Sounds like an excellent idea. You might consider using infrared instead of laser, or possible a sonic sensor? Whereas the infrared would be invisible to the human eye, it wil probably require a sensor on the other side. Whereas a sonic sensor may be capable of detecting a car without a sensor on the other side of the street. RU looking to build from scratch parts or boughten parts? (ie.. Component level building or puchase of pieces to build your idea?