Its another argument for driving with a camera. This isn't expensive to do, a mini DV one from fleabay with a 8GB Micro SD card will record 2-3 hours including sound. When its full it starts to delete the oldest file so you get the last 2-3 hours continuously.
Price of camera and card ~$35.
The look on the other driver's face when you say "well, lets review the film from my in car camera" - priceless.
EDIT - As others have tapped sometimes all those cameras have a use, can you see if there are cameras next time you are there and then ask whoever looks after the road for footage ? They will start to rotate tapes or whatever they store it on though so this needs to be done quickly.