                         RECOMMENDED FOR FULL-TEXT PUBLICATION
                             Pursuant to Sixth Circuit I.O.P. 32.1(b)
                                      File Name: 18a0183p.06

                  UNITED STATES COURT OF APPEALS
                                FOR THE SIXTH CIRCUIT



 JAMIE L. WILDEN, Legal Guardian of Janice T.            ┐
 Wilden; JACOB YEAGER, Legal Guardian of Vincent         │
 Yeager,                                                 │
                             Plaintiffs-Appellants,      │
                                                         │
                                                          >    No. 17-6306
       v.                                                │
                                                         │
                                                         │
 LAURY TRANSPORTATION, LLC, et al.,                      │
                                          Defendants,    │
                                                         │
 GREAT DANE LIMITED PARTNERSHIP, aka Great Dane          │
 Trailers, Inc., aka Great Dane Trailers Limited         │
 Partnership,                                            │
                                                         │
                              Defendant-Appellee.
                                                         ┘

                        Appeal from the United States District Court
                     for the Western District of Kentucky at Louisville.
                     No. 3:13-cv-00784—David J. Hale, District Judge.

                                   Argued: July 31, 2018

                            Decided and Filed: August 23, 2018

                   Before: BOGGS, CLAY, and ROGERS, Circuit Judges.

                                      _________________

                                          COUNSEL

ARGUED: Paul F. Ferguson, Jr., THE FERGUSON LAW FIRM, Beaumont, Texas, for
Appellants. John L. Tate, STITES & HARBISON, PLLC, Louisville, Kentucky, for Appellee.
ON BRIEF: Paul F. Ferguson, Jr., THE FERGUSON LAW FIRM, Beaumont, Texas, Kevin C.
Burke, Jamie K. Neal, BURKE NEAL, PLLC, Louisville, Kentucky, Ronald P. Hillerich,
Louisville, Kentucky, Allen K. Gruner, Louisville, Kentucky, for Appellants. John L. Tate,
Marjorie A. Farris, STITES & HARBISON, PLLC, Louisville, Kentucky, for Appellee.
 No. 17-6306                     Wilden, et al. v. Laury Transp., et al.                   Page 2


                                       _________________

                                            OPINION
                                       _________________

          ROGERS, Circuit Judge. This Kentucky state-law products-liability case was brought on
behalf of Janice T. Wilden and her young son, both of whom were involved in a serious traffic
accident with an eighteen-wheel tractor-trailer. Janice Wilden suffered severe brain damage
when her sedan was pulled beneath the side of the trailer in what is known as a “side-underride”
crash.     The only remaining defendant is Great Dane Limited Partnership, the trailer’s
manufacturer. At issue on appeal is the district court’s exclusion of plaintiffs’ expert-witness
testimony about an alternative design that allegedly would have prevented, or at least mitigated,
Janice Wilden’s injuries. That alternative design is a so-called “telescoping side guard.” An
ordinary, fixed-position side guard would block the space underneath the side of the trailer so
that, in the event of a crash, automobiles would not go underneath. A telescoping side guard
would also slide and expand to protect the space opened up when a truck’s sliding rear-axle—
which trucks use to meet state and federal weight-per-axle regulations—is moved toward the rear
of the truck. The problem is that, although elements of the telescoping design have existed for
some time, and computer simulations suggest that the design could work, nobody has ever built
or tested one in the real world. Primarily on that basis, the district court held that the testimony
of the two expert witnesses was unreliable and thus inadmissible under Federal Rule of Evidence
702, and therefore granted summary judgment to Great Dane. In the context of this case,
including the total absence of real-world, physical-prototype testing and the fact that neither of
the experts had designed (let alone built) a telescoping side guard prior to this litigation, the
district court did not abuse its discretion in excluding the evidence. Summary judgment was thus
proper.

          On June 24, 2013, a sedan driven by nineteen-year-old Janice Wilden and containing her
infant son crashed into the side of a trailer, manufactured by defendant Great Dane, which was
being pulled behind a tractor that allegedly failed to yield the right-of-way. According to a
police accident report, the tractor-trailer was turning left into the northbound lanes of the
Greenbelt Highway—a divided highway in Louisville—when Wilden’s Chevrolet, which was
 No. 17-6306                         Wilden, et al. v. Laury Transp., et al.                            Page 3


traveling south on that highway, struck the left side of the trailer. The district court described the
resulting accident as follows:

        The Chevrolet’s right-front edge struck the left back tandem ax[le] of the trailer,
        and the remainder of the car went underneath the trailer, pushing past the
        windshield, a type of car-and-truck collision known as “underride.” The trailer’s
        rear wheels were in their most rearward possible position at the time of the crash.
        Perry Ponder, Plaintiffs’ expert, estimates that Wilden was traveling at 38 miles
        per hour and at a 63-degree angle relative to the trailer’s roadside floor rail when
        she struck the trailer.

Wilden v. Laury Transp., LLC, No. 3:13-cv-784-DJH-CHL, 2016 WL 4522670, at *1 (W.D. Ky.
Aug. 29, 2016) (record citations and footnote omitted). That version of the facts is not in dispute
for purposes of this appeal.          Tragically, Wilden suffered severe and debilitating injuries,
including brain damage. Her son’s injuries fortunately were not severe. The trailer involved in
the accident was manufactured by Great Dane in 1998 and lacked protection against side
underride.

        Tanya Wilden (Janice’s legal guardian) and Jacob Yeager (the legal guardian of Janice’s
son) brought this suit in Jefferson County Circuit Court. The case was subsequently removed to
federal district court, where Tanya Wilden was replaced as Janice Wilden’s legal guardian by
Jamie Wilden, the appellant here. (For convenience, we refer to the plaintiffs collectively as
“Wilden.”) The only remaining defendant is Great Dane and the only remaining claim is a
products-liability claim under Kentucky law. The specific claim is one of “crashworthiness,”1
which has three elements: “(1) an alternative safer design, practical under the circumstances;
(2) proof of what injuries, if any, would have resulted had the alternative, safer design been used;
and (3) some method of establishing the extent of enhanced injuries attributable to the defective
design.” Toyota Motor Corp. v. Gregory, 136 S.W.3d 35, 41 (Ky. 2004).

        To prove the existence of an alternative safer design, Wilden offered the opinions of two
experts, Perry Ponder and Bruce Enz, who proposed to testify to the feasibility of a telescoping


        1“In  a crashworthiness or enhanced injury case, the plaintiff claims not that a defect in a motor vehicle
caused a collision, but that a defect in the vehicle caused injuries over and above those which would have been
expected in the collision absent the defect. The claim, in essence, is that the design of the vehicle failed to
reasonably protect the occupant in a collision.” Toyota Motor Corp. v. Gregory, 136 S.W.3d 35, 41 (Ky. 2004).
 No. 17-6306                        Wilden, et al. v. Laury Transp., et al.                           Page 4


side guard. In Ponder’s description, a telescoping side guard is “a horizontal bar underneath the
side of the trailer that would expand or slide rearward as the trailer tandems are repositioned
rearward underneath the trailer.” Critically, only a telescoping side guard would have prevented
or reduced the extent of Janice Wilden’s injuries. The rear axle on modern tractor-trailers can be
extended rearward to comply with various state and federal regulations. Here, according to
Ponder, the trailer’s wheels were in the “most rearward possible position” at the time of the
accident, and Janice Wilden’s sedan hit the trailer near the rear axle. With a regular, non-
telescoping side guard, there would still have been a 78-inch gap between the rear tires and the
side guard, large enough to fit Janice Wilden’s 72-inch-wide sedan. Thus, only a telescoping
design would have extended far enough back to the rear wheels so as possibly to prevent
underride from occurring there. In his expert report prepared for this litigation, Ponder proposed
a design for a telescoping side guard. Ponder’s report also included another, similar telescoping
design developed (but apparently never built) by Strick Trailers in 2000.

        Non-telescoping, fixed-position side guards exist, though they are not an industry
standard and are not legally required.            Indeed, Enz built some years before this lawsuit.
However, neither Ponder’s design, nor the Strick design, nor any other telescoping side guard
has ever been built. As evidence of prior telescoping designs, Wilden points to a 1968 report
sponsored by the federal government and a 1977 patent, but Wilden concedes that both of these
designs “call for telescoping guards in an upward rather than lateral manner.” Thus, even if
these designs could be built and had been on the trailer in this case, they could not have extended
horizontally to prevent underride from occurring. Wilden also cites a 2006 patent application by
several people including Enz, but which Enz himself stated did not incorporate a telescoping
design.2 Additionally, Wilden observes that there are a few patents—such as the 2000 Strick
Trailers Patent mentioned in Ponder’s report—for side guards with telescoping elements
(although it is not at all clear from Wilden’s briefs that these designs could telescope horizontally
to accommodate a sliding rear axle, as would have been necessary here). At any rate, regardless
of the precise mechanics of these designs, it is undisputed that no horizontally telescoping side

        2When    Great Dane’s lawyer asked Enz in his deposition whether the 2006 patent “had a telescoping
section,” Enz responded, “It did not, although that would have been intended to be part and parcel to the—to the
unit.” Enz also testified that this patent application was denied.
 No. 17-6306                     Wilden, et al. v. Laury Transp., et al.                    Page 5


guard has ever actually been built or physically tested, let alone used in the trucking industry.
Indeed, the most testing that Ponder did, even for this litigation, was to test the load-bearing
capabilities of his telescoping design via finite element analysis—a mathematical, computerized
method for determining how a product will react to real-world conditions. Ponder did not,
however, conduct his finite element analysis using the 63-degree angle of impact that occurred in
Janice Wilden’s accident. Enz did not perform any tests on Ponder’s design.

       Great Dane moved in limine to exclude the opinions of Ponder and Enz and also for
summary judgment.        Performing its gatekeeping role under Daubert v. Merrell Dow
Pharmaceuticals, Inc., 509 U.S. 579 (1993), the district court found Ponder’s and Enz’s opinions
to be unreliable and thus inadmissible under Federal Rule of Evidence 702. In reaching this
conclusion, the district court considered the following factors relating to reliability: (1) “whether
a ‘theory or technique . . . can be (and has been) tested’”; (2) “whether the theory or technique
enjoys ‘general acceptance’ within a ‘relevant scientific community’”; and (3) whether the
design was prepared solely for this litigation. Wilden, 2016 WL 4522670, at *2 (ellipses in
original) (quoting Johnson v. Manitowoc Boom Trucks, Inc., 484 F.3d 426, 429 (6th Cir. 2007)).

       The district court first concluded that the telescoping side guard was not adequately
tested, and thus this factor weighed against admissibility. The court found that, although Ponder
and Enz were highly qualified, neither had expertise in telescoping side guards in particular. The
court therefore determined that “testing is necessary.” Id. at *3. The court then concluded that
the telescoping side guard had not been adequately tested in view of “the complexities behind
industrial equipment, the extensive testing necessary to prudently test underride side guards prior
to retail, and the relatively minimal testing that Ponder and Enz conducted,” which consisted
solely of computer modeling. Id. at *4 (internal citations omitted).

       The court also reviewed the second factor—general acceptance—and concluded that it
too weighed against admissibility:

               Unlike Johnson, where the expert’s proposed alternative was accepted and
       in use within the relevant industry, Ponder and Enz’s prototype of the proposed
       telescoping side guard has never been built. Although Ponder’s report states that
       the design principles have been known for a long time, this does not mean that the
       telescoping side guard proposed by Ponder and Enz has been accepted by their
 No. 17-6306                     Wilden, et al. v. Laury Transp., et al.                  Page 6


        peers. Nor could it be, since it only exists as Ponder and Enz’s concept. This
        factor therefore also weighs against admitting Ponder’s and Enz’s testimonies.

Id. (record citations omitted). The court also discussed the prepared-for-litigation factor, but
considered that factor “neutral” because, although Ponder and Enz had not personally developed
a telescoping design before this litigation, the concept appeared to have at least existed. Id. at
*4-5.

        All told, the district court concluded that the evidence was inadmissible:

               In sum, Ponder’s and Enz’s testimonies fail to meet any of the relevant
        Daubert factors. The testing was insufficient by Enz’s own standards. The
        proposed alternative design is not used in the relevant industry; it is new and not
        generally accepted. And even though the idea of an expanding side guard appears
        to have already existed, the telescoping bar was not created by Ponder or Enz
        prior to this litigation. The Court therefore finds that these opinions are
        unreliable. As such, they are inadmissible under FRE 702.

Id. at *5 (record citations omitted). Finally, concluding that the opinions of Ponder and Enz
were essential to Wilden’s claims because only a telescoping design would have prevented
underride, the court granted summary judgment to Great Dane. Id.

        Wilden now appeals.

        The district court did not abuse its discretion in excluding the testimony of Ponder and
Enz.    Although the record shows that Ponder did perform some computerized testing and
modeling, it was not unreasonable for the district court to require physical-prototype testing
given that nobody has ever built a telescoping side guard.           Further, while the idea of a
telescoping side guard has existed for some time, the design that Ponder offers here was created
for this litigation. For these and other reasons, the expert evidence was permissibly excluded by
the district court. Moreover, because Wilden’s claim cannot survive without that evidence,
summary judgment was proper.
 No. 17-6306                     Wilden, et al. v. Laury Transp., et al.                      Page 7


       Federal Rule of Evidence 702 governs the admissibility of expert testimony. It provides:

       A witness who is qualified as an expert by knowledge, skill, experience, training,
       or education may testify in the form of an opinion or otherwise if:
           (a) the expert’s scientific, technical, or other specialized knowledge will help
               the trier of fact to understand the evidence or to determine a fact in issue;
           (b) the testimony is based on sufficient facts or data;
           (c) the testimony is the product of reliable principles and methods; and
           (d) the expert has reliably applied the principles and methods to the facts of
               the case.

As interpreted in Daubert, Rule 702 grants district courts “discretion in determining whether . . .
a proposed expert’s testimony is admissible, based on whether it is both relevant and reliable.”
Johnson, 484 F.3d at 429. The Supreme Court has identified several non-exclusive factors that
lower courts may consider in assessing reliability: (1) whether a theory or technique can be (and
has been) tested; (2) whether the theory or technique has been subjected to peer review and
publication; (3) whether the technique has a high known or potential rate of error; and
(4) whether the technique enjoys general acceptance within the relevant scientific, technical, or
other specialized community. Daubert, 509 U.S. at 593–94; Kumho Tire Co. v. Carmichael,
526 U.S. 137, 147-50 (1999). We have approved the use of an additional factor: whether the
expert prepared his or her opinion “solely for purposes of litigation.” See Johnson, 484 F.3d at
434.
       Here, the district court considered three of the factors and determined (1) that the
telescoping side guard was inadequately tested, (2) that it was not generally accepted, and
(3) that the prepared-for-ligation factor was “neutral.” On balance, therefore, the district court
concluded that Ponder’s and Enz’s testimony was unreliable and accordingly inadmissible. We
review that determination for abuse of discretion, and we are “highly deferential when assessing
not just a trial court’s analysis of each factor, but also the trial court’s initial selection of which
factors are relevant to the case at hand.” Id. at 430; see also Kumho Tire Co., 526 U.S. at 152–
53.
       Taking the testing factor first, it was permissible for the district court to determine that
the telescoping design was inadequately tested.              Nobody—engineer, manufacturer, or
 No. 17-6306                    Wilden, et al. v. Laury Transp., et al.                    Page 8


otherwise—has ever built or tested a physical prototype of a telescoping side guard. Ponder’s
testimony was quite clear on this point:

       Q. You are not aware of anyone who has built a guard with a telescoping section
          incorporated as part of the design; correct?
       A. Actually constructed it? That’s correct.
       Q. And that’s—whether it’s you or another consultant like yourself or any trailer
          manufacturer or anyone else, an academic at some university, anywhere in
          the world; correct? You’re not aware of anybody having attempted to build a
          side guard with [a] telescoping component; correct?
       A. Well, the industry certainly hasn’t done it, and I’m not aware that’s correct.
       Q.     So no one, to your knowledge, in the world has attempted to build a guard
              with a tele[s]coping section; correct?
       A. That’s right, to my knowledge.
       Q. And, therefore, no one has been able to take a constructed guard and perform
          any type of testing on it to see how strong or what type [of] loads might be
          applied to the telescoping section; correct?
       [objection]
       A. Correct.

Enz gave similar testimony:
       Q.     Okay. You have not built and tested a telescoping guard component, have
              you?
       A.     No, sir.
       ....
       Q. To your knowledge, nobody in the world has built a telescoping guard for the
          side of a trailer, correct?

       A. Not to my knowledge.

       Moreover, Enz testified that, before selling a trailer with side-underride protection in
1998, a reasonably prudent engineer would have “come up with a concept,” and then would have
“develop[ed] the concept . . . perhaps even [had] a prototype made discussing some of the pros
and the cons, and seeing if they can eliminate the cons, or if they are valid. And then obviously
the prototype has got to come along, and then testing.” As the district court recognized, Enz’s
 No. 17-6306                    Wilden, et al. v. Laury Transp., et al.                     Page 9


own standards were not met here, where there was no physical testing on a prototype of a
telescoping side guard. See Wilden, 2016 WL 4522670, at *3-4.

       It is unsurprising that Enz would have specified such high standards for a new concept,
given that underride protection presents a complex engineering challenge. As the National
Highway Traffic Safety Administration (“NHTSA”) explained in its rule requiring rear
underride guards:

       Underride occurs to some extent in most collisions in which a passenger vehicle
       crashes into the rear end of a large trailer because most trailer beds are higher than
       the hoods of passenger vehicles. In the worst cases, referred to as passenger
       compartment intrusion (PCI) or “excessive underride” crashes, the passenger
       vehicle underrides so far that the rear end of the trailer strikes and enters its
       passenger compartment. PCI collisions generally result in passenger vehicle
       occupant injuries and fatalities caused by occupant contact with the rear end of
       the trailer.
       The solution to PCI is upgrading underride guards to make them stronger, but this
       introduces another concern. Even if guards succeed in preventing PCI, overly
       rigid guards may stop the passenger vehicle too suddenly, resulting in excessive
       occupant compartment deceleration forces and killing or injuring passenger
       vehicle occupants.

Federal Motor Vehicle Safety Standards Rear Impact Guards; Rear Impact Protection, 61 Fed.
Reg. 2004, 2004 (Jan. 24, 1996). Thus, “[t]he key engineering challenge in designing such a
guard involves a trade-off between the strength of a rear guard and its capacity to absorb
energy.” Rapp v. Singh, 152 F. Supp. 2d 694, 696 (E.D. Pa. 2001). Too strong and the guard
will stop cars too quickly; too flexible and the guard will not adequately prevent underride.
Underride protection must therefore strike a careful balance between rigidity and energy
absorption.

       Physical-prototype testing helps find that balance. Indeed, dynamic crash testing played
an important role in the development of the federal standards for rear underride guards, which
are now an industry standard. The NHTSA first proposed rigid rear underride guards that could
withstand 75,000 pounds of static force. See Rear Underride Protection; Trailers and Trucks
With Gross Vehicle Weight Rating Over 10,000 Pounds, 34 Fed. Reg. 5383, 5384 (Mar. 19,
1969). Later, this was revised to 50,000 pounds. See Rear Underride Protection; Trucks and
 No. 17-6306                     Wilden, et al. v. Laury Transp., et al.                   Page 10


Trailers, Notice of Proposed Rule Making, 35 Fed. Reg. 12,956, 12,957 (Aug. 14, 1970). But
after dynamic crash testing, the NHTSA concluded that such guards were not effective because,
even where excessive underride was prevented, the “rigid guards increase[d] the deceleration
forces experienced by car occupants in a crash and thus increase[d] the risk of injury due to
hazards other than underride.”      Federal Motor Vehicle Safety Standards; Rear Underride
Protection, 46 Fed. Reg. 2136, 2138 (Jan. 8, 1981). The final rule provided for rear guards far
different from those originally proposed, requiring each guard to withstand 22,480 pounds of
force at certain points, and half that amount at others. See Federal Motor Vehicle Safety
Standards Rear Impact Guards; Rear Impact Protection, 61 Fed. Reg. at 2009-10. The guards
also must displace at least 5 inches and must absorb at least 5,650 joules during that
displacement. Id. at 2011. In short, designing underride protection is complicated and crash
testing helps get it right.

        There is also reason to believe that designing side underride protection, and especially a
telescoping side guard, is even more complicated than rear underride protection. Trailers are
much longer than they are wide, so side guards must protect a larger area than rear guards.
Moreover, while rear collisions usually involve a perpendicular impact as the car runs straight
into the rear of the trailer, side collisions happen at a wider range of impact angles, such as the
63-degree angle at which Janice Wilden’s sedan struck the trailer in this case. Finally, sliding
rear-wheel assemblies mean that a telescoping design is needed, which makes the design more
complicated by introducing moving parts and extra joints. Thus, physical-prototype testing is
even more important in this context.

        In view of these considerations, the district court did not abuse its discretion by requiring
physical-prototype testing. The concept here exists only in theory, rendered on paper and in
computers, but never given physical form. Particularly due to the difficulty of achieving the
right balance between strength and flexibility—as demonstrated by the history of rear-guard
 No. 17-6306                          Wilden, et al. v. Laury Transp., et al.                              Page 11


regulation—and the even greater difficulties of designing side guards, it made good sense for the
district court to require some physical testing on the facts of this case.3

         Precedent supports this conclusion. In Johnson, 484 F.3d at 426, we upheld the district
court’s exclusion of expert testimony about an interlocking-outrigger system, which, according
to the expert, would have prevented a truck-mounted crane from falling over onto the victim.
Typically, truck-mounted cranes (also known as “boom truck cranes”) are braced with several
spider-like legs (called “outriggers”) that anchor into the ground to stabilize the crane while it is
in use. Id. at 427. The accident in Johnson occurred when one of the outriggers was retracted
while the crane was in use, causing it to tip over onto the victim. The plaintiff’s expert would
have testified about the feasibility of a so-called interlocking-outrigger system, which would
have prevented the crane from operating if any of the outriggers were not in contact with the
ground. Id. at 428. Such a system had been used on a smaller kind of crane for some time, but
had not been used on the much larger crane at issue in Johnson. Therefore, the plaintiff’s expert
proposed to testify about retrofitting the system onto the larger crane. However, the expert “did
not actually test his schematic, . . . in other words, he conducted no empirical research to
determine just how functional his proposed retrofit of the [large crane] might be.” Id. We
concluded that the district court did not abuse its discretion in excluding the expert’s testimony,
reasoning that testing was necessary under the circumstances because “the design of industrial
equipment is a complex process and changes to prevent one problem could create other
problems, thus increasing the overall danger of using a product.” Id. at 431 (quoting Brown v.
Raymond Corp., 432 F.3d 640, 648 (6th Cir. 2005)). As discussed above, the same concerns are
present here.
         Wilden cites several out-of-circuit cases that purportedly show that a district court may
never require physical-prototype testing, but these cases do not actually support that extreme
proposition. Wilden first points to Quilez-Velar v. Ox Bodies, Inc., 823 F.3d 712 (1st Cir. 2016),
another case in which Ponder served as an expert witness. Quilez-Velar rejected the defendant’s
argument that “Ponder’s testimony should have been excluded under Daubert because the expert

         3This    conclusion is only reinforced by the fact that the merits question here is not whether a telescoping
design would be practicable today, but whether such a design would have been practicable in 1998 when Great Dane
sold the trailer involved in the accident.
 No. 17-6306                    Wilden, et al. v. Laury Transp., et al.                  Page 12


must have actually tested the alternative design, either physically or using computer modeling,
and Ponder did not do so,” reasoning that this “argument rests on a profound misunderstanding
of Daubert, which eschews such per se approaches.” Id. at 718. But, crucially, that case was
about rear guards, see id. at 717, which have been on trucks for decades, and have been
physically tested many times. Telescoping side guards, on the other hand, have never been built
or tested. Quilez-Velar thus helpfully demonstrates the kind of case where prototype testing
might not be needed. As the district court here explained, “the need for testing is lessened if the
proposed alternative design is simple or is already in use in the industry.” Wilden, 2016 WL
4522670, at *3 (emphasis added) (internal quotation marks and citation omitted).               The
telescoping side guard is neither simple nor in use; the rear guard is at least the latter. Thus,
although it might well be an abuse of discretion for a district court to require physical-prototype
testing of a fixed-position rear guard when such devices have long been in use, here it was
reasonable to expect some physical testing of the telescoping design that has never been built.
This conclusion accords with Quilez-Velar’s observation that the Daubert inquiry “eschews . . .
per se approaches,” 823 F.3d at 718, and “depends upon the particular circumstances of the
particular case at issue,” id. (quoting Kumho, 526 U.S. at 150). Moreover, the standard of review
cut the opposite way in Quilez-Velar. There, the district court had admitted Ponder’s evidence
and the defendant thus had to show that such admission amounted to an abuse of discretion.
Quilez-Velar’s conclusion—that a district court may, in some circumstances, admit expert
evidence despite a lack of physical testing—says little here, where the question is whether a
district court may, in other circumstances, exclude expert evidence because of the absence of
such testing.

       Wilden also cites Unrein v. Timesavers, Inc., 394 F.3d 1008, 1012 (8th Cir. 2005), in
which the Eighth Circuit observed that its cases “do not require that experts manufacture a new
device or prototype in order for their opinion to be admitted. The question is whether the
expert’s opinion is sufficiently grounded to be helpful to the jury.”       But this statement is
consistent with our approach.     We, too, do not have a bright-line rule requiring physical-
prototype testing. Nor do we adopt such a rule here. Rather, we simply recognize that each case
must be decided on its own facts, which sometimes require testing and sometimes do not.
Moreover, the court in Unrein went on to affirm the district court’s exclusion of the disputed
 No. 17-6306                    Wilden, et al. v. Laury Transp., et al.                   Page 13


expert testimony, reasoning that although the expert “proposed using a safety trip cord, a
commonly used device, he did not prepare drawings showing how it would be integrated into the
[industrial] sander or present photographs showing its use with similar machines.” Id. The court
also noted that “[a]n expert proposing safety modifications must demonstrate by some means
that they would work.” Id. Unrein thus supports affirmance here, where there is little indication
of whether the telescoping design would work.

       Wilden further cites Brochtrup v. Mercury Marine, 426 F. App’x 335, 339 (5th Cir.
2011), in which the Fifth Circuit discussed a decision of the Texas Supreme Court, General
Motors Corp. v. Sanchez, 997 S.W.2d 584 (Tex. 1999). The language that Wilden quotes from
Sanchez, however, dealt with a question of substantive Texas products-liability law, not the
admissibility of expert evidence under federal law. See id. at 590-92. In sum, the dicta Wilden
has pulled from out-of-circuit cases provide scant support for reversal here because none of it
suggests that a district court lacks the discretion to require physical-prototype testing under
appropriate circumstances.

       Wilden also argues that, even if prototype testing might sometimes be required, such
testing was not necessary here due to the extensive expertise that Ponder and Enz have in the
area of side-underride protection. However, this argument fails. First, there is no per se rule in
this circuit that physical testing is unnecessary whenever an expert has sufficient expertise with
the particular matter in question. Wilden relies on our statement in Johnson that “[o]ne way to
overcome the testing requirement might be to show that the expert has significant technical
expertise in the specific area in which he is suggesting an alternative design.” 484 F.3d at 431.
But Johnson noted that this proposition came from an out-of-circuit district court case, Bah v.
Nordson Corp., No. 00CIV9060DAB, 2005 WL 1813023 (S.D.N.Y. Aug. 1, 2005), and then
went on to recognize that “even if the logic of Bah were binding on this Court (which it is not,
seeing as it stems from a district court in a different circuit than our own), it would not apply to
the case at hand.” Johnson, 484 F.3d at 432. There is thus no rule in this circuit that expertise
necessarily excuses the need to test a design. Moreover, this statement was dictum. In Johnson,
we concluded that the expert in question was a generalist without specific expertise, and we
therefore affirmed the district court’s exclusion of expert evidence without deciding whether to
 No. 17-6306                       Wilden, et al. v. Laury Transp., et al.                   Page 14


adopt Bah’s reasoning. See id. Additionally, even if Bah were correct that an expert’s testimony
is reliable so long as the expert has “‘extensive experience’ with the very types of machines at
issue in the case,” id. (quoting Bah, 2005 WL 1813023, at *8), that would not matter here
because neither Ponder nor Enz—nor indeed anyone—can be said to have “extensive
experience” with telescoping side guards which have never been built.

           Next, Wilden contends that even if some testing was required to validate the telescoping
design, Ponder and Enz satisfied that requirement through mathematical modeling and computer
testing.     But the district court here had discretion to require physical-prototype testing.
Computer modeling might sometimes be enough; indeed, it might have sufficed had this case
been about the industry-standard rear guard. Because a telescoping side guard has never been
built, however, the district court was permitted to require more to ensure the reliability of
Ponder’s and Enz’s opinions.

           Wilden offers various arguments as to why computerized testing was sufficient, but they
are not persuasive. First, Wilden argues that Ponder’s computer modeling was enough for
admissibility because the modeling complied with the Federal Motor Vehicle Safety Standards
for rear underride guards. See 49 C.F.R. § 571.223. Even though that regulation appears to call
for some physical testing, see id. (“S6. Guard Test Procedures”), Wilden contends that physical
testing is not in fact required because, according to a 1997 letter from the NHTSA, rear guard
designs can be validated by “other kinds of testing or even engineering analysis.” See Letter
from John Womack, Acting Chief Counsel, NHTSA, to Frank Smidler, Dir. of Eng’g, Wabash
Nat’l Corp. (April 29, 1997). This argument is unavailing, however, because even if Wilden’s
interpretation of the 1997 letter is correct (which is doubtful), general NHTSA standards for
testing rear guards are not particularly helpful in determining how to validate the novel
telescoping side guard. Second, Wilden maintains that a report commissioned by the European
Union uses finite element analysis to test side-underride protection. Great Dane contends that
this report does not in fact support Wilden’s position because crash tests were later used to
validate the report’s numerical model. Regardless of who is correct, this one EU report is not
enough to show that the district court lacked discretion to require physical testing here.
 No. 17-6306                        Wilden, et al. v. Laury Transp., et al.                         Page 15


        It is true that Johnson recognized that non-physical testing may sometimes suffice: “We
can imagine innumerable tests that could have been conducted by [the expert]—all well short of
building a full-fledged prototype of the Manitowoc 2592, but all well beyond drawing a one-
page diagram—that would have demonstrated the practicality of his proposed design.” 484 F.3d
at 433. We need not disagree with this statement to affirm the district court here. That non-
physical testing perhaps would have sufficed in Johnson says nothing about this case. Johnson is
factually distinguishable because there someone had at least built an interlocking-outrigger
system, just not one as big as would have been required to fit the crane in question. Johnson was
thus about adapting a safety device from a small crane to a large crane, a difference in degree
rather than in type. In contrast, the telescoping side guard exists only on paper and in computers.
The design at issue in Johnson was not as novel as the one here, and for that reason different
testing may have been sufficient.4

        Accordingly, on the particular facts of this case, the district court’s determination that the
testing factor weighed against admissibility was not an abuse of discretion.

        Moving on to the general-acceptance factor, the district court reasoned that, because the
telescoping design had “never been built,” it could not be “generally accepted” within the
trucking industry. Wilden, 2016 WL 4522670, at *4-5. This was a sensible conclusion. Of
course, “general acceptance” refers most naturally to scientific methodology rather than product
design, but as the Supreme Court held in Kumho, 526 U.S. at 149-50, and as we reaffirmed in
Clay v. Ford Motor Co., 215 F.3d 663 (6th Cir. 2000), “general acceptance . . . may be
considered by the district court even when the proffered expert testimony is not scientific,” id. at
667. The parties dispute the precise meaning of this factor. We have previously equated general
acceptance with “industry custom.” Johnson, 484 F.3d at 434. Wilden argues that the proper
inquiry is not whether the trucking industry has already adopted telescoping side guards, but
whether they are generally accepted as being feasible. We need not decide which is the correct
inquiry because, regardless of what it means to be “generally accepted” in this context, the
telescoping design is not. It cannot be an industry custom to use something that has never once

         4Moreover, the above-quoted statement is dictum, as the expert in Johnson conducted no testing and the
panel thus had no occasion to consider the efficacy of a particular form of testing.
 No. 17-6306                    Wilden, et al. v. Laury Transp., et al.                    Page 16


been built. Nor has Wilden shown that the telescoping side guard is even generally accepted as
being feasible. Wilden points to the Strick Design and a few patents, but these items are few in
number and show, at most, that the telescoping side guard existed as a concept. In Daubert, the
Supreme Court observed that “a known technique which has been able to attract only minimal
support within the community may properly be viewed with skepticism.” 509 U.S. at 594
(internal quotation marks and citation omitted). Because that characterization applies to the
telescoping side guard, which has been built by no one, the district court permissibly concluded
that the general-acceptance factor also weighs against admissibility.

       Although not one of the considerations mentioned in Daubert itself, we have recognized
that “expert testimony prepared solely for purposes of litigation, as opposed to testimony flowing
naturally from an expert’s line of scientific research or technical work, should be viewed with
some caution.” Johnson, 484 F.3d at 434. This factor likewise cuts against admissibility here.
The district court found that, although Ponder and Enz were not “quintessential experts for hire,”
Wilden, 2016 WL 4522670, at *4 (alteration omitted) (quoting Johnson, 484 F.3d at 435), and
that they were unquestionably qualified, the particular telescoping design proffered here was
“conceived after the start of this litigation” and was not a product of their independent work, id.
The court concluded, however, that because the telescoping concept might have existed prior to
the litigation (even if Ponder’s particular design did not), this factor was “neutral.” Id. at *5.
Wilden argues that this factor favors admission because both Ponder and Enz have “participated
in the design of side underride guards completely apart from litigation.” Yet neither of them
developed a telescoping side guard until the start of this litigation. Indeed, Ponder stated in his
deposition that he made drawings of his telescoping design sometime after November 2013,
which means the drawings were created at least several months after this lawsuit was filed.
Wilden also points out that in 2006 Enz and others devised a patent for an extendable side guard.
But Enz explicitly testified that it was not a telescoping design. Because neither Ponder nor Enz
developed a telescoping design until hired to do so (apparently in response to the particular
accident in this case), Ponder’s design was not a natural outgrowth of his independent research.
Thus, this factor too weighs against admissibility, though not as heavily as the others.
 No. 17-6306                          Wilden, et al. v. Laury Transp., et al.                            Page 17


        The district court did not consider the other two factors mentioned in Daubert—error rate
and peer review—because, according to the district court, “there is little if any published or peer-
reviewed information regarding the proposed alternative design and the third factor [error rate] is
irrelevant to the case at hand.” Id. at *2. Wilden does not dispute the court’s conclusion as to
the error-rate factor but argues that the peer-review and publication factor is relevant and favors
admissibility. However, contrary to Wilden’s contention, to the extent that the peer-review
factor is relevant it cuts the opposite way. Wilden cites several academic articles, including
many authored at least in part by Ponder or Enz, and many of which do appear to come from
peer-reviewed journals. But Wilden does not argue that any of those articles address telescoping
side guards, as opposed to the problem of side underride in general. Moreover, although Wilden
argues that “telescoping side guards have appeared in published works for decades,” this is
mostly a reference to various patents that supposedly incorporated telescoping elements, and
patents are not peer-reviewed.5 Thus, even if this factor were relevant, it would weigh against
admissibility. What is more, our review of the district court’s choice of factors is “highly
deferential.” Johnson, 484 F.3d at 430. “[W]hether Daubert’s specific factors are, or are not,
reasonable measures of reliability in a particular case is a matter that the law grants the trial
judge broad latitude to determine.” Kumho, 526 U.S. at 153. Under that standard of review,
Wilden has not provided nearly enough for us to second-guess the district court’s decision to
exclude the peer-review and publication factor from its analysis.

        Finally, Wilden advances the policy argument that affirming in this case will unduly
hamper future products-liability litigation due to the expense of building physical prototypes.
But, contrary to Wilden’s contention, by affirming here we do not create a bright-line rule that
plaintiffs must always physically test proposed alternative designs. Rather, as has long been the
case, “‘the gatekeeping inquiry must be tied to the facts of a particular case,’ depending on ‘the
nature of the issue, the expert’s particular expertise, and the subject of his testimony.’” Johnson,


        5Almost     all communication regarding whether to grant a patent occurs only between the applicant and the
patent examiner. In fact, “[e]xaminers may neither consult the public, talk to experts, nor, in many cases, even use
the Internet.” Beth Simone Noveck, “Peer to Patent”: Collective Intelligence, Open Review, and Patent Reform,
20 Harv. J. L. & Tech. 123, 124 (2006). Moreover, “independent third-party input is greatly restricted; it must be
made by mail, within a two-month window, for a fee of $180, and without commentary. Not surprisingly, third-
party input is rarely given.” Id. at 136.
 No. 17-6306                     Wilden, et al. v. Laury Transp., et al.                  Page 18


484 F.3d at 430 (quoting Kumho, 526 U.S. at 150). As the First Circuit noted in Quilez-Velar,
Daubert “eschews . . . per se approaches.” 823 F.3d at 718. We do not apply a per se approach
today, but instead reach only the common-sense conclusion that the district court had the
discretion to require physical-prototype testing of this particular never-built design.

       Wilden also argues that the district court’s grant of summary judgment was erroneous,
but the evidentiary issue is dispositive. Indeed, Wilden does not contend that summary judgment
can be avoided without Ponder’s and Enz’s testimony.

       For these reasons, the district court’s judgment is affirmed.
