traffic light sensors

[EthanS]

Do most traffic lights have sensors to trigger a change in the light when a car comes to an intersection, or is it mostly a timer-run system?

[ricepad]

Most traffic signals have loop detectors, which are basically metal detectors buried in the pavement. They sense a large mass of metal (your car), and then trigger the cycle. Generally speaking, timed signals are more common on one-way streets, typically in downtown areas (at least in California).

[Acco40]

The next time you come up to an intersection, look at the pavement as you roll up to it.

You will see what looks to be a "cutout" square or strip of pavement with a thin metal strip surrounding it... these "buttons" as I've heard them called will trigger the light to change sooner than if you stay off of them. They are almost always in the left-hand turn lanes.

It irks me when people don't use them! Waste my life will you?!

These "buttons" seem to be everywhere I've traveled in the U.S., so look for 'em you're out and about.

[occ]

I dont think most lights have them (its expensive to install), but intersections where at certain times there would be almost no traffic from one direction (say, in the middle of the night) do. There was one near my old office, i'd come down the hill (I worked the night shift) and at the same exact moment, the perpendicular light would turn yellow and then my light would change.