There is an international effort underway to do precisely this sort of thing, among many other traffic/radio related things. The Federal Gov't has allocated bandwidth (in the 5.9 GHz band) and there's an IEEE standards committee (802.11p) working on "Wireless Access in the Vehicle Environment" (WAVE radio.) My wife is on the committee and is doing research on (among other things) how well base stations can handle floods of cars arriving & trying to connect simultaneously. There are test intersections set up in San Francisco (down by the ballpark) and I have had the joy of spending my Sunday afternoons driving our minivan, festooned with magnetic roof antennas connected to a pile of WAVE radio modules, back and forth up 3rd Street dozens of times while she does load tests.