                  FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

LORI OBERSON, Legal Guardian for        
Brian Musselman, an incapacitated
person; KIMBERLEE MUSSELMAN,
individually and as the Natural
Mother of Devon Musselman, a
minor,
               Plaintiffs-Appellees-
                  Cross-Appellants,
                 v.
UNITED STATES DEPARTMENT OF
AGRICULTURE, FOREST SERVICE,                  Nos. 04-35268
             Defendant-Third-Party                 04-35315
                Plaintiff-Appellant-
                    Cross-Appellee,            D.C. No.
                                            CV-99-00048-DWM
                and                          OPINION AND
STATE OF MONTANA, by and                         ORDER
through the Department of Fish,
Wildlife and Parks; WEST
YELLOWSTONE CHAMBER OF
COMMERCE,
                 Defendants-Third-
                    Party Plaintiffs,
                 v.
JAMIE LOUIS LEINBERGER; PATRICK
B. KALAHAR; TIM A. JOHNSON,
           Third-Party Defendants.
                                        
        Appeal from the United States District Court
                for the District of Montana
        Donald W. Molloy, District Judge, Presiding

                             2865
2866                     OBERSON v. USDA
                  Argued and Submitted
           December 7, 2005—Seattle, Washington

                      Filed March 20, 2006

       Before: Ronald M. Gould and Marsha S. Berzon,
         Circuit Judges, and William W Schwarzer,*
                    Senior District Judge.

                  Opinion by Judge Schwarzer




  *The Honorable William W Schwarzer, Senior United States District
Judge for the Northern District of California, sitting by designation.
                     OBERSON v. USDA                   2869


                OPINION AND ORDER

SCHWARZER, Senior District Judge:

   I.   CERTIFICATION OF QUESTIONS OF LAW

  Pursuant to Rule 44(c) of the Montana Rules of Appellate
Procedure, we respectfully request the Montana Supreme
Court to exercise its discretion to adjudicate the following
questions of Montana law:

  1. Does the gross negligence standard of care in the
snowmobile liability statute, MONT. CODE ANN. § 23-2-653
(1996), violate the Montana equal protection clause, MONT.
CONST. art. II, § 4?
2870                   OBERSON v. USDA
   2. If the snowmobile liability statute’s gross negligence
standard is unconstitutional, does the recreational use statute’s
willful or wanton misconduct standard of care, MONT. CODE
ANN. § 70-16-302(1) (1996), apply in its place?

   3. If neither the snowmobile liability statute nor the recre-
ational use statute provide an applicable standard of care, does
the ordinary care standard, MONT. CODE ANN. § 27-1-701,
apply?

   The answers to the certified questions will be determinative
of the appeal pending in our court in this action. We have
found no controlling Montana appellate decisions, constitu-
tional provisions, or statutes for these questions. We acknowl-
edge that your Court may decide to reformulate the questions,
and that our phrasing of the questions is not intended to
restrict your Court’s consideration of this request. Relevant
facts concerning the certified questions are stated below. We
would be grateful for any guidance your Court can give us,
whether or not directly responsive to the questions as we have
phrased them.

          II.   PROCEDURAL BACKGROUND

   Brian Musselman (Musselman) was gravely injured in a
snowmobile accident on a National Forest trail. Lori Oberson,
his legal guardian, and others brought this action against the
United States under the Federal Tort Claims Act (FTCA), 28
U.S.C. §§ 2671-2680, alleging that the United States Forest
Service negligently failed to correct or warn of a dangerous
condition on the Big Sky snowmobile trail where Musselman
was injured. The United States filed a third-party complaint
against Jamie Leinberger (Leinberger), Patrick Kalahar (Kala-
har) and Tim Johnson (Johnson), alleging that their negli-
gence caused Musselman’s injuries. Default was taken against
Johnson. Kalahar settled with plaintiffs and was dismissed
before trial. Following a bench trial, the district court entered
judgment for plaintiffs, awarding damages of $11,296,800
                            OBERSON v. USDA                             2871
and apportioning liability 40% to the Forest Service, 50% to
Leinberger and 10% to Musselman.1

   The Forest Service appealed, citing as error the district
court’s refusal to apply the discretionary function exception of
the FTCA and its finding of negligence. Plaintiffs cross-
appealed from the district court’s liability allocation of 50%
to third-party defendants and its methodology for calculating
life expectancy.2 We affirm the district court in all respects
except that we leave for future resolution the determinative
question of the appropriate standard of care pending the dis-
position of this request for certification.
  1
    The opinion of the district court is reported at 311 F. Supp. 2d 917 (D.
Mont. 2004).
  2
    Plaintiffs advance three contentions on their cross-appeal. We find
each to be without merit.
   First, they contend that the district court erred in holding Kalahar and
Leinberger jointly and severally liable for Musselman’s injuries and
thereby apportioning 50% of the liability to Leinberger, the remaining
tortfeasor. The court correctly rejected plaintiffs’ argument that a meeting
of the minds had to be proved to establish joint and several liability, rely-
ing on the leading case of Summers v. Tice, 199 P.2d 1 (Cal. 1948), which
held that where two tortfeasors are each negligent but it cannot be deter-
mined whose negligence was the actual cause of the injuries, each is liable
for the entire damage. See Azure v. City of Billings, 596 P.2d 460, 470
(Mont. 1979) (citing Tice, 199 P.2d 1).
   Second, they contend that the district court violated the “empty chair”
rule by factoring Kalahar’s negligence in the allocation of 50% liability to
Kalahar and Leinberger. The court, however, specifically acknowledged
that no liability can be apportioned to a defendant who has settled and
allocated liability solely to Leinberger. See Azure, 596 P.2d at 469.
   Third, they contend that the district court erred in accepting the Forest
Service’s expert’s estimate of Musselman’s life expectancy, which did not
take into account the quality of medical care provided to Musselman. The
court’s life expectancy finding is a finding of fact reviewed for clear error.
The finding was supported by substantial evidence and has not been
shown to be clearly erroneous. Sines v. United States, 430 F.2d 644, 645
(9th Cir. 1970).
2872                  OBERSON v. USDA
                        III.   FACTS

A.     THE ACCIDENT

   On February 25, 1996, Musselman, an expert snowmobiler,
joined friends to ride on snowmobile trails in Yellowstone
National Park and Gallatin National Forest. After dark, Mus-
selman and his friends rode to a restaurant some nine miles
north of West Yellowstone, Montana. Musselman, joined by
Kalahar, Johnson, and Leinberger, rode on the Big Sky Trail,
a groomed snowmobile trail managed by the U.S. Forest Ser-
vice. During the trip, Johnson and Kalahar were riding com-
petitively at speeds up to 60 mph.

   At the restaurant, Musselman and his friends joined a group
of some twenty people to cook steaks, drink beer, and tell sto-
ries. Musselman and Kalahar each drank at least three beers,
Johnson consumed three to four beers, and Leinberger con-
sumed between four and eight beers. Upon leaving the restau-
rant around 10:00 p.m., Leinberger was heavily impaired by
alcohol, Johnson and Kalahar were impaired to a lesser
degree, and Musselman was not impaired and was fully able
to operate his snowmobile.

   Musselman and Johnson left the restaurant first, followed
by Kalahar and Leinberger. No member of the group had pre-
viously traveled on the stretch of the Big Sky Trail immedi-
ately adjacent to the restaurant. The Forest Service had posted
a speed limit of 45 mph on its Yellowstone-area trails, but
was aware that snowmobilers regularly traveled this stretch of
trail at speeds in excess of 60 mph. Musselman took off
quickly from the restaurant and continued at a pace keeping
him in front of the other riders. He was not exceeding 45 mph
when he reached the accident site. Johnson, Kalahar, and
Leinberger approached the accident site at approximately 50-
55 mph.

  The stretch of trail approaching the accident site is flat and
smooth for roughly a quarter of a mile. It then drops suddenly
                          OBERSON v. USDA                          2873
down a steep hill, the site of the accident. There it loses sev-
enteen feet of elevation over approximately eighty feet, an
11.5 degree pitch or 25% slope. Musselman was the first rider
to come to the hill, negotiating it safely and landing his snow-
mobile under control slightly off the trail. Johnson was next
over the hill, crashing his machine at the bottom. After John-
son’s crash, Musselman got off his machine for reasons
unknown, although the district court found that he was likely
trying to help Johnson or warn the approaching riders of the
drop. As Musselman stepped on the trail, Kalahar and Lein-
berger came flying over the hill side-by-side. One of the rid-
ers hit Musselman’s head, causing catastrophic brain injuries.
The injury left Musselman unable to swallow, speak, under-
stand complex communication, and independently conduct
activities of daily living. He currently resides in an adult care
facility.

   The court had difficulty reconstructing the scene immedi-
ately following the accident given the shambolic post-
accident investigation.3 However, it found that Musselman
was lying on the groomed portion of the trail between his and
Leinberger’s machine, with his head facing away from the
hill. Leinberger’s machine was still on the groomed trail with
its front end, windshield, handlebars, tunnel area, skis, cowl-
ing, rear bench rest, and motor mounts damaged. Kalahar’s
machine suffered broken engine mounts, a damaged wind-
shield, and bent handlebars. Johnson’s machine had a dam-
aged windshield, which he replaced the morning after the
accident before the machine could be inspected.

   There were no eyewitnesses to the accident. Leinberger tes-
tified that he did not believe he hit Musselman because he
  3
   The investigators lost notes they had taken that night, took only two
photographs of the scene, did not prevent the snowmobiles from being
moved, spoiled the blood sample drawn from Musselman the night of the
accident, and did not order blood drawn from the other riders involved in
the accident.
2874                   OBERSON v. USDA
“didn’t hit an object that was going to move.” Kalahar testi-
fied that he was positive he did not hit Musselman. Physical
evidence and attempts at reconstruction revealed only that a
snowmobile track hit Musselman in the helmet while he was
on the trail and that the impact came from the right to left
side.

   Plausible explanations can be given for either Leinberger or
Kalahar hitting Musselman. Leinberger’s machine tumbled
after the crest of the hill either because he applied his brakes
before becoming airborne or because he hit Musselman while
airborne, forcing the nose of his snowmobile down. Alterna-
tively, the line of travel for Kalahar’s machine places it in line
with Musselman’s likely location at the moment of the acci-
dent. Additionally, Kalahar reported hitting the ground and
bouncing, an account consistent with hitting Musselman and
then hitting the ground.

B.     THE TRAIL

   Musselman’s accident occurred at a sudden and steep drop
on the Big Sky Trail. At the time of the accident, there were
no signs warning of the hill or instructing riders to slow down.
Numerous witnesses suggested at trial that it was unusual for
such a drop not to be signed and that a trail of such abrupt
steepness was rare.

   The Forest Service has sole responsibility for identifying
and correcting hazards on the Big Sky Trail. To identify haz-
ards, the Forest Service engages in a process called “warrant-
ing.” The program manager for the Gallatin National Forest
described the process as “identifying the hazards that our
average, prudent, reasonable rider would not expect based on
a spectrum of users that we had out there.” Forest Service
employees identify hazards during warranting by riding the
trail at 35 mph during the day. Upon identifying a hazard, the
Forest Service closes the trail, corrects the hazard, or warns
the user. The Forest Service does not warrant trails at night,
                           OBERSON v. USDA                             2875
on the assumption that reflectors and a rider’s judgment as to
the appropriate speed will provide sufficient protection.

   The Forest Service originally warranted the Big Sky Trail
in 1993, at which point it did not identify the hill as a hazard.4
The speed limit at the site of the accident, as on all other trails
in the region, was then 35 mph. A month before the accident,
the Forest Service implemented a new speed limit of 45 mph
solely for consistency with the speed limit in effect in adja-
cent Yellowstone National Park. The Forest Service did not
warrant the trails at 45 mph.

   Sixteen days before Musselman’s accident, two snowmo-
biles and a snow grooming machine were involved in an acci-
dent at the hill in question. The snowmobiles were traveling
in clear weather at night, going 35-40 mph, which the investi-
gation report characterized as too fast based upon the condi-
tions. The two snowmobiles collided with the grooming
machine immediately after coming over the crest of the hill.
In a Forest Service re-creation following the accident, an
observer 100 feet back from the hill could not see the groom-
ing machine, equipped with two headlights and a flashing
light on top, when it was at the bottom of the hill. Although
the collision caused only minor damage to the involved
machines and no personal injury, the district court found that
the accident could have resulted in serious injury to the riders.

C.    THE DISTRICT COURT DECISION

  The district court held that the Forest Service’s decisions
whether to warrant the trails and how it would conduct the
  4
   Following the accident, in November 1996, the Forest Service rewar-
ranted the trail, making several adjustments to the site of the accident. The
Service realigned the approach to the hill, replacing the straight, flat
approach with a curve leading through trees. Additionally, signs were
added indicating the upcoming curve and subsequent hill. The warranting
form also noted that the hill can “sneak up on a person” at night and that
the hill’s crown was scheduled for reduction.
2876                   OBERSON v. USDA
process were grounded in policy considerations and were the
type of discretionary decisions Congress intended to shield
from tort liability. Once the decision to warrant the trails was
made, however, the agency had a duty to accomplish that task
with reasonable care. The Service’s failure to warn of the haz-
ardous hill was not the result of a decision grounded in public
policy or of a choice among competing policy considerations.
The court concluded that the Forest Service’s failure to cor-
rect or warn of this hazard was not the type of discretionary
decision that is shielded from tort liability.

   Having determined that the plaintiffs could sue the Forest
Service, the district court considered whether the Service
owed them a duty under Montana law. It held that the exis-
tence of a duty was not negated by the Montana snowmobil-
ing statutes relieving parties of liability for risks “inherent in
the sport of snowmobiling.” MONT. CODE ANN. §§ 23-2-651,
653-54 (1996). The hill, it found, was not an inherent risk as
a “variation in terrain” because the Forest Service had notice
of the danger and could have eliminated it with ordinary care
by altering the trail and/or posting a warning.

   The court then addressed the applicable standard of care.
First, it held the Montana snowmobile liability statute’s gross
negligence standard, MONT. CODE ANN. § 23-2-653(2) (1996)
(amended 1997), to be unconstitutional under Montana’s
equal protection clause, MONT. CONST. art. II, § 4, on the
authority of Brewer v. Ski-Lift, Inc., 762 P.2d 226 (Mont.
1988). Next, the court declined to apply Montana’s recre-
ational use statute in its place, holding the statute’s willful or
wanton misconduct standard, MONT. CODE ANN. § 70-16-
302(1), to be preempted in this instance by the more specifi-
cally applicable snowmobile liability statute. In the absence of
other applicable standards, the court imposed a duty of ordi-
nary care. The court found the ordinary care standard, adopted
in the 1999 revision of the snowmobile statute in place of the
gross negligence standard, to be constitutional. Finally, the
district court held that the Forest Service breached its duty of
                      OBERSON v. USDA                     2877
ordinary care to plaintiffs and that the breach contributed to
Musselman’s injuries.

   The court next considered whether any of the other parties
shared responsibility for the accident. It concluded that Mus-
selman exposed himself to severe injury by walking onto the
trail and, further, that Leinberger and Kalahar, given their
speed and alcohol-impaired state, and the evidence of contact
with Musselman, breached their duty to avoid injury to others.
Since it proved impossible to determine whether it was Lein-
berger or Kalahar who hit Musselman, the court held both
jointly and severally liable for Musselman’s injuries.

   The court apportioned 40% of the liability to the Forest
Service, 10% to Musselman, and 50% to Kalahar and Lein-
berger. Since Kalahar had settled with Musselman and been
dismissed from the suit as a result of his settlement with Mus-
selman, the court assigned the 50% share of liability solely to
Leinberger on the ground that he and Kalahar had acted in
concert. In calculating damages, the district court used a
future life expectancy of 12.8 years. After reducing the
amount by 10% for Musselman’s liability, the court entered
judgment for plaintiffs in the amount of $10,167,120.

                    IV.   DISCUSSION

A. FTCA’s DISCRETIONARY FUNCTION
EXCEPTION

   [1] The United States “can be sued only to the extent that
it has waived its immunity.” United States v. Orleans, 425
U.S. 807, 814 (1976). The FTCA waives the federal govern-
ment’s immunity from suit for a discrete class of lawsuits. 28
U.S.C. §§ 2671-80 (2005). This waiver, however, is limited
by the discretionary function exception, precluding “[a]ny
claim based upon an act or omission of an employee of the
Government . . . based upon . . . a discretionary function or
duty.” 28 U.S.C. § 2680(a). The Supreme Court has estab-
2878                   OBERSON v. USDA
lished a two-part test for application of the discretionary func-
tion exception. See United States v. Gaubert, 499 U.S. 315,
322-25 (1991); Berkovitz v. United States, 486 U.S. 531, 536-
37 (1988). First, courts inquire as to whether the challenged
action was discretionary, as opposed to being governed by
mandatory statute, policy, or regulation. Whisnant v. United
States, 400 F.3d 1177, 1180-81 (9th Cir. 2005) (summarizing
Berkovitz test). Second, if the court finds the action to have
been discretionary, it then determines whether the action
involved “a decision susceptible to social, economic, or politi-
cal policy analysis.” Id. at 1181; see also O’Toole v. United
States, 295 F.3d 1029, 1033-34 (9th Cir. 2002). If the chal-
lenged action was both discretionary and policy driven, the
discretionary function exception bars FTCA claims. We
review de novo the district court’s application of the discre-
tionary function exception. Marlys Bear Med. v. United
States, 241 F.3d 1208, 1213 (9th Cir. 2001).

   [2] There is no serious claim that the Forest Service’s
actions in this case were mandated by statute, policy or regu-
lation. The question is whether its discretionary actions were
policy driven. The Forest Service argues that the absence of
a warning sign at the hill was the direct result of the 1993
warranting process protected by the discretionary function
exception. But as the Forest Service’s brief explains, the war-
ranting process was conducted at a route design speed of 35
mph and did not identify the hill as a hazard. Later, in 1996,
the Service raised the speed limit to 45 mph—the limit in
effect at the time of the accident—simply to conform to the
speed limit in Yellowstone Park, resulting in the creation of
the hazardous condition at the hill. Because the trails were
never warranted at the higher speed limit of 45 mph, the Ser-
vice is not shielded by the warranting process.

   [3] The Service’s argument that the decision not to rewar-
rant the trails in 1996 was protected by the discretionary func-
tion exception misses the point. Undoubtedly, its decision
whether to warrant trails is protected, as is its decision to
                      OBERSON v. USDA                        2879
adopt a 45 mph speed limit. But not having warranted the
trails at 45 mph, the Service cannot claim that its failure to
warn of hazardous conditions at that speed is protected by the
discretionary function exception.

   [4] Inasmuch as the warranting process does not shield the
Service under the circumstances of this case, we must con-
sider whether the failure to post a warning at the hill is pro-
tected by the discretionary function exception. Whether it is
protected depends on whether the Service’s action—or
inaction—was grounded in social, economic or political pol-
icy. Other than its (misplaced) reliance on the 1993 warrant-
ing process, the Service offers no evidence to show that its
failure to post a warning was the result of a policy decision.
This case is not like Childers v. United States, 40 F.3d 973
(9th Cir. 1995), where the decision not to post signs and to
close portions of Yellowstone National Park were the result
of policy decisions regarding how best to manage the park
during winter. The court explained:

    Unable to maintain all the trails in the park, cogni-
    zant that posting warning signs would inadvertently
    attract visitors to unmaintained trails, and unable to
    post signs throughout the park, NPS could only
    decide to close large portions of the park, or to keep
    the park open, provide visitors with information on
    the hazards, and take steps to discourage visitors
    from going to hazardous areas.

Id. at 976. Similarly, in Valdez v. United States, 56 F.3d 1177
(9th Cir. 1995), the failure to install warning signs alongside
a potentially hazardous stream was held to “implicate[ ] a
choice between the competing policy considerations of maxi-
mizing access to and preservation of natural resources versus
the need to minimize potential safety hazards.” Id. at 1180;
see also Blackburn v. United States, 100 F.3d 1426, 1434 (9th
Cir. 1996) (following Valdez and Childers, holding the deci-
sion how to warn public of the hazard of diving off a bridge
2880                  OBERSON v. USDA
in Yosemite National Park involved considerations of visitor
enjoyment, preservation of historical features, minimizing
manmade intrusions and protection of wildlife and the envi-
ronment).

   [5] The case before us is different. It is more nearly analo-
gous to cases such as Summers v. United States, 905 F.2d
1212, 1215-16 (9th Cir. 1990), holding the discretionary func-
tion did not protect the Park Service where it had failed to
warn visitors of the danger of stepping on hot coals in a fire
ring in the Golden Gate National Recreation Area. The court
found that “NPS’s failure to identify and warn of the danger
to barefoot visitors of hot coals on park beaches resembles
more a departure from the safety considerations established in
Service policies . . . than a mistaken judgment in a matter
clearly involving [policy] choices.” Id. at 1216. Similarly,
Faber v. United States, 56 F.3d 1122, 1127 (9th Cir. 1995),
held that the Forest Service’s failure to post a sign warning of
danger of diving off a waterfall in a National Forest was not
protected by the discretionary function exception. The court
stated: “It would be wrong to apply the discretionary function
exception in a case where a low-level government employee
made a judgment not to post a warning sign . . . .” Id. at 1125;
see also Seyler v. United States, 832 F.2d 120, 123 (9th Cir.
1987) (stating “we doubt that any decision not to provide ade-
quate signs would be of the nature and quality that Congress
intended to shield from tort liability”) (internal quotation
marks and citation omitted). In Sutton v. Earles, 26 F.3d 903,
910 (9th Cir. 1994), the court held the Navy’s decision not to
post speed limit signs after creating a hazard to navigation not
protected by the discretionary function exception, stating that
“[a] decision not to warn of a specific, known hazard for
which the acting agency is responsible is not the kind of
broader social, economic or political policy decision that the
discretionary function exception is intended to protect.” Here,
the Forest Service knew of the hazard through its own investi-
gation, which disclosed that sixteen days prior to Mussel-
man’s accident the hill in question had been the site of a
                             OBERSON v. USDA                        2881
potentially serious collision between a snow grooming
machine and two snowmobiles. In the absence of any evi-
dence that the failure to post a warning or remedy the hazard
was the product of a policy choice, we conclude that the dis-
cretionary function exception did not shield the Forest Service
from liability.

B.     THE GOVERNMENT’S NEGLIGENCE

   “A negligence action requires proof of four elements: (1)
existence of a duty; (2) breach of the duty; (3) causation; and
(4) damages.” Gentry v. Douglas Hereford Ranch, Inc., 962
P.2d 1205, 1209 (Mont. 1998).5 The district court addressed
the four elements required for a negligence action. In holding
that the government had breached its duty to Musselman, the
district court applied a standard of ordinary care under Mon-
tana law. As we discuss later in this order, the government
disputes the application of that standard, contending that the
standard is either gross negligence or willful or wanton mis-
conduct. Because we leave this issue for determination by the
Montana Supreme Court by way of this certification, we defer
its resolution pending a response from that Court. However,
our order resolves all other issues and leaves only the standard
of care as determinative of this appeal.

  1.        The Government’s Duty

       a.    Inherent risk

   [6] To establish a claim of negligence, a plaintiff must first
demonstrate the existence of a duty of care. Gaudreau v. Clin-
ton Irrig. Dist., 30 P.3d 1070, 1073 (Mont. 2001). “The exis-
tence of a legal duty can be determined as a matter of law.”
Lopez v. Great Falls Pre-Release Servs., Inc., 986 P.2d 1081,
1087 (Mont. 1999). The Forest Service owed a duty of care
to all persons which it could reasonably foresee would use the
  5
     The Forest Service does not dispute the trial court’s damage award.
2882                   OBERSON v. USDA
snowmobile trail. Id.; see Busta v. Columbus Hosp. Corp.,
916 P.2d 122, 134 (Mont. 1996) (“Duty . . . is measured by
the scope of the risk which negligent conduct foreseeably
entails” (quoting Mang v. Elaisson, 458 P.2d 777, 781-82
(1969)).

   The government argues that Musselman cannot show the
existence of a duty because Montana’s snowmobile statute
places all legal responsibility for risks “inherent in the sport
of snowmobiling” on the snowmobiler. MONT. CODE ANN.
§ 23-2-654 (1996). It argues that the statute relieved the gov-
ernment of liability for the failure to warn Musselman of the
“variation in terrain” on which he was injured. § 23-2-654(1).
The government, however, overstates the law.

   [7] We review de novo a district court’s interpretation of
state law. See Rabkin v. Or. Health Scis. Univ., 350 F.3d 967,
970 (9th Cir. 2003). To determine whether the government
had a duty to warn Musselman of the hill’s hazardous nature,
we must interpret the meaning of “inherent risk” in the Mon-
tana snowmobile statute. Courts considering the meaning of
“inherent risk” or similar language in similar statutes have
held that language to shield defendants from liability for risks
that are integral parts of a sport and, therefore, could not be
eliminated by a defendant with ordinary care. See Brewer v.
Ski-Lift, Inc., 762 P.2d 226, 231 (Mont. 1988); see also Bou-
chard v. Johnson, 555 N.W.2d 81, 84 (N.D. 1996); Knight v.
Jewett, 834 P.2d 696, 705-06 (Cal. 1992); Clover v. Snowbird
Ski Resort, 808 P.2d 1037, 1047 (Utah 1991); Wright v. Mt.
Mansfield Life, Inc., 96 F. Supp. 786, 791-92 (D. Vt. 1951).
Here, the risk under consideration is not a variation in terrain
but the lack of a warning sign at a hazardous variation in ter-
rain. Given the Service’s comprehensive safety program
designed to standardize hazard notification on trails, the negli-
gent failure to post a warning sign is not an integral part of
the sport of snowmobiling. The risk from such a failure could
be eliminated by the exercise of ordinary care. We conclude
that the snowmobile statute’s “inherent risk” provision did not
                       OBERSON v. USDA                       2883
shield the Forest Service from liability for failure to provide
a warning of a known hazard on the Big Sky Trail.

    b.   The standard of care

   To determine whether the government breached its duty to
Musselman, the district court analyzed the Forest Service’s
conduct under the ordinary care standard. The district court
arrived at this standard of care only after rejecting two alter-
native standards prior to trial. To the extent relevant to the
questions certified to the Montana Supreme Court, we recite
the grounds upon which the district court based its pretrial rul-
ings.

   [8] First, the district court barred affirmative defenses
based upon the 1996 snowmobile statute’s gross negligence
standard, MONT. CODE ANN. § 23-2-653, holding that standard
of care to violate the Montana equal protection clause, MONT.
CONST. art. II, § 4. In so doing, the district court relied on
Brewer, in which the Montana Supreme Court held unconsti-
tutional a statute eliminating legal recourse against ski area
operators for skiers suffering injury by virtue of their partici-
pation in the sport of skiing “regardless of the cause [of
injury] and regardless of the presence of negligence or inten-
tional conduct on the part of the ski area operators.” 762 P.2d
at 230. According to the district court, the “special rights” cre-
ated by the 1996 snowmobile statutes gross negligence stan-
dard were the same as those created by the statute considered
in Brewer— which removed all liability—and similarly were
not rationally related to the statutes purpose. Here, the district
court based its analogous treatment of the ski area operator
and snowmobile statutes upon its unpublished order in Riska
v. USDA, CV-96-63-BU-DWM (D. Mont. Oct. 14, 1997)
(holding 1996 snowmobile statute’s gross negligence standard
unconstitutional under Brewer). We have found no Montana
court decisions resolving the constitutionality of the 1996
snowmobile statute’s gross negligence standard or Brewer’s
2884                   OBERSON v. USDA
applicability to gross negligence standards of care in other
statutes.

   [9] Second, the district court barred affirmative defenses
based upon the recreational use statute’s willful or wanton
misconduct standard, MONT. CODE ANN. § 70-16-302(1).
According to the district court, the parties agreed that the
Montana legislature “carved snowmobiles out of the general
recreational use statute by enacting the specific snowmobile
liability statutes.” The government argued the recreational use
statute’s willful or wanton misconduct standard still should
apply in place of the snowmobile statute’s rejected gross neg-
ligence standard. The district court, however, held the recre-
ational use statute to be preempted by the more specific
snowmobile liability statute in this instance. Having found the
recreational use statute’s standard of care inapplicable, the
district court instructed the parties to proceed under the catch-
all ordinary care standard, MONT. CODE ANN. § 27-1-701, cit-
ing Brewer, 762 P.2d at 230, and Mead v. M.S.B., Inc., 872
P.2d 782, 786-87 (Mont. 1994). We have found no Montana
court decisions addressing whether the snowmobile liability
statute’s standard of care preempts the recreational use stat-
ute’s standard in this context.

   [10] The Montana Supreme Court’s decision regarding the
appropriate standard of care will be dispositive of this appeal.
If the Court endorses the application of the ordinary care stan-
dard, we find no reason to overturn the district court’s deci-
sion. However, if the Court holds the gross negligence or
willful or wanton misconduct standard applicable, we must
remand the case to the district court for a new trial under the
appropriate standard of care.

  2.   Causation

   The government contends first that there was no causal
relationship between the absence of a warning and Mussel-
man’s injury. We review a district court’s findings of both
                       OBERSON v. USDA                      2885
cause-in-fact and proximate cause for clear error. Husain v.
Olympic Airways, 316 F.3d 829, 835 (9th Cir. 2002) (review-
ing mixed questions of law and fact regarding proximate
cause); United States v. Hackett, 311 F.3d 989, 991 (9th Cir.
2002) (reviewing factual findings regarding causation).

   [11] “[A] party’s conduct is a cause-in-fact of an event if
the event would not have occurred but for that conduct.” Gen-
try, 962 P.2d at 1209 (internal quotation marks and citation
omitted). The record established that when approaching the
hill at only 35 mph (below the speed limit), a snowmobile
would leave the ground and sail over the crest of the hill with
the operator losing control and unable to see objects at the
foot of the hill, as occurred in the earlier collision of a snow-
mobile with the snow groomer at the same location. As the
district court found, “[p]lacing a sign on the approach to this
hill would have changed the riders’ expectancies and
informed them that the approaching hill was far steeper than
the usual gentle grade.” Oberson v. United States, 311 F.
Supp. 2d 917, 959 (D. Mont. 2004). Musselman, having
landed off the trail, saw Johnson on the trail and reentered it,
as the court found, to assist or warn the other snowmobilers
of the danger. He was hit when Kalahar and Leinberger,
unwarned, sped over the hill’s crest and lost control of their
snowmobiles as a result of the unexpected steep grade. The
court found that “but for the hill and the associated problems
found in the facts, the accident would not likely have
occurred.” Cause-in-fact, therefore, was established on the
record which showed that the Service’s conduct, “in a natural
and continuous sequence . . . helped produce [Musselman’s]
injury and [the injury] would not have occurred without it.”
Busta, 916 P.2d at 139.

   The Forest Service’s principal contention is that Kalahar
and Leinberger’s speed and intoxication combined with Mus-
selman’s carelessness in entering the trail were independent
intervening causes. Even if its failure to post a warning sign
were found to be a cause-in-fact of Musselman’s injuries, the
2886                   OBERSON v. USDA
Forest Service argues, the riders’ acts were unforeseeable
intervening causes, sufficient to prevent a finding that the For-
est Service’s failure to warn was a proximate cause of the
accident.

   Where, as here, the existence of independent intervening
acts is alleged, causation requires proof of both cause-in-fact
and proximate cause. Gentry, 962 P.2d at 1209. “When two
or more causes concur to bring about an event, then cause-in-
fact is established by the ‘substantial factor’ test.” Sletteland
v. Roberts, 16 P.3d 1062, 1067 (Mont. 2000) (citation omit-
ted). Under that test, a party held to have contributed to an
event is not “absolved from that responsibility upon the
ground that the identical harm would have occurred without
[its involvement].” Rudeck v. Wright, 709 P.2d 621, 628
(Mont. 1985). As the foregoing discussion shows, the Ser-
vice’s maintaining the hazardous and unwarned condition of
the hill contributed to the event in which Musselman was
injured.

   [12] Proximate cause is established when a party could rea-
sonably foresee that its conduct would result in injury. Busta,
916 P.2d at 135. An independent intervening act will not bar
liability if it is “one that the defendant might reasonably fore-
see as probable or one that the defendant might reasonably
anticipate under the circumstances.” Estate of Strever v.
Cline, 924 P.2d 666, 672 (Mont. 1996). However, “[t]he par-
ticular resulting injury need not have been foreseeable.”
Hinkle v. Shepherd Sch. Dist. # 37, 93 P.3d 1239, 1245
(Mont. 2004).

  [13] That serious accidents involving snowmobiles could
occur at the site of the hill was foreseeable. The district court
found that the potential severity of the earlier snow groomer
accident, even though no one was injured, placed the Forest
Service on notice that serious injury could result from its fail-
ure to post a warning. That riders would operate snowmobiles
negligently was foreseen by the Service. Its awareness of rid-
                          OBERSON v. USDA                            2887
ers’ high speeds was a motivation for implementing a trail
warranting process. And its awareness of intoxicated persons
operating snowmobiles is reflected in the Code of Federal
Regulations’ express prohibition of operation of a snowmo-
bile while intoxicated in a National Park or Forest. 36 C.F.R.
§§ 2.18, 4.23 (1996). Thus, Kalahar’s and Leinberger’s negli-
gence was foreseeable.

   [14] The Montana Supreme Court has long recognized the
rescue doctrine.6 Under that doctrine, “one who, observing
another in peril, voluntarily exposes himself to the same dan-
ger in order to protect him . . . may recover for any injury sus-
tained in effecting the rescue against the person through
whose negligence the perilous condition has been brought
about.” Bracey v. Nw. Improvement Co., 109 P. 706, 707
(Mont. 1910); see also Brown v. Columbia Amusement Co.,
6 P.2d 874, 878 (Mont. 1931). Although the cases speak
broadly in terms of liability, they recognize that liability is
intertwined with causation in the application of the doctrine.
Thus, in Kiamas v. Mon-Kota, Inc., 639 P.2d 1155, 1159
(Mont. 1982), the court held that causation had not been
established where action was no longer required to avert a
threatened harm. In its discussion of “Scope of Liability
(Proximate Cause),” the Restatement of Torts (Third) (Pro-
posed Final Draft) states, with respect to rescuers, “The aspect
relevant to scope of liability provides that an actor whose tor-
tious conduct puts the actor or another at risk, is subject to lia-
bility to a third person who is injured while attempting to
come to the aid of the actor or the other imperiled person.”
§ 32 cmt. b (2005). Musselman’s entry onto the trail, the dis-
trict court found, was a response to the peril created by the
Service to warn or give aid to other snowmobilers. As such,
it was a foreseeable consequence of the Service’s negligence.
   6
     “Danger invites rescue. The cry of distress is the summons to relief.
The law does not ignore these reactions of the mind in tracing conduct to
its consequences. It recognizes them as normal. It places their effects
within the range of the natural and probable.” Wagner v. Int’l Ry. Co., 133
N.E. 437, 437 (N.Y. 1921) (Cardozo, J.).
2888                     OBERSON v. USDA
                          V.   NOTICE

   The clerk of this court shall forward a copy of this order,
under official seal, to the Montana Supreme Court, along with
copies of all briefs and excerpts of record that have been filed
with this court. The parties shall notify the clerk of this court
within fourteen (14) days of any decision by the Montana
Supreme Court to accept or to decline certification. If the
Montana Supreme Court accepts certification, the parties shall
then notify the clerk of this court within fourteen (14) days of
the issuance of the Court’s opinion. Submission of the ques-
tions presented in this appeal will be vacated by separate
order pending the Montana Supreme Court’s response to this
request.

                   VI.     COUNSEL LIST

  The following is a list of counsel appearing this matter:

Counsel for Plaintiffs/Appellants Lori Oberson, Legal Guard-
ian for Brian Musselman, Kimberlee Musselman, individually
and as Natural Mother of Devon Musselman, a minor,
     Tom L. Lewis
     Lewis Slovak & Kovacich
     P.O. Box 2325
     Great Falls, MT 59403
and
     Andrew D. Huppert
     Petit Hock & Huppert
     P.O. Box 8718
     111 N. Higgins Avenue
     Missoula, MT 59807

Counsel for Defendant/Third-Party Plaintiff/ Appellee United
States Department of Agriculture, Forest Service
     Bernard F. Hubley
     USHE - Office of the U.S. Attorney
                      OBERSON v. USDA                   2889
      901 Front Street
      Helena, MT 59626
and
      H. Thomas Byron, III
      U.S. Department of Justice
      Civil Division/Appellate Staff
      950 Pennsylvania Avenue, NW
      Washington, DC 20530-0001

Counsel for Defendant/Third-Party Plaintiff State of Montana,
by and through the Department of Fish, Wildlife and Parks
    Lucy T. France
    Garlington Lohn & Robinson, PLLP
    P.O. Box 7909
    199 West Pine
    Missoula, MT 59807-7909

Counsel for Defendant/Third-Party Plaintiff West Yellow-
stone Chamber of Commerce
     Gig A. Tollefsen
     Berg Lilly & Tollefsen
     1 West Main Street
     Bozeman, MT 59715

Counsel for Third-Party Defendant Jamie Louis Leinberger
    Jamie Louis Leinberger
    303 South Dean
    Bay City, MI 48706

Counsel for Third Party Defendant Patrick B. Kalahar
    Marshal L. Mickelson
    Corette Pohlman Allen Black & Carlson
    Mayer Building
    129 West Park Street
    P.O. Box 509
    Butte, MT 59703

Third-Party Defendant Tim A. Johnson
     No Appearance
2890    OBERSON v. USDA
       Respectfully submitted,

       Ronald M. Gould, Marsha S. Berzon
       Circuit Judges and
       William W Schwarzer, Senior United
       States District Judge for the Northern
       District of California, sitting by
       designation

       Ronald M. Gould, Circuit Judge,
       Presiding
