Whats the correct way of animating vehicles?

I want some cars and busses to populate my roads, they are NOT driven my a player, just decoration... NPC cars if you like. Im finding it hard to animate the transform of the vehicles, without them going through geometry or flaying. My scene is counrt roads, not flat cities. I tried adding the wheel collidors, but that just means that my vehicles just run down the hill, fun.. but not what I want. What would be the best way to do this? I would like to edit only the direction transforms, but let physics do the pulling down onto the ground. How can I achieve this? Mark