Help! i need to limit the movement area

Hi, I’m hoping that someone is able to help me with my problem. I am creating a prototype system for children to encourage mouse based navigation. The interface is based on a street map with a number of houses on it. Its up to the kids to move a car around the street map, but I only want the car to move in specific areas of the map i.e. the on the street and not over the houses. I hope that makes sense! In theory it sounds like it can be done, but I’m really stuck on how to do this Flash.
Can anyone help me please??? Cheers