Here is an interesting piece in the Wired How-To wiki, telling you how to trigger green lights as you are driving.
The first instruction, for "demand-actuated" lights, is pretty helpful:
Look for the triggerThis method won't work for every light. For example, in city block grids, the lights are more often than not programmed by timer -- that is, they're set to coordinate with the lights near them so that most poor suckers won't get caught every hundred yards (see Hackers). Other coordinated traffic systems are "actuated-coordinated" which means they will not immediately turn green even if your vehicle is detected. This allows for major corridors to remain coordinated while you wait for your time in the cycle. Lights that aren't pre-programmed are called "demand-actuated" lights, because they only change their state when a vehicle rolls up to them.
To find out if you're sitting at a demand-actuated stoplight, look for the inductive-loop detector embedded in the asphalt in front of you. An inductive-loop detector is a loop, or double-loop, of wire that's been embedded into cut pavement and then tarred over. It detects the presence of conductive materials, like the steel or iron used to make your car, or aluminum or titanium in a bicycle. Aluminum bicycle rims, positioned appropriately over the sensor, are adequate.
You can also look at the traffic signal arm for other methods of detection such as cameras or radar sensors. These are newer and much more reliable technology.
One of those little things most people don't know.