I don't drive currently, but when we've traveled with the dogs, we usually just held the little dogs in our laps. We went to Alabama to stay with friends once with Holly, and she just sat in the back seat with me.
When we go to the park/petsmart, I hold Violet on a leash in the back.
When I'm finally able to drive myself, I hope to get a small suv/station wagon/jeep that I can fit a crate in the back of.
We currently have a ford freestar van, and I *may* be inheriting it when we get another family car, and if I do, I can fold the entire back seat down (it has 2 front seats, 2 middle seats, and then a large back seat in the far back that seats 3 people, and I wouldn't need it) to give me ample space for 2 or more crates.