I was involved in a major car accident that totaled my new vehicle due to falling asleep at the wheel on a highway. Obviously it scared me enough that I'll never fall asleep in a car again, but it was due to sleep deprivation from being up many hours irresponsibly the night before.

If you're falling asleep often at the wheel I would recommend you tell your primary. He'll likely refer you to a sleep doctor. I didn't have major symptoms, but after a recommendation from my primary I discovered I have a bad case of sleep apnea.

If you're falling asleep at the wheel often (and btw, you're doing the right thing by pulling over and napping) you should go to a sleep doctor. You might not be getting a proper amount of sleep, for whatever reason, and they might be able to help.