This is under discussion right now, on the insulin-pumpers mailing list.
It's scary that doctors don't talk to diabetics about driving. I check my blood sugar before driving unless I just finished eating and I've used checked before the meal.
I also check frequently when making long trips, at least once an hour, as I can think of few things worse than going low while driving. I also keep Snickers minatures in the car and use them to keep from having an emergency -- they are about 5 grams of carb each of slow sugar. I also keep a couple of bags of lifesavers in the car for fast sugar.