Some babies seem to fall asleep the minute they feel the car's motor start; others scream. The problem with car travel for some babies seems to be boredom. For a very young baby, pictures to look at may help. Prop a book or a plastic picture holder designed for the car on the seat facing your baby, or try hanging a mirror designed for crib use from the back headrest so your baby can admire herself. Company in the back seat or music also keeps your baby content. Bring toys for older babies; hook them to the car seat with plastic links or they'll all end up on the floor in minutes. Stopping to change a diaper or soothe the baby usually doesn't help, unless you're planning a long stop.
If you're going on a long trip, try to plan the trip around your baby's regular sleep schedule. Figure that she might be happily awake in the car seat for as much as an hour — but not much more. If you leave an hour before naptime, she'll be awake for an hour and fall asleep, if you're lucky, for two hours. Then stop to feed her, change her, maybe eat dinner yourself, and get her back in the car for another hour awake and hope that brings you to your destination.
If your baby is hungry, don't remove her from her car seat while the car is in motion. Stop to nurse. The best news about car travel is that children under the age of two rarely suffer from motion sickness. This particular treat may be in your future, but is not something you have to deal with right now.