch’s motion for sum-
mary judgment (R. 123), and the case proceeded to trial.
  At trial, John Lowrey, a personal injury attorney
with more than thirty years’ experience, testified on
Mihailovich’s behalf as an expert regarding the standard of
care for attorneys handling personal injury cases. Lowrey
opined that Laatsch had deviated from that standard of
No. 01-3885                                               21

care in a number of respects, including the following: (1) He
had failed to properly investigate the Mihailovich accident
and ascertain the underlying facts. (2) He had failed to
depose the County employees who were responsible for the
care and maintenance of the Kedzie curve. He had not
deposed anyone named in the County’s answer to the
interrogatories he had propounded other than Waters, nor
had he pursued the leads that Waters himself had supplied
in his deposition. (3) He had failed to speak to Olympia
Fields officials about the circumstances that led them to
pass the 1984 resolution. Village president Healy, for
example, “would have been a gold mine,” in Lowrey’s view.
Tr. 449. (4) Laatsch had failed to engage a highway expert
early on in the discovery phase of the case, not simply to
supply him with an opinion as to the state of the Kedzie
curve but also to help him identify the information neces-
sary in order to render such an opinion. Although Laatsch
had contacted Berg, he had not provided Berg with the
information that the engineer needed in order to form an
opinion about the Kedzie curve. (5) By informing the trial
court in March 1997 that the parties had engaged in
extensive discovery, Laatsch had misled the court about the
actual amount of discovery taken (which was modest). (6)
Also in March 1997, Laatsch had acceded to an earlier trial
date (January 13, 1998) than Laatsch’s level of preparation
warranted. (7) He had allowed discovery deadlines to pass
without disclosing Mihailovich’s witnesses. (8) Laatsch had
withdrawn from the case in January 1998, eleven years into
the litigation, with the trial date fast approaching and the
case file in a state of disrepair.
  Lowrey also opined, based on a review of the depositions
of Village President Healy, Village trustee Field, Village
police chief Unger, County engineer Reno, Harry Abbott,
the County Highway Department’s Chief Engineer of
Design, and other witnesses, as well as the Olympia Fields
resolution regarding the Kedzie curve, the available in-
22                                               No. 01-3885

formation regarding the Mihailovich accident, and records
of the accidents that had occurred at the Kedzie curve, that
Mihailovich’s negligence suit against the County was a
winnable case.
  In an effort to show that Mihailovich would not have won
her suit against the County even if Laatsch had taken the
steps that Mihailovich’s legal expert criticized him for not
taking, Laatsch presented the testimony of accident re-
constructionist Kenneth Baker. Baker had reviewed the
plans for the construction of the Kedzie curve in the early
1970s as well as the specifications for the Chevrolet Nomad
that the Mihailoviches were driving at the time of the
accident, and he had entered the pertinent data from both
into a computer simulation program designed to recreate
electronically the conditions at the time of the accident.
Based on his study and the results of the simulation, Baker
opined that there were no defects in the design of the
Kedzie curve. Baker admitted, however, that his knowledge
of the actual condition of the curve at the time of the
accident was limited. He had not inspected the scene of the
accident, he could not describe the condition of the pave-
ment at the time of the accident, he had not reviewed any
of the internal correspondence between County employees
regarding the condition of the curve, and he did not know
whether the surface of the curve was highly polished and
what its properties were with respect to the retention of
water. Baker also acknowledged that the coefficients of
friction he had assigned to the curve for purposes of his
review might have been inconsistent with the true condition
of the curve. He further allowed that if the pavement of the
curve allowed oil to rise to its surface when wet, the actual
coefficient of friction at the time of the accident might well
have been lower (and the surface thus more slippery) than
he had assumed. Tr. 735.
  At the conclusion of the trial, the jury returned a general
verdict in Laatsch’s favor, precipitating this appeal.
No. 01-3885                                                23

                             II.
   Under Illinois law, in order to prevail on a claim of
attorney malpractice, a plaintiff must succeed in proving
four elements: (1) an attorney-client relationship giving rise
to a duty on the attorney’s part; (2) a negligent act or
omission by the attorney amounting to a breach of that
duty; (3) proximate cause establishing that but for the
attorney’s negligence, the plaintiff would have prevailed in
the underlying action; and (4) actual damages. E.g., Woidtke
v. St. Clair County, Ill., 335 F.3d 558, 562 (7th Cir. 2003)
(Illinois law); Cleveland v. Rotman, 297 F.3d 569, 572 (7th
Cir. 2002) (Illinois law); Mitchell v. Schain, Fursel &
Burney, Ltd., 773 N.E.2d 1192, 1193-94 (Ill. App. Ct. 2002).
These elements effectively demand that the malpractice
plaintiff present two cases, one showing that her attorney
performed negligently, and a second or predicate “case
within a case” showing that she had a meritorious claim
that she lost due to her attorney’s negligence. E.g., Ignarski
v. Norbut, 648 N.E.2d 285, 289 (Ill. App. Ct. 1995);
Sheppard v. Krol, 578 N.E.2d 212, 214 (Ill. App. Ct. 1991);
Claire Assocs. v. Pontikes, 502 N.E.2d 1186, 1190 (Ill. App.
Ct. 1986).
    Because legal malpractice claims must be predicated
    upon an unfavorable result in the underlying suit, no
    malpractice exists unless counsel’s negligence has re-
    sulted in the loss of the underlying action. Plaintiff
    is required to establish that but for the negligence of
    counsel, he would have successfully prosecuted or
    defended against the claim in the underlying suit.
    Damages will not be presumed, and the client bears the
    burden of proving he suffered a loss as a result of the
    attorney’s alleged negligence.
Ignarski, 648 N.E.2d at 288 (citations omitted).
 Laatsch, of course, was no longer representing
Mihailovich when the Illinois court entered orders barring
24                                               No. 01-3885

her from presenting any witnesses and then granting sum-
mary judgment in favor of the County; by that time
McKenna had stepped into the case. However, in cases of
successive representation, Illinois law recognizes that a
prior attorney’s negligence may be the proximate cause of
a plaintiff’s damages where the plaintiff’s underlying claim
is no longer viable when his representation ends. See
Mitchell, 773 N.E.2d at 1194-95. As noted, this is
Mihailovich’s theory here. By the time Laatsch withdrew
from the case, she asserts, her claim against the County
was no longer viable given a trial date six months’ hence
and the minimal degree to which Laatsch had prepared the
case for trial.
  As our summary of the testimony above reveals, much
of the trial was devoted to the merits of Mihailovich’s
negligence claim against the County—the “case within
a case” whose meritorious nature Mihailovich had to es-
tablish in order to prevail on her malpractice claim against
Laatsch. Indeed, all of the issues that Mihailovich has
raised on appeal relate to her efforts to prove that aspect of
her malpractice claim. In briefing those issues, both parties
have assumed that Mihailovich presented sufficient evi-
dence as to the other elements of the claim, including proof
that Laatsch breached the duty of care that he owed
Mihailovich as her attorney. Having thoroughly reviewed
the record, we agree that Mihailovich presented evidence
adequate to support a jury finding on that point and the
other elements of her malpractice claim. Because the jury
rendered a general verdict in Laatsch’s favor, we must
therefore assume that it might have found against
Mihailovich based solely on her failure to show convincingly
that she had a meritorious claim against the County and
had lost that claim due to Laatsch’s alleged negligence. Any
error that prejudicially interfered with Mihailovich’s ability
to prove that aspect of her malpractice case would therefore
require a new trial. We thus turn our attention to the first
No. 01-3885                                                25

and primary issue she has raised on appeal, which concerns
the district court’s decision to exclude all evidence of other
accidents that had occurred at the Kedzie curve. Because
we conclude that the district court erred in excluding that
evidence and that a new trial is warranted on that basis, we
need not resolve the other issues Mihailovich has presented.
See Fed. R. Civ. P. 61. We shall, however, devote some
attention at the conclusion of our opinion to the exclusion
of Mihailovich’s proffered expert testimony, a matter that
we believe warrants reconsideration on remand.


  A. Exclusion of Other Accidents
  As we have noted, the district court granted Laatsch’s
motion in limine to exclude all evidence regarding other
accidents at the Kedzie curve. See Tr. 247-48, 685-88.
Mihailovich’s purpose in offering that evidence was to
establish that the curve was dangerous when wet. Tr. 685.
But in the court’s view, there was already ample evidence
in the record establishing that the surface of the curve
became slick when wet, including testimony that the
County itself had installed a “Slippery When Wet”sign at
the curve in 1984-85 to warn motorists of that condition.
Tr. 686. Indeed, this was a common-sense proposition that
the jury was likely to appreciate without such evidence. Tr.
685-86. Therefore, the court viewed the probative worth of
the other-accidents evidence as “minuscule to say the least.”
Tr. 685. At the same time, looking to this court’s decision in
Kelsay v. Consolidated Rail Corp., 749 F.2d 437, 442-46 (7th
Cir. 1984), the court saw a significant potential for undue
prejudice in this evidence. By her own admission,
Mihailovich was not contending that the condition of the
roadway was the sole cause of all other accidents at the
curve; and the variables that might have contributed to
those other accidents (including the direction of travel,
weather, speed, vehicle weight, seasonal conditions, and
26                                                No. 01-3885

other traffic) were “almost infinite.” Tr. 687. The potential
for a collateral trial to occur as to the causes of those
accidents thus struck the district court as substantial. Tr.
687-88. Finally, in the absence of evidence that any of the
motorists involved in the other incidents had filed suit
against the County, the court found no reason to believe
that those accidents bespoke negligence on the County’s
part. Tr. 688.
   Although relevant evidence is presumptively admissible,
see Fed. R. Evid. 402, a court has the authority to exclude
it if the risks posed by the introduction of the evidence sig-
nificantly outweigh its probative worth.
     Although relevant, evidence may be excluded if its pro-
     bative value is substantially outweighed by the danger
     of unfair prejudice, confusion of the issues, or mislead-
     ing the jury, or by considerations of undue delay, waste
     of time, or needless presentation of cumulative evi-
     dence.
Fed. R. Evid. 403. Rule 403 thus calls upon the district
court to weigh the need for and probative value of the evi-
dence against potential harm that its admission might
cause. Id., Advisory Committee Note (1972); see also, e.g.,
Manuel v. City of Chicago, 335 F.3d 592, 596 (7th Cir.
2003). “ ‘The balancing of probative value and prejudice is
a highly discretionary assessment, and we accord the dis-
trict court’s decision great deference, only disturbing it if no
reasonable person could agree with the ruling.’ ” Id., quoting
United States v. Thomas, 321 F.3d 627, 630 (7th Cir. 2003).
  In order to show that she would have won her suit against
the County but for Laatsch’s alleged malpractice,
Mihailovich had to prove that the County, which was re-
sponsible for maintaining the Kedzie curve in a reasonably
safe condition for its normal and intended uses, negligently
discharged that responsibility. See generally DiBenedetto v.
Flora Tp., 605 N.E.2d 571, 574 (Ill. 1992); First Nat’l Bank
No. 01-3885                                                 27

in DeKalb v. City of Aurora, 373 N.E.2d 1326, 1331 (Ill.
1978); 745 Ill. Comp. Stat. 10/3-102, 10/3-105; see also, e.g.,
Storen v. City of Chicago, 27 N.E.2d 53, 55 (Ill. 1940);
Santelli v. City of Chicago, 584 N.E.2d 456, 459 (Ill. App.
Ct. 1991). Her theory in relevant part was that years of
wear had polished the surface of the Kedzie curve, so that
when precipitation dampened the pavement and oil rose to
its surface, the pavement became extremely slick; thus, by
failing to make appropriate repairs or changes to the road,
notwithstanding requests that it do so, the County had
permitted a hazard to arise and persist that caused motor-
ists to lose control of their vehicles when the pavement was
wet. See, e.g., Tr. 771, 813.
  In this context, there is no doubting that the other acci-
dents proffered were relevant to establish that the worn
pavement of the curve constituted a hazard for motorists. If,
as Mihailovich theorized, the Kedzie curve was dangerous
and had been for some time, then one would naturally
expect there to have been other accidents besides hers over
the years. In fact, as the record in this case reveals, the
number of accidents that occur at a particular location is
one of the first indicators that officials concerned with
roadway safety look to for an indication that the location is
dangerous.
  We know, of course, from the proffers of excluded testi-
mony that the reason why the Olympia Fields board of
trustees passed the 1984 resolution calling on the County
to address the curve was the high number of accidents that
had occurred at that curve; employees of the County’s
Highway Department themselves had cited the high acci-
dent rate in calling for studies of the curve in 1983, more
than a year before the Olympia Fields board passed its
resolution.
 More than this, the frequency with which accidents occur
would appear to be an accepted reference point in civil
28                                                  No. 01-3885

engineering for the purpose of identifying potentially haz-
ardous roadway locations. The February 21, 2001 report of
civil engineer Paul Box, which Mihailovich submitted in
opposition to Laatsch’s motion to exclude the evidence of
prior accidents, noted that although there is no national
standard as to the number of accidents necessary in order
to label a particular location as “hazardous,” experts in
the field had conducted research and developed a graph
indicating that the occurrence of more than two accidents
over a period of three years signaled that the location was
“hazardous,” while the occurrence of more than ten acci-
dents in three years indicated that it was “very hazardous.”
R. 105, Box Report at 6 ¶ 18, citing J.I. Taylor and H.T.
Thompson, Determining Hazardousness of Spot Locations,
Transportation Research Record 630 (1977); see also J.I.
Taylor and H.T. Thompson, Federal Highway Administra-
tion, Identification of Hazardous Locations, Report Nos.
FHWA-RD-77-81 through 83 (1977) (identifying accident
rate as among factors relevant to hazardousness of particu-
lar location). Assuming that Box has correctly interpreted
and applied this criterion, the total of ninety-four accidents
that occurred at the Kedzie curve in the four and one-half
years surrounding the Mihailovich accident obviously would
exceed the Taylor & Thompson threshold for a “very hazard-
ous” location by many multiples. If the frequency of acci-
dents at a particular location constitutes a category of data
that both experts in civil engineering and officials charged
with the maintenance of roadways consider important, then
that information would be highly relevant to the jury’s own
assessment of whether or not the curve posed a hazard.9


9
  It appears that the data regarding other accidents at the Kedzie
curve played a role in changing Box’s mind as to whether or not
the curve was hazardous at the time of the Mihailovich accident.
Box was one of the two experts that attorney McKenna had
                                                   (continued...)
No. 01-3885                                                    29

  Indeed, our own precedents recognize that other accidents
are generally deemed admissible both to prove the existence
of a defect or danger in a location or a product and to show
that the defendant had notice of the defect or danger, so
long as the other accidents are “substantially similar” to the
accident at issue in the litigation. Weir v. Crown Equip.
Corp., 217 F.3d 453, 457 (7th Cir. 2000); Ross v. Black &
Decker, Inc., 977 F.2d 1178, 1185 (7th Cir. 1992); Estate of
Carey by Carey v. Hy-Temp Mfg., Inc., 929 F.2d 1229, 1235
n.2 (7th Cir. 1991); Nachtsheim v. Beech Aircraft Corp., 847
F.2d 1261, 1268 (7th Cir. 1988); see also Walker v. Trico
Mfg. Co., 487 F.2d 595, 599 (7th Cir. 1973); Lever Bros. Co.
v. Atlas Assur. Co., 131 F.2d 770, 777 (7th Cir. 1942).
Where, as here, the other accidents are offered as proof of
a dangerous condition,10 courts insist on a greater degree of
similarity between those accidents and the one at issue
than they do when the other incidents are offered solely to



(...continued)
contacted in 1998 when he provisionally took over the representa-
tion of Mihailovich in her suit against the County. Based on the
information that McKenna provided, which did not include any
information about the other accidents that had occurred at the
curve, Box told McKenna that Mihailovich had “no case.” Tr. 296-
97. Later, in the course of the instant malpractice suit, Box was
supplied with much more information about the history of the
Kedzie curve, including the other accidents that had occurred
there. Based on the more expansive information with which he
was provided, Box concluded that the curve was dangerous at the
time of the Mihailovich accident. R. 105, Box Report at 6-7.
10
  Evidence of other accidents arguably could have served as proof
that the County was on notice of the alleged hazard. However,
Mihailovich did not offer the evidence for that purpose. In any
case, there was other evidence in the record establishing that the
County was on notice, including the Village’s 1984 resolution and
the County Highway Department’s own internal investigation into
the safety of the curve.
30                                                     No. 01-3885

establish notice of a potential danger to the defendant.
Nachtsheim, 847 F.2d at 1268 n.9. This is not to say,
however, that the accidents must correspond with one
another in every detail. “In applying this standard, we have
emphasized that ‘substantially similar’ does not mean
‘identical,’ and that the range between similar and identical
is a matter to be addressed on cross-examination.”
Buscaglia v. United States, 25 F.3d 530, 533 (7th Cir. 1994),
citing Carey, 929 F.2d at 1235 n.2. The particular defect or
danger alleged by the plaintiff will serve to define the
degree of commonality that there must be among the
accidents in order for them to be considered substantially
similar. Jackson v. Firestone Tire & Rubber Co., 788 F.2d
1070, 1083 (5th Cir. 1986). Looking to the established facts
underlying both the plaintiff’s accident and the other acci-
dents she has proffered, the court must consider whether
those facts reasonably support an inference that all of the
accidents share a common cause—i.e., the danger that the
plaintiff has alleged. See Nachtsheim, 847 F.2d at 1269. If
the facts do support such an inference, then the other
accidents are admissible barring other factors suggesting
that their admission will result in undue prejudice. See id.
  Laatsch suggests at the outset that Mihailovich did not
make enough of a record regarding the other accidents at
the Kedzie curve to show substantial similarity between
those accidents and her own, but we disagree. The record
before us includes a year-by-year tabulation, prepared by
Unger, then the chief of police of Olympia Fields, of the
accidents that had occurred at the Kedzie curve from
January 1, 1983 through July 15, 1987;11 police reports
for some (but by no means all) of the accidents that had



11
  So long as they satisfy the criterion of substantial similarity,
accidents that occurred after as well as before the plaintiff ’s acci-
dent are admissible to prove the existence of a danger. Ross, 977
F.2d at 1185; Nachtsheim, 847 F.2d at 1268 n.8.
No. 01-3885                                                31

occurred at the curve in 1981 through 1986; and the
compilations and analyses of the accident data reflected in
the reports of both Box and Berg. Mihailovich cited and
presented this information to the district court prior to
trial, both in opposing (successfully) Laatsch’s motion for
summary judgment and in opposing (unsuccessfully) the
County’s motion in limine to exclude evidence concerning
the other accidents. See, e.g., R. 62 (Plaintiff’s Local
Rule 56.1 Statement of Undisputed Material Facts)
Exs. 6 (Berg Report), 24 (Kedzie curve accident data);
R. 105 (Plaintiff’s Response to Defendant’s Motion In
Limine No. 2) Attachment (Box Report). The police reports
for some of the accidents that occurred at the curve in 1981
through 1986 were attached to Mihailovich’s malpractice
complaint. R. 1 Ex. J. Collectively, this data, coupled with
the other evidence of record, tends to show that: (1) the
other accidents occurred along the Kedzie curve; (2) they
occurred relatively close in time to the Mihailovich accident;
(3) the condition of the curve was no different at the time of
the Mihailovich accident than it was at the time of the other
accidents (the curve itself had not been resurfaced since the
1970s, and would not be resurfaced until 1988 or 1989); (4)
a majority of the accidents (roughly three-quarters of the
total from 1983 through mid-July 1987) occurred on wet
pavement, just as the Mihailovich accident did; and (5)
although the police reports (and Berg’s analysis of the
reports for twenty-four of the twenty-nine accidents that
occurred in the eighteen months immediately prior to the
Mihailovich accident) reveal variations in precisely where
each vehicle left the roadway and what happened as a
result (for example, some vehicles collided with oncoming
traffic, while others came to rest on either side of the
roadway), the reports reveal a pattern of drivers losing
control of their vehicles while on the curve, particularly
when the roadway was wet.
  Keeping in mind the particular danger that Mihailovich
has alleged was present at the curve—excessive slipperi-
32                                               No. 01-3885

ness when the pavement was wet—we are satisfied that the
other accidents are substantially similar to the Mihailovich
accident and therefore are admissible as proof that the
curve posed a hazard to motorists. We readily grant
Laatsch’s point that there is a great deal that the record
does not tell us about these accidents. The gross accident
data compiled by Unger, for example, reveals only the date
and time that each accident occurred, the number of
vehicles involved, whether the accident resulted in a
fatality, injury, or property damage, the weather conditions
at the time of the accident, and the condition of the road
surface. See R. 62 Ex. 24. Although Unger’s compilation was
apparently derived from police reports regarding these
accidents (the compilation lists a report number for each
incident), the record before us does not include all of these
reports. The thirty-five or so police reports attached to the
complaint reveal a good deal more information about those
accidents, including in particular the basic facts of each
accident, and also the identities of the motorists involved,
the types of vehicles they were driving, where the accident
occurred, whether alcohol was involved, the apparent
physical condition of the motorists, and so forth. But the
reports do not recount precisely what each driver was doing
immediately before the accident occurred (how fast he was
driving, for example, whether and how he applied his
brakes, and so forth), they do not establish with precision
where along the S-curve each incident took place, and they
do not purport to assign fault for the accident or to identify
each of the factors that may have contributed. Even so, the
data do reveal a distinct pattern to the other accidents: they
occurred at an average rate of twenty per year in the four
and one-half years surrounding the Mihailovich accident
(January 1983 through mid-July 1987); a substantial
majority (at least seventy-five percent) occurred when the
pavement was wet; and (per the police reports) they almost
uniformly involved a loss of control by a driver with the
result that the vehicle slid, spun, or skidded into a vehicle
No. 01-3885                                               33

proceeding in the opposite direction, into a vehicle stopped
directly ahead of him, or off the road altogether. Berg’s
analysis also indicates that in twenty-one of the twenty-four
accidents occurring the eighteen months prior to the
Mihailovich accident for which records were available, the
subject vehicle had been proceeding southbound into the S-
curve. Given the frequency and regularity with which
motorists, and in particular southbound motorists, were
losing control of their vehicles at the curve when the
pavement was wet, it is a fair inference that both the
condition of the curve’s pavement when wet and the nature
of the southbound approach to the curve, played some role
in these other accidents. The same inference can be drawn
from what we know about the Mihailovich accident: it
involved a southbound motorist, it occurred on the curve,
when the pavement was wet, and it involved a loss of
control that resulted in the van leaving the road. In these
respects, the other accidents may be deemed substantially
similar to the Mihailovich accident. Evidence concerning
the other accidents was therefore admissible, barring a
reason which would have justified its exclusion under Rule
403.
  As our summary of the district court’s rationale for
excluding the other accidents indicates, the court had two
basic concerns: it saw the probative worth of evidence con-
cerning the other accidents as minimal, while the potential
for prejudice in such evidence to be great. As we explain
below, the court was off-base in its first concern, and in
articulating its second concern, the court appears to have
been led astray by our fact-specific holding in Kelsay.
  In weighing the probative value of the evidence, the
district court described the relevance of the prior accidents
as “minuscule” (Tr. 685), reasoning that other evidence
amply established that the surface of the Kedzie curve was
slippery when wet and that jurors in any event would have
appreciated that fact as a matter of common sense. In that
34                                                No. 01-3885

observation, the court was mistaken. Mihailovich’s theory
as to the hazard posed by the curve was not simply that
it was slippery when wet, but that it was unusually and
dangerously so, i.e., to a degree that motorists would not
ordinarily anticipate. That so many other accidents oc-
curred on the curve, and that approximately three-quarters
of them occurred when the pavement was wet, lends
support to the notion that the curve was unreasonably slick
when wet. Indeed, the fact that the accidents continued
unabated even after the County installed a “Slippery When
Wet” advisory sign at the curve in 1984 might be construed
as an indication that the degree of slipperiness exceeded
even the expectations of motorists who were placed on
notice of the potential hazard. The reaction of local officials
to the history of accidents occurring at the curve likewise
could be construed as a sign that the degree of danger was
unusual.
  Indeed, we must point out that, in view of the factual
backdrop of the case, proof of the other accidents that oc-
curred on the curve was one of the few ways in which
Mihailovich could have established that the roadway was in
an unreasonably dangerous condition. A more direct way of
proving the supposed hazard would have been to measure
objectively the slipperiness of the pavement, as
by ascertaining the coefficient of friction on the surface
of the curve when it was wet. But that was no longer
possible by the time Laatsch withdrew from representing
Mihailovich—the curve had been reconstructed and re-
surfaced between 1988 and 1989, and Laatsch had not
engaged an expert to inspect the curve and conduct appro-
priate testing to determine the coefficient of friction prior to
that time. So Mihailovich had to pursue more indirect ways
of establishing the alleged hazard. The jury of course knew
that the Olympia Fields trustees believed the curve to be
hazardous and that they had conveyed that concern to the
County by way of the 1984 resolution. But the district court
permitted the jury to consider that resolution solely as proof
No. 01-3885                                                35

that the County had notice of a potential hazard, not as
proof that the roadway was, in fact, dangerous. Tr. 233.
Beyond this, the jury heard only abbreviated testimony that
the surface of the roadway had a tendency to become oily
and slick when wet. Consequently, there was little, if any,
evidence indicating whether or not the surface of the Kedzie
curve in fact became unusually or dangerously slippery
when wet and what effect the wet surface had upon motor-
ists who negotiated the curve. Not surprisingly, this was a
point that Laatsch’s counsel seized upon in his closing
argument to the jury. E.g., Tr. 795.
  So, when it engaged in the Rule 403 balancing process,
the court did not give the other-accidents evidence the
weight it deserved in terms of its probative value. At the
same time, on the other side of the scale, the court in as-
sessing the potential reasons for excluding the evidence did
not properly focus on the key consideration, which is
substantial similarity between the other accidents and the
one at issue in this case. Instead, referencing our opinion in
Kelsay as its primary guidepost, the court found that the
justifications that Kelsay cited for the exclusion of other-
accident evidence in that case also weighed in favor of
exclusion here.
  But our decision in Kelsay does not mandate the exclusion
of the other-accident evidence tendered here. Kelsay, which
involved the collision of a train with an automobile at a
railroad crossing, affirmed (over a dissent) the district
court’s decision to exclude evidence of two fatal accidents
that had occurred at the same crossing twelve and thirty
years earlier. We cited four factors that in our view sup-
ported the exclusion of the prior-accident evidence: (1) key
differences in the conditions and circumstances of the prior
accidents, (2) the absence of evidence that lawsuits had
been filed on behalf of the deceased motorists in the prior
accidents, which suggested that the drivers themselves may
have been at fault, (3) the danger that the jury might infer
36                                              No. 01-3885

from the prior accidents alone that the railroad crossing at
issue was hazardous, and (4) the plaintiff’s ability to show
through other evidence that the conditions at the crossing
had remained unchanged for a number of years, so that the
potential danger would have been evident to the railroad.
749 F.2d at 443. Nothing in our opinion suggested that this
was an exhaustive list of pertinent considerations or that
the factors we identified would apply with the same force in
any case involving other-accident evidence. It bears men-
tioning in that regard that we had first identified these
factors in Gardner v. Southern Ry. Sys., 675 F.2d 949, 952
(7th Cir. 1982), as being among the “many factors” that the
district judge “undoubtedly” had considered in excluding
other-accident evidence; Gardner itself did not cite any
precedents that focused on these factors. The governing
standard, however, as cases both before and after Gardner
and Kelsay make clear, was and is whether the other
accidents are substantially similar to the accident at issue
in the instant suit. See, e.g., Buscaglia, 25 F.3d at 533;
Carey, 929 F.2d at 1235 n.2; Walker, 487 F.2d at 599; Lever
Bros., 131 F.2d at 777.
  Moreover, Kelsay involved a materially distinct set of
circumstances. First, the prior accidents involved in that
case were, as the court noted, quite remote in time from the
incident under litigation, and in each case there was
at least one apparent circumstance suggesting that some-
thing other than the hazardous nature of the railroad
crossing was at fault. In one of the instances, the windows
of the car had been heavily soaped, which obviously would
have interfered with the driver’s view of the crossing; and
in the other instance, the motorist had arrived at the
crossing after the train did, suggesting that the driver’s
view of the crossing and the oncoming train was not a cause
of the collision. By contrast, the accidents that Mihailovich
sought to introduce occurred with regularity in the years
just before and just after her accident, with no grossly
No. 01-3885                                                37

apparent distinctions that would explain away the condition
of the roadway as a cause of those accidents.
  Second, the prior accidents involved in Kelsay had both
resulted in fatalities, so the lack of evidence that lawsuits
had been filed in those cases was more telling than it would
be here, where the vast majority of the other accidents did
not result in deaths. As litigious as Americans may be, one
cannot reasonably expect every motorist who loses control
of her car on a slippery road to file a lawsuit. Moreover,
whether or not another accident resulted in litigation really
is of very limited use in weighing the probative value of
that accident. As Judge Posner pointed out in his dissent in
Kelsay, the question with respect to other accidents is not
whose fault they were in a legal sense but what they can
tell the jury about the dangerousness of the location where
they occurred. 749 F.2d at 451 (Posner, J., dissenting).
  Third, although it is quite possible that a jury might infer
from the prior accidents themselves that the Kedzie curve
was in fact dangerous, that inference would not be as
inappropriate as it might have been in Kelsay. The other
accidents proffered here, which are neither isolated nor
remote in time from the Mihailovich accident, fall into a
pattern suggesting that the Kedzie curve indeed did become
dangerously slippery when wet. That is an inference that
the Village and some County employees themselves drew
from the pattern, and it is also an inference that at least
two experts in the field, Box and Berg, drew from a review
of the evidence. In Box’s view, for example, the fact that a
distinct majority of the reported accidents occurred on wet
pavement “represents an extremely unusual circumstance”
that pointed to the need for the pavement to be addressed
by the County. R. 105, Box Report at 7 ¶ 5. Moreover, it
appears that when the curve was reconstructed in 1988-
1989, the accident rate dropped dramatically to at most a
few accidents per year. Although the district court excluded
evidence concerning the reconstruction pursuant to Fed. R.
38                                               No. 01-3885

Evid. 407—and we assume that it did so correctly—the
impact that the reconstruction had on the occurrence of
accidents nonetheless is something that we may consider in
determining whether or not the court abused its discretion
in excluding evidence of other accidents. The noteworthy
reduction in the number of accidents following reconstruc-
tion tends to confirm that the prior condition of the curve in
fact did have something to do with the accidents that
occurred there. All of this indicates that evidence concern-
ing the other accidents would not have been unduly prejudi-
cial in the sense that it might have led the jury to draw
unwarranted inferences from those accidents about the
condition of the curve.
  Fourth, although exclusion of the other-accidents evidence
did not prevent Mihailovich from proving that the condition
of the Kedzie curve had been evident for some time prior to
her accident—such that the County was on notice of the
potential danger—that is really beside the point.
Mihailovich, as the district court recognized, was not
seeking to introduce the evidence for purposes of notice, but
to prove that the pavement of the curve was dangerously
slippery when wet. Almost none of the evidence that was
admitted by the court in this case went to that important
point. Exclusion of the other-accidents evidence deprived
Mihailovich of highly probative proof of the condition of the
curve and its impact on motorists.
  The district court found it noteworthy that Mihailovich,
by her own admission, was not asserting that the condition
of the roadway was necessarily the sole cause of the other
accidents and that the number of other variables that might
have contributed to the other accidents—including the
direction of the vehicle, the weather, the speed, vehicle
weight, seasonal conditions, and other traffic—was “almost
infinite.” Tr. 687. That the condition of the curve was not
necessarily the sole cause of the other accidents was an
entirely reasonable and prudent concession for Mihailovich
No. 01-3885                                                 39

to make, given that the available data concerning those
accidents (including the police reports) do not undertake to
assign and/or apportion fault or causation. But so long as
the evidence regarding those accidents reasonably supports
the inference that the condition of the roadway was at least
a cause of those accidents—and we believe it does—then
they remain probative of whether the road was unreason-
ably hazardous and whether the condition of the road was
a cause of the Mihailovich accident as well. Likewise, the
number of variables that might have contributed to the
other accidents does not demand their exclusion. The
standard for admission, as we have pointed out, is not
identity or near identity but substantial similarity. We are
satisfied that the record reveals the other accidents to have
been substantially similar to the Mihailovich accident.
Laatsch would certainly be free to highlight for the jury the
potential differences in the types of variables that the
district court cited, but those differences go to the weight of
the other-accidents evidence rather than its admissibility.
  Finally, although the district court was (rightly) con-
cerned about the potential for testimony regarding the
other accidents to devolve into a collateral trial over the
circumstances in those accidents, there are alternatives
to exclusion of any and all evidence about those accidents
that would suffice to avoid this peril. The occurrence of
other accidents at the curve, and the range of circumstances
that might have caused them, is not so complex a subject as
to require an overly expansive evidentiary record.
Mihailovich could establish the pertinent facts regarding
these accidents without delving deeply into the circum-
stances of each occurrence, and Laatsch similarly could
identify the potential distinctions between those accidents
and the Mihailovich accident as well as the types of condi-
tions besides the road surface that might have contributed
to the other accidents without having to litigate who was at
fault in each instance. As always, the court remains free to
40                                               No. 01-3885

establish reasonable limits on this line of inquiry in order
to prevent it from becoming a sideshow.
  Although, for all of these reasons, we conclude that the
district court abused its discretion in excluding all evidence
regarding other accidents at the Kedzie curve, we must also
consider whether the error was prejudicial. Pursuant to
Federal Rule of Civil Procedure 61, an error in the admis-
sion or exclusion of evidence does not supply a basis for
setting aside a verdict unless the error was “inconsistent
with substantial justice.” “Our cases hold that ‘evidentiary
errors satisfy this standard only if a significant chance
exists that they affected the outcome of the trial.’ ” Shick v.
Ill. Dep’t of Human Servs., 307 F.3d 605, 611 (7th Cir.
2002), quoting Hasham v. Cal. State Bd. of Equalization,
200 F.3d 1035, 1048 (7th Cir. 2000); see also Harris v. City
of Chicago, 266 F.3d 750, 755 (7th Cir. 2001).
  In this case, we believe that there is a significant chance
that the exclusion of the other-accident evidence affected
the outcome of the trial. In order to prove her “case within
a case”—i.e., that she would have prevailed on her claim
against the County but for Laatsch’s alleged negligence—
Mihailovich was required to present evidence that the
Kedzie curve was dangerous. As we have explained, the
most direct method of establishing the hazard—having an
expert inspect the curve and conduct tests to determine the
condition of its pavement when wet—was foreclosed to her.
The Kedzie curve as it was at the time of the Mihailovich
accident in 1986 no longer exists; it was reconstructed in
1988-1989, ten years before she filed the instant suit.
During the time that Laatsch represented Mihailovich prior
to the reconstruction, he did not cause any testing or
inspections to be done that might have preserved data
about the coefficient of friction on the curve’s pavement
when wet. So Mihailovich was confined to indirect means of
proving that the curve was dangerous. Primary among
these methods was proof that other accidents occurred
No. 01-3885                                               41

regularly at the curve when the pavement was wet. Barred
from introducing any evidence regarding such accidents,
Mihailovich was left solely with proof that Village officials
and County employees perceived the curve to be a potential
hazard and no explanation as to why they held that percep-
tion. Had the jury known that both the Village and some
employees of the County perceived the curve to be danger-
ous because motorists were regularly losing control of their
vehicles when the pavement was wet, it might well have
assessed her prospects of prevailing against the County
differently.
  We therefore conclude that Mihailovich is entitled to a
new trial at which she may introduce proof of the other
accidents that occurred at the Kedzie curve. The district
court, of course, remains free to establish reasonable limits
on the type and extent of other-accident evidence that
Mihailovich may introduce. For example, to the extent that
police reports reveal at least the rudimentary details about
each accident, and in that way permit the parties to es-
tablish points of comparison with the Mihailovich accident,
the court might decide to permit evidence solely regarding
those accidents for which such reports are available.12
We also wish to reiterate that the admission of this line of
evidence need not portend a mini-trial as to each and every
one of the other accidents. Mihailovich may present data
regarding the number and basic nature of these accidents,
and Laatsch may identify the limits of the facts known
about those accidents, the distinctions between those
accidents and the Mihailovich accident, and the range of
circumstances other than the condition of the curve that
might account for those accidents, without inquiry into this
area becoming unduly prolonged.


12
  As we have noted, the appellate record contains only an in-
complete set of police records. But we assume that additional
records may be available to the parties.
42                                             No. 01-3885

  B. Exclusion of Expert Testimony by Berg
  In advance of trial, Laatsch moved to bar William Berg
from testifying as an expert pursuant to Fed. R. Evid. 702.
Mihailovich had identified Berg as a civil engineer from
whom she intended to elicit testimony as to, among other
things, the condition of the Kedzie curve at the time of the
accident, the County’s asserted negligence in maintaining
the curve, and the likely cause of the Mihailovich accident.
Having reviewed a report summarizing Berg’s opinions
on these subjects and having deposed him on multiple
occasions, Laatsch contended that Berg should not be
permitted to testify because his analytical methodology did
not satisfy the requirements outlined in Daubert v. Merrell
Dow Pharm., Inc., 509 U.S. 579, 113 S. Ct. 2786 (1993), and
Kumho Tire Co. v. Carmichael, 526 U.S. 137, 119 S. Ct.
1167 (1999).
  Berg expressed three basic opinions with respect to
the condition of the Kedzie curve at the time of the
Mihailovich accident and the cause of that accident. In sum,
Berg opined: (1) The County was not maintaining the
Kedzie curve in a reasonably safe condition as of July 1,
1986. (2) The hazardous nature of the curve resulted in part
from the widening and resurfacing of Kedzie Avenue to the
north of the S-curve in 1981. As a result of that project,
southbound motorists were approaching the curve at speeds
greater than the advisory speed of thirty miles per hour for
the curve itself and were engaging in frequent applications
of their brakes after they entered the curve. The higher
speeds and more frequent brake applications resulted in a
higher frictional demand in the curve. When coupled with
low or reduced skid resistance in the curve when the
pavement was wet, this posed a safety hazard to motorists.
(3) The configuration of both the S-curve itself as well as
the southbound approach to the curve were substantial
causal factors of the Mihailovich accident. The prevailing
conditions at the time of the accident and the sequence of
No. 01-3885                                               43

events were substantially similar to the continuing pattern
of wet-pavement accidents involving southbound vehicles.
See R. 109 Ex. B; Tr. 620-21.
  Berg, a civil engineer, employed a “human factors” model
in reaching these conclusions. Broadly speaking, a human
factors analysis focuses on the interaction between human
behavior and the design of a machine, product, or in this
case, a particular roadway. An expert engaging in such
an analysis will consider whether, in light of predictable
human behavior, the design or condition of the subject item
poses a potential hazard. See, e.g., Richard J. Kholmn,
Using the Human Factors Expert in Civil Litigation, 40 AM.
JUR. TRIALS 629 §§ 1-2. Berg’s opinions reflect this approach
to the extent they incorporate certain notions about how
motorists likely would have been behaving as they entered
the Kedzie curve as it existed at the time of the Mihailovich
accident.
  One of the key assumptions underlying Berg’s analysis is
that southbound motorists typically would have entered the
curve at speeds of between thirty-five and forty-five miles
per hour. R. 109 Ex. B; R. 62 Ex. 6 at 3 ¶ 6; Tr. 620. That
assumption was based on two circumstances. First, the
posted speed limit on Kedzie Avenue to the north of the
reverse curve was forty-five miles per hour. That limit
dropped to thirty-five miles per hour at 203rd Street, or
approximately 1700 feet north of the S-curve. Id. Second,
the 1981-82 widening and resurfacing of Kedzie Avenue to
the north of the curve had created a roadway that would
cause drivers to feel safer and to drive faster. In short, due
to the higher posted speed limit and more commodious
roadway immediately to the north of the curve, southbound
motorists were likely to be driving at a rate somewhere
between thirty-five miles per hour and forty-five miles per
hour as they entered the Kedzie curve, through which the
advisory speed was thirty miles per hour. Id. ¶¶ 6-7; Tr.
626-27.
44                                               No. 01-3885

  The curve itself required motorists to adjust both the path
and speed of their vehicles. In Berg’s view, the location,
configuration, and other characteristics of the Kedzie curve
gave rise to what he described as an “expectancy violation”
and a “detection/recognition” problem for motorists. Id.; R.
109 Ex. B ¶ 3; Tr. 620. Berg defined an expectancy violation
as an unusual situation that catches the motorist by
surprise. Tr. 575-76. He believed that the Kedzie curve
posed an expectancy violation for southbound motorists in
the sense that it required motorists to suddenly both slow
and turn their vehicles after having traveled along a stretch
of wide, straight roadway. R. 62 Ex. 6 at 3 ¶ 7. A detec-
tion/recognition problem is one that a motorist will have
difficulty recognizing. Tr. 576. Berg thought that the Kedize
curve also posed this type of problem, because roadside
vegetation coupled with a downward grade of 2.5 percent
beginning near the middle of the north curve made it
difficult for motorists to visualize the location and tightness
of the curve. R. 62 Ex. 6 at 3 ¶ 7.
  Under these circumstances, Berg explained, motorists
required warnings to alert them to the presence and loca-
tion of the reverse curve, the need to reduce their speed to
thirty-five miles per hour or less, and also to the fact that
the surface of the curve was slippery when wet. Id. at 3 ¶ 8.
The need for the latter warning was demonstrated by the
pattern of accidents that had occurred on the curve when
the pavement was wet. Id. at 3 ¶ 4. As we know, following
the field studies that the County conducted in 1984 in
response to the concerns expressed by its own employees
and by the Village of Olympia Fields about the accident rate
at the curve, additional signage had been installed to alert
motorists to each of these circumstances. In Berg’s opinion,
those signs were sufficient to provide motorists with the
information they needed about those aspects of the curve.
Id. ¶ 8.
No. 01-3885                                                45

  However, accident records for the eighteen-month period
beginning in January 1985 (after the additional warning
signs were installed) and ending in June 1986 revealed that
the wet pavement safety problem continued unabated. Of
the twenty-nine accidents reported to have occurred at the
curve during that period, twenty-two had taken place on
wet pavement and the remaining seven had occurred when
the pavement was covered with ice or snow. Based on the
police reports available for twenty-four of these accidents,
Berg had prepared a collision diagram indicating what had
happened to the vehicles involved in these accidents.
According to Berg, the diagram demonstrated a clear
pattern: twenty-one of the twenty-four vehicles involved
had been proceeding southbound on Kedzie Avenue and,
after entering the curve, had either left the roadway on the
inside or outside of the curve or had crossed the center line
and collided with a northbound vehicle. Id. at 3-4 ¶ 9; see
also Tr. 621-22. Because Berg was relying on the police
reports for data about these accidents, he lacked informa-
tion as to what the driver in each case had done
(in particular, whether he or she had applied the vehicle’s
brakes) at a particular moment. Tr. 576-77. But what the
available data made clear to Berg was that in each case the
driver had lost control of the vehicle after entering the
curve. R. 62 Ex. 6 at 3-4 ¶ 9.
  Berg acknowledged that the available information about
the Mihailovich accident did not permit him to determine
where, precisely, the Mihailovich’s van had left the road. Tr.
630-31. But Berg indicated that what was known about the
Mihailovich accident placed it “dead center” within the
pattern that he discerned in the history of accidents at the
curve—that is, the vehicle was proceeding southbound on
Kedzie Avenue, the pavement was wet, and the driver lost
control of the vehicle after entering the curve. R. 109 Ex. B
¶ 3; Tr. 578, 620, 630.
46                                               No. 01-3885

  As we have noted, Berg opined that two factors in combi-
nation were the probable cause of this pattern of accidents:
(1) low or reduced pavement skid resistance on the curve,
and (2) high frictional demand for vehicles traversing the
curve. R. 109 Ex. B ¶ 2. Berg surmised that the surface of
the curve had low or reduced skid resistance based on the
fact that a distinct majority of the accidents occurred when
the pavement was wet. Tr. 574-75. He thought that the
reduction in skid resistance was likely due either to a
polishing of the pavement resulting from a high volume of
cornering traffic and/or to the poor drainage of surface
water resulting from the low superelevation rate of the
curve and the descending 2.5 percent grade. Tr. 584-85.13
Berg attributed the high frictional demand to the relatively
high speeds (thirty-five to forty-five miles per hour) of
vehicles approaching the curve, frequent brake applications
by motorists after they entered the curve, and the reduced
effectiveness of the advisory signs present at the curve.
Again, Berg assumed that southbound motorists were likely
traveling at a rate of at least thirty-five to forty-five miles
per hour on approach to the curve based on the higher
speed limit and the wider and straighter roadway on Kedzie
Avenue north of the curve. Tr. 626-27. He assumed that
frequent brake applications occurred on the curve given
that motorists were, by his estimate, approaching the curve
at speeds in excess of the advisory speed for the curve itself
(thirty miles per hour) and given the sheer number of cars
passing through the curve on a daily basis (6,200 cars per
day). Tr. 573-74. Finally, he assumed that the warning
signs were not as effective as they otherwise might have
been because the curve did not appear to pose a problem



13
  Berg admitted that the actual coefficient of friction on the
surface of the curve as of the date of the Mihailovich accident
could not be ascertained, because the roadway had since been
realigned and repaved. Tr. 624.
No. 01-3885                                               47

when dry and because the recommended speed of thirty
miles per hour through the curve was not dramatically
lower than the speed of thirty-five to forty-five miles per
hour at which he believed motorists likely were approaching
the curve. See R. 62 Ex. 6 at 3-4 ¶¶ 9-10; Tr. 621-22.
Consequently, motorists who were used to driving through
the curve when it was dry might not have paid as much
attention as they should have to the warning signs posted
at the curve.
  Appropriate countermeasures to address the ongoing wet-
pavement safety problem, in Berg’s view, would have
included applying a skid-resistant pavement overlay within
the curve and/or to increase the superelevation of the curve.
In addition, the County might have conducted a speed study
of southbound traffic to determine whether there was a
need for either additional enforcement of regulatory speed
limit or for measures, such as a flashing beacon or rumble
strips, to reinforce the warnings that were already in place.
Id. at 4 ¶ 11; see also Tr. 628.
  Berg noted that after the Mihailovich accident occurred,
the advisory speed through the curve was reduced to
twenty-five miles per hour. Yet, accident records for the
year following the accident again showed that the wet-
pavement problem persisted. R. 62 Ex. 6 at 4 ¶ 12.
  What ultimately did solve the problem, Berg noted, was
the reconstruction of the curve in 1989. The key aspects of
the reconstruction which, to Berg’s mind, improved the
safety of the curve were a new road surface coupled with a
larger curve radius and superelevation at the north curve
which reduced frictional demand on the pavement. Berg
noted that as a result of these changes, drivers could safely
navigate the curve at speeds of up to forty-five miles per
hour. Id. ¶ 13; Tr. 624-25.
  Finally, Berg opined that the County had been negligent
in failing to identify and address the root causes of the
48                                               No. 01-3885

problem at the Kedzie curve sooner than it did. He indi-
cated that since the 1960s, it has been a standard civil
engineering practice to prepare collision diagrams for
roadway segments that reflect unusually high frequency of
accidents. In his view, the County’s Highway Department
should have had, but did not have, appropriate procedures
in place prior to the Mihailovich accident in order to ensure
that collision diagrams were prepared when the circum-
stances warranted. R. 62 Ex. 6 at 4 ¶ 14. Berg believed that
had the County prepared such a diagram for the Kedzie
curve in 1983, when the high accident rate for that location
was first noticed by County employees, the timetable for
identifying the cause of the accidents and for implementing
improvements sufficient to rectify the problem could have
been advanced so that those improvements would have been
made prior to the Mihailovich accident in 1986. Had the
curve been reconstructed prior to July 1, 1986 as it later
was in 1989, for example, the Mihailovich accident likely
would not have occurred, in Berg’s opinion. Id. at 4-5 ¶ 15.
  Having reviewed Berg’s three principal opinions, his
deposition testimony, and his testimony on voir dire, the
district court barred him from testifying as an expert but
gave little explanation of its reasons for doing so. After
noting the factors identified as pertinent in Daubert, the
court simply stated that “it’s clear that none of these factors
are even remotely present to support [Berg’s] purported
analysis and methodology in reaching these three conclu-
sions or opinions . . . .” Tr. 590-91.
  When it will assist the trier of fact, Rule 702 permits a
witness whose knowledge, skill, experience, training, or
education has granted him expertise in a particular field to
testify in the form of an opinion regarding one or more
issues in the case, provided that “(1) the testimony is based
upon sufficient facts or data, (2) the testimony is the
product of reliable principles and methods, and (3) the
witness has applied the principles and methods reliably to
No. 01-3885                                               49

the facts of the case.” Fed. R. Evid. 702. The district court
functions as a gatekeeper with respect to testimony prof-
fered under Rule 702 to ensure that the testimony is
sufficiently reliable to qualify for admission. See Kumho
Tire Co., 526 U.S. at 147, 119 S. Ct. at 1174. The Supreme
Court in Daubert identified five criteria that bear on that
assessment, including whether the theoretical framework
or technique underlying the witness’s testimony (1) is
subject to verification through testing, (2) whether it has
been subjected to peer review and publication, (3) what its
known or potential rate of error is, (4) whether there are
standards controlling its application, and (5) whether it
is generally accepted within the relevant expert community.
509 U.S. at 593-94, 113 S. Ct. at 2796-97. “[T]his list is
neither definitive nor exhaustive, but rather flexible to ac-
count for the various types of potentially appropriate expert
testimony.” Deputy v. Lehman Bros., Inc., 345 F.3d 494, 505
(7th Cir. 2003), citing Kumho Tire Co., 526 U.S. at 141, 119
S. Ct. at 1171. We review de novo whether the district court
correctly applied the Daubert framework, but assuming that
it did so, we review the court’s decision to admit or exclude
testimony under Rule 702 for abuse of discretion. E.g.,
Deputy, 345 F.3d at 505.
  Review of the district court’s decision to exclude Berg’s
testimony is hampered by the fact that the court simply
stated without any explanation that Berg’s methodology
satisfied none of the criteria identified in Daubert. We do
not doubt that the court was aware of the relevant consider-
ations, in view of the fact that the court correctly recited
each of the Daubert factors. But given that the Daubert
framework is a flexible one that must be adapted to the
particular circumstances of the case and the type of tes-
timony being proffered, a court excluding expert testimony
would do well to articulate with reasonable specificity the
reasons why it believes the testimony is insufficiently
reliable to qualify for admission under Rule 702. The lack of
50                                              No. 01-3885

such explication makes it difficult to meaningfully review
the lower court’s decision. It is not immediately apparent
here, for example, in what respects Berg’s methodology
departed from those of other civil engineers and why the
analysis underlying his opinions is not reliable. Berg
appears to have answered each of the questions posed to
him on voir dire about his methods and assumptions and
identified the data that he relied upon.
  The admissibility of Berg’s testimony therefore should
be freshly addressed on remand. Although we express no
opinion as to the appropriate outcome of that analysis,
we do make the following observations, which the district
court should consider in evaluating the reliability of Berg’s
methodology.
  First, we note that the human factors analysis that
colored Berg’s opinions vis-à-vis the Kedzie curve is not
novel. It is a recognized analytical approach that is applied
in a variety of contexts and may yield legitimate insights as
to the hazards that particular products and situations
(including roadways) may pose in light of predictable
human behavioral patterns. See Kohlman, supra, Using the
Human Factors Expert in Civil Litigation, 40 AM. JUR.
TRIALS 629.
  Second, a key assumption underlying Berg’s evaluation of
the curve is that drivers approaching the Kedzie curve from
the north frequently would have done so at speeds in excess
of thirty or thirty-five miles per hour and thus would have
applied their brakes as they entered the curve in order to
slow down. The district court appears to have had some
doubt as to the legitimacy of this assumption. See Tr. 569-
74. However, given that the stretch of Kedzie Avenue
immediately to the north of the S-curve was more straight,
wide, and recently paved than the curve itself, not to
mention subject to a higher posted speed limit of forty-five
miles per hour, common sense and experience suggests that
No. 01-3885                                             51

southbound motorists frequently might have been traveling
at speeds in excess of thirty-five miles per hour as they
entered the curve. More to the point, the assumption that
they were is consistent with the contemporaneous observa-
tions of both County engineers and Olympia Fields police.
Recall that when Joseph Marsik, the County Highway
Department’s Chief Engineer of Maintenance, forwarded
Frank Reno’s memorandum about the possible problem
with the curve to Chief Engineer of Transportation and
Planning Arthur Kaindl, he wrote, inter alia, “It is our
belief that the traffic is going too fast on the straight
roadway, entering the curve, and then has a problem
negotiating the curve.” Tr. 500. The proffer of Olympia
Fields police officer Mark Fazzini likewise reveals a
perception that motorists were approaching the curve at
speeds substantially in excess of the advisory speed of
thirty miles per hour. According to Mihailovich’s counsel,
Fazzini would have testified:
   [T]he police department literally set up speed traps at
   the highway to try to get drivers to slow down because
   they knew that cars coming from the north down into
   this curve had been traveling through a semirural area
   and that, as we know in this case, Kedzie Avenue just
   north of the curve, was resurfaced in 1981, +82 at Biden
   and this resurfacing went right up to the beginning of
   this north curve and the cars would come into the curve
   at a high rate of speed because of their assurance they
   were on a good, sound highway.
Tr. 253. Thus, the notion that southbound drivers often
would be applying their brakes as they entered the curve,
and among other things thereby creating a higher frictional
demand on the roadway surface, does not, on its face,
appear to be implausible. Whether frequent brake applica-
tions would have resulted in a polishing of the roadway
surface, as Berg further hypothesized, is not a matter to
which we can speak. Yet, there is ample evidence in the
52                                              No. 01-3885

record that the surface of the curve was worn and smooth,
with a tendency to become quite slick when wet. E.g., Tr.
529, 661, 663; see also Tr. 252, 434-35.
  Third, Berg’s hypothesis that the slipperiness of the
pavement caught motorists by surprise notwithstanding the
posted warning signs appears, at the least, to be consistent
with the facts. The data in the record do reflect an ongoing
pattern of wet-pavement accidents that continued unabated
notwithstanding the warning signs that were added or
modified in 1984 (calling motorists’ attention to the curve,
advising a speed of thirty miles per hour through the curve,
and warning that the pavement was slippery when wet) and
again in 1986 (reducing the advisory speed to twenty-five
miles per hour). That pattern suggests that the signs,
although sufficient in the abstract to alert motorists to a
potential hazard (as Berg himself agreed), proved inade-
quate in practice to avert accidents.
  Fourth and finally, there appears to be at least some
evidentiary support, although weak, for Berg’s opinion that
the defects he identified in the roadway likely were at least
partially responsible for the Mihailovich accident. That
accident does fit the pattern revealed by the data in that it
involved a southbound vehicle on wet pavement leaving the
roadway (suggesting a loss of control) shortly after entering
the curve. Moreover, although Mihailovich could not speak
to what her husband was or was not doing as the driver of
the vehicle in the crucial seconds before the accident, she
did testify that he was driving at a “regular” rate of speed,
keeping pace with other traffic, and that there was nothing
unusual in the manner of his driving. It is a reasonable
inference from her testimony and the pattern of ongoing
accidents occurring at the curve under similar conditions
that Mr. Mihailovich was not driving in a manner out of
character with other traffic, and that he lost control of the
vehicle due to the combination of the higher frictional
demand his vehicle imposed as it entered the curve and the
No. 01-3885                                               53

reduced skid resistance posed by the wet pavement. See
Jahn v. Equine Servs., PSC, 233 F.3d 382, 390-92 (6th Cir.
2000) (Cudahy, J.) (in order to qualify for admission,
expert’s opinion as to causation need not eliminate all other
potential causes; expert’s opinion as to probable cause
admissible so long as it is based on facts and sound method-
ology); see also R. 123 at 9-10 (denying Laatsch’s motion for
summary judgment as to causation of Mihailovich accident).
  We leave it up to the district court on remand to deter-
mine whether Berg’s methodology reflects sufficient in-
tellectual and professional rigor to render his opinions
trustworthy enough to reach the jury. The parties should be
fully prepared to demonstrate to the court the respects in
which Berg’s methodology and conclusions do or do not
conform to the evidence or to civil engineering standards.


                            III.
  For the reasons set forth above, the judgment in favor of
the defendants-appellees is VACATED, and the case is
REMANDED for a new trial.
A true Copy:
      Teste:

                        ________________________________
                        Clerk of the United States Court of
                          Appeals for the Seventh Circuit




                    USCA-02-C-0072—3-5-04
