Anybody know the cause and prevention of falling asleep driving?

Find the cause first: Get an evaluation by a sleep specialist to find and fix the underlying cause (sleep apnea, sleep deprivation, etc). Short of that, opening the car window and shewing (gum, beef jerky, etc) may prove helpful in some individuals. Caffeine may help as well but it takes time: 30-50 minutes to kick in after consumption; and it should not be taken close to bedtime.