How would I go about spawning parts with velocity in a certain direction?

I'm working on a test game, I'm trying to make cars that travel back and fourth across a road. I don't know if the method in question would be the right way to do this, but I just want some way I could have cars travel across a road and then end up back at their starting point (to loop it). If anyone has any idea of how I would go about this, help would be greatly appreciated.