                    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,
                 v.
UNITED STATES DEPARTMENT OF
AGRICULTURE, FOREST SERVICE,                    Nos. 04-35268
   Defendant-Third-Party Plaintiff-                  04-35315
                          Appellant,
                and                              D.C. No.
                                              CV-99-00048-DWM
STATE OF MONTANA, by and                         AMENDED
through the Department of Fish,                  OPINION*
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

  *This amended opinion supersedes our previous opinion in this matter
reported at 441 F.3d 703 (9th Cir. 2006)

                                1469
1470                     OBERSON v. USDA
                  Argued and Submitted
           December 7, 2005—Seattle, Washington

                     Filed January 30, 2008

     Before: Ronald M. Gould and Marsha S. Berzon,
Circuit Judges, and William W Schwarzer,** 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                    1473


                        COUNSEL

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
     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
1474                 OBERSON v. USDA
       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


                        OPINION

SCHWARZER, District Judge:

           I.   PROCEDURAL BACKGROUND

  Brian Musselman (Musselman) was gravely injured in a
                           OBERSON v. USDA                             1475
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 Lori Oberson as guardian for Musselman against
the United States in the amount of $4,518,720 and against
Leinberger for $5,648,400.1

   The Forest Service timely appealed, citing as error the dis-
trict court’s refusal to apply the discretionary function excep-
tion 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 calcu-
lating life expectancy.2 We certified to the Montana Supreme
  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 proven 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
1476                        OBERSON v. USDA
Court the determinative question of the appropriate standard
of care by way of an earlier published opinion in this matter.
Having received that court’s response, Oberson v. U.S. Dep’t
of Agric., 171 P.3d 715 (Mont. 2007), we now amend our
prior opinion and affirm the district court’s judgment in all
respects.

                              II.    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 Forest Service.
During the trip, Johnson and Kalahar were riding competi-
tively 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

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, 644-
645 (9th Cir. 1970).
                       OBERSON v. USDA                      1477
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
down a steep hill to the site of the accident, losing seventeen
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 left his machine for reasons
unknown, although the district court assumed he was likely
trying to help Johnson or warn the approaching riders of the
trail’s drop. As Musselman entered on the trail, Kalahar and
Leinberger came flying over the hill side-by-side. One of the
two riders hit Musselman’s head, causing catastrophic brain
injuries. The injury left Musselman unable to swallow, speak,
understand complex communication, and independently con-
duct 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-
1478                      OBERSON v. USDA
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
“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 on 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.
  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.
                           OBERSON v. USDA                             1479
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 drop 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,
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.
  4
   Following the accident, in November 1996, the Forest Service rewar-
ranted the trail, making several adjustments at 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 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.
1480                   OBERSON v. USDA
   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
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 Service 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 it had jurisdiction, the district court
then applied the Montana snowmobiling statute. MONT. CODE
ANN. §§ 23-2-651, 653, 654 (1996). It first held the gross neg-
ligence standard of the statute as originally enacted in 1987 to
be unconstitutional on the authority of Brewer v. Ski-Lift, Inc.,
762 P.2d 226 (Mont. 1988). It then applied the standard estab-
lished by the statute as amended in 1999, imposing a duty of
reasonable care on snowmobile area operators “except for the
                       OBERSON v. USDA                       1481
risks inherent in the sport of snowmobiling.” § 23-2-653(3).
The term inherent risk is defined in the statute as including
“variations in terrain.” MONT. CODE ANN. § 1346(b)(1). The
court rejected the Forest Service’s argument that the hazard-
ous hill on the trail was a “variation in terrain,” reasoning that
the Service had notice of the danger and could have elimi-
nated it with ordinary care by altering the trail or posting a
warning. It concluded that by failing to reduce the steep grade
of the hill or warning snowmobilers of the hazard, the Forest
Service breached its duty 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 this settlement, the court
assigned the 50% share of liability solely to Leinberger on the
ground that he and Kalahar had acted in concert. In calculat-
ing damages, the district court used a future life expectancy
of 12.8 years. After reducing the amount by 10% for Mussel-
man’s liability, the court entered judgment for plaintiffs
against the United States in the amount of $4,518,720 and
against Leinberger in the amount of $5,648,400.
1482                    OBERSON v. USDA
                      III.   DISCUSSION

A.     FTCA’s DISCRETIONARY FUNCTION
       EXCEPTION

   [1] The District Court’s jurisdiction arose under 28 U.S.C.
§ 1346(b)(1) imposing liability on the United States for “per-
sonal injury . . . caused by the negligent . . . act or omission
of any employee of the Government while acting within the
scope of his office or employment, under circumstances
where the United States, if a private person, would be liable
to the claimant in accordance with the law of the place where
the act or omission occurred.” Under section 2680(a), how-
ever, no liability lies for “[a]ny claim . . . based upon the exer-
cise or performance or the failure to exercise or perform a
discretionary function or duty on the part of a federal agency
or an employee of the Government . . . .” Pursuant to the two-
prong test announced in United States v. Gaubert, 499 U.S.
315, 324-25 (1991), we ask first “ ‘whether the alleged
wrongful conduct violated a specific and mandatory regula-
tion or statute. If so, the conduct is outside the realm of dis-
cretion. If there is no mandatory regulation or statute
involved, we then ask whether the conduct was susceptible to
being based upon social, economic, or political policy.’ ” Bolt
v. United States, No. 06-35993, 2007 WL 4225791, at *3 (9th
Cir. Dec. 3, 2007) (quoting Bibeau v. Pac. Nw. Research
Found., 339 F.3d 942, 945 (9th Cir. 2003)). If the conduct
involves choice or discretion, we must ask “ ‘whether that
judgment is of the kind that the discretionary function excep-
tion was designed to shield.’ ” GATX/Airlog Co. v. United
States, 286 F.3d 1168, 1174 (9th Cir. 2002) (quoting
Berkovitz v. United States, 486 U.S. 531, 536 (1988)). The
government bears the burden of proving that the discretionary
function exception applies. Id.

  [2] There is no serious claim that the Forest Service’s
actions in this case were mandated by regulation or statute.
The question is whether its discretionary actions were policy
                       OBERSON v. USDA                      1483
driven. The Forest Service argues that the absence of a warn-
ing 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 warranting 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 Yellow-
stone Park, resulting in the creation of the hazardous condi-
tion at the hill. Because the trails were never warranted at the
higher speed limit of 45 mph, the Service 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. But not having war-
ranted the trails at 45 mph, the Service cannot claim that its
failure to warn of hazardous conditions at that speed is pro-
tected 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 unlike 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 was the result of pol-
icy 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
1484                   OBERSON v. USDA
    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 the public of the hazard of diving off a
bridge in Yosemite National Park involved considerations of
visitor enjoyment, preservation of historical features, minimi-
zation of manmade intrusions and protection of wildlife and
the environment).

   The case before us presents a different context. It is more
nearly analogous to cases such as Summers v. United States,
905 F.2d 1212, 1215-16 (9th Cir. 1990), holding that the dis-
cretionary function exception 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 Recre-
ation 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 con-
siderations established in Service policies . . . than a mistaken
judgment in a matter clearly involving [policy] choices . . . .”
Id. at 1216. Similarly, in Faber v. United States, 56 F.3d
1122, 1127 (9th Cir. 1995), we held that the Forest Service’s
failure to post a sign warning of danger of diving off a water-
fall in a National Forest was not protected by the discretionary
function exception. We stated that “[i]t would be wrong to
apply the discretionary function exception in a case where a
                           OBERSON v. USDA                          1485
low-level government employee made a judgment not to post
a warning sign . . . .” Id. at 1125. And in Sutton v. Earles, 26
F.3d 903, 910 (9th Cir. 1994), we held the Navy’s decision
not to post speed limit signs after creating a hazard to naviga-
tion was not protected by the discretionary function excep-
tion, 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.” See also Seyler v. United States, 832 F.2d 120,
123 (9th Cir. 1987) (stating “we doubt that any decision not
to provide adequate signs would be of the nature and quality
that Congress intended to shield from tort liability”) (internal
quotation marks and citation omitted).

   [5] Here, the Forest Service knew of the hazard through its
own investigation, which disclosed that sixteen days prior to
Musselman’s accident the hill in question had been the site of
a 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 FOREST SERVICE’S NEGLIGENCE

   [6] We turn next to the question whether Montana law
imposed a duty on the Forest Service to remedy or warn of the
hazardous condition on the trail. See Conrad v. United States,
447 F.3d 760, 767 (9th Cir. 2006) (in assessing liability under
the FTCA, court applies the law of the state in which the
alleged tort occurred). Under Montana law, a negligence
action requires proof of four elements: existence of a duty;
breach of the duty; causation; and damages. Gentry v. Doug-
las Hereford Ranch, Inc., 962 P.2d 1205, 1209 (Mont. 1998).5
  5
     The Forest Service does not dispute the trial court’s damage award.
1486                   OBERSON v. USDA
   [7] In its certification decision, the Montana Supreme Court
essentially adopted the reasoning of the district court. Ober-
son v. U.S. Dep’t of Agric., 171 P.3d 715 (Mont. 2007).
Applying the rationale of Brewer to the snowmobiling statute,
the court held that its gross negligence standard violated the
equal protection clause of the Montana Constitution. Id. at
720 (citing MONT. CONST. art. II, § 4). Next, it rejected the
Service’s argument that in the absence of the gross negligence
standard, the willful or wanton misconduct standard of the
recreational use statute, MONT. CODE ANN. § 70-16-302(1)
(1995), should apply. It held that while it had struck the gross
negligence standard, the snowmobile statute otherwise
remained in effect governing the duties of snowmobile area
operators. Accordingly, “[t]he more general recreational use
statute’s willful or wanton misconduct standard of care, § 70-
16-302(1), MCA (1995), has no application under these cir-
cumstances.” Id. at 721-22. Finally, the court concluded, fol-
lowing its decision in Mead v. M.S.B., Inc., 872 P.2d 782
(Mont. 1994), that in the absence of the gross negligence stan-
dard, “the general statutory duty of due care, set forth in § 27-
1-701, MCA, provides the standard of care to govern the For-
est Service’s actions in this case.” Id. at 722.

   [8] Under Montana law the duty element of negligence
turns primarily on foreseeability. Lopez v. Great Falls Pre-
Release Servs., Inc., 986 P.2d 1081, 1086-87 (Mont. 1999).
Whether the Forest Service owed a duty to Musselman
depends on whether it could reasonably foresee an unreason-
able risk of harm to users of the snowmobile trail. Id.; see also
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.
Eliasson, 458 P.2d 777, 781-82 (Mont. 1969)). The Forest
Service argues that whatever risk was foreseeable, there was
no breach of 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(3) (1996). It argues that the statute relieved the
                       OBERSON v. USDA                      1487
government of liability for the failure to warn Musselman of
the “variation[ ] in terrain” on which he was injured. § 23-2-
654(1). We agree with the district court that the argument is
foreclosed by Brewer, which limited the range of inherent
risks to those “that are essentially impossible to eliminate by
the ski area operator.” Brewer, 762 P.2d at 231. The risk
under consideration is not a variation in terrain but a hazard-
ous condition of the trail which could with reasonable care
have been readily eliminated by altering the trail or posting a
warning sign.

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

   [9] “[A] party’s conduct is a cause-in-fact of an event if the
event would not have occurred but for that conduct.” Gentry,
962 P.2d at 1209 (internal quotation marks and citation omit-
ted). 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 snowmobile
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, 311 F. Supp. 2d at 959. Musselman,
having landed off the trail, saw Johnson on the trail and reen-
tered it, as the court found, to assist or warn the other snow-
mobilers of the danger. He was hit when Kalahar and
Leinberger, unwarned, sped over the hill’s crest and lost con-
trol of their snowmobiles as a result of the unexpected steep
1488                   OBERSON v. USDA
grade. 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.”
See Busta, 916 P.2d at 139.

   The Service’s principal contention is that Kalahar’s and
Leinberger’s speed and intoxication combined with Mussel-
man’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
Service argues, the riders’ acts were unforeseeable interven-
ing causes, sufficient to prevent a finding that the 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.

   [10] 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.”
                          OBERSON v. USDA                            1489
Hinkle v. Shepherd Sch. Dist. No. 37, 93 P.3d 1239, 1245
(Mont. 2004). 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
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 Forest Service. Its awareness
of riders’ high speeds was a motivation for implementing a
trail warranting process. And its awareness of intoxicated per-
sons operating snowmobiles is reflected in the Code of Fed-
eral Regulations’ express prohibition of operation of a
snowmobile while intoxicated in a National Park or Forest.
36 C.F.R. §§ 2.18, 4.23 (1996). Thus, Kalahar’s and Leinber-
ger’s negligence was foreseeable.

   [11] 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-
  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.).
1490                   OBERSON v. USDA
posed Final Draft) states, with respect to rescuers, “[t]he
aspect relevant to scope of liability provides that an actor,
whose tortious conduct puts the actor or another at risk, is
subject to liability to a third person who is injured while
attempting to come to the aid of the actor or the other imper-
iled person.” § 32 cmt. b (2005). Musselman’s entry onto the
trail, the district court found, was a response to the peril cre-
ated by the Forest Service to warn or give aid to other snow-
mobilers. As such, it was a foreseeable consequence of the
Service’s negligence.

                       CONCLUSION

  For the reasons stated, the judgment is affirmed.
