An IEEE working group is looking at adding Wi-Fi to cars: The effort actually has its own letter, 802.11p. The idea is to allow emergency vehicles to be able to change traffic lights when they're nearby and for the radios in cars to offer feedback to a central system that detects traffic jams and also may change traffic signals to speed things up.
While the idea sounds good, it may be difficult to get automobile manufacturers to build the radios into cars and even harder to get municipalities or other groups to build the networks and backend systems that collect data.