Why would crossing a road matter? Small gauge wires duct taped to a road surface would work just fine, road vehicles are designed specifically to distribute their weight very evenly across the road surface, otherwise we'd be ripping up our roads every other year. You also didn't mention a road, want us to read your mind for more suggestions that fit your specific needs without explaining under what specific situations you expect this to work?
You want an IR or RF solution? What price do you are you limiting yourself to and how 'simple' does it need to be? This forum is FILLED with requests like this and no attention to the first rule of engineering "Keep it simple stupid"
A simple pair of wires to drive a transistor to drive the relay (or other driving transistor) would be as simple as you can possible get, dirt cheap, and perfectly practical. Doubt me? Try it.