It is critical to take the time to call the police and make a report.
This is the ONLY data transportation planners have to use in determining safe design and other mitigants for safety, like policies against distracted or intoxicated driving.
This data is collected by the Montana Department of Transportation and is used in many ways.
Report the incident to an officer by calling 911 or the nonemergency line 406-657-8200.
If the car leaves, get the license plate number if you can, but it is not absolutely necessary.
The only thing that IS necessary is that you stay at the scene until an officer arrives to take you information and describe the area.