I used to live next to a couple of idiots that would flip cigarettes out on I-5 headed south, and laugh about the black spots when they drove back north.
Not very easy to start a fire with a smoke even though they sure get blamed for a bunch. Probably the number one fire starter coming from vehicles is metal making contact with the road. Safety chains seem to be the number one suspect. Had one guy dragging a chain down I-20 in west Texas and started 5 fires in 22 miles. He was pulling his boat to the lake. Bad brakes and bearings can send out allot of sparks, see allot of fires started close to off ramps.