It seems that in the US, some traffic lights, usually the ones in busy intersections have this special sensor that can detect transmitters placed on emergency vehicles. When the sensor detects the special transmitter it will make the traffic light green, so the emergency vehicles can pass without stopping and loosing precious time.
The guys over at I-Hacked.com have info on how to build one of this transmitters so you can turn the traffic lights green yourself. But be aware, even building and owning one of this transmitter is illegally, you don’t even have to use it to get in trouble, so be careful, consult your local laws before starting this project.
DIY Mobile Infra Red Transmitter: [Link]