I recently read it costs about $150,000 to put in traffic lights here in California. That's a heavy duty pole, the lights, the power, the connection to other lights and the vehicle sensors. Seems to me modern technology should be able to make that a lot cheaper. New lights are all LED (the energy savings pay for themselves quickly) but that should also mean smaller safer power lines with special digging under the road. (Wonder if they could do it with light pipes and have no electricity up there.)
However, as it turns out, we want electricity up there. It seems there would be a natural marraige between traffic lights and a wireless data network. Years ago I tried to suggest to Metricom that they buy a traffic light control software package and offer free traffic light computerization to any city.
That's because you could then mount your wireless nodes on the lights. You get everything you want -- you are at the major intersections (by definition, almost) and there is power there. A tiny amount of your bandwidth can control the lights from a computer. You're up reasonably high. Metricom used streetlights for their power and altitude.
And while I am loathe to suggest putting cameras at the intersections, ones watching the cars and not the people don't bother me nearly as much and are already there. These could allow human beings to direct traffic at important intersections during rush hour and unusual traffic flow.
A temporary light with wireless camera and control could also do this for special locations that need traffic control on weekends. Police forces can't afford to send an officer out, even though the time saved by the drivers would be huge, and besides it's risky work. However, remote tele-operators could easily do the job and save lots of time and bad traffic.