making a traffic system

I have a city roadmap, and i was wondering: how do I make a simple traffic system where cars stop at lights etc?

On the light stopping part I would direct you to using triggers, Specifically OnTriggerEnter to detect when object reach's a light. and OnTriggerStay to update weather the object need's to keep stopped or proceed. Collider class

[Edited 1] Way-points would work for a system of paths with cars following them like your wanting.

There are agent-driven traffic simulations which Civil(?) Engineers use. Try looking into those, to see what tricks they use.