You don't need to send the data to cars for wi-fi equipped traffic lights to be useful.

I can imagine hooking up a traffic light with both a wi-fi device and cameras to measure traffic flow. Traffic light A could communicate with traffic light B to let each one know how much traffic was on the way to the other light, so that it could adjust its programming. If you did this on a large scale, I would expect it to reduce traffic by a lot.

Of course, I don't know if the reduction in traffic would be greater than the cost of equipping all traffic lights with wi-fi devices and cameras.