This can't be done with an AI script. You'll have to use triggers to force AI pathing for your cars on the road. Driving lanes could be setup using different regions. Border, intersection, and curves detection using regions should do it.
You could look at a tower defense map how its done. This implements pathing along a fixed path. If you're using lanes (oppossing traffic) then you'd have to make the movement more accurate (car driving in a straight line)