Its called a magnetic sensing loop, there is one aproximately 1/4 mile from the signal, and 2 or 3 closer to the light in each lane. You may have noticed a black stop sign shaped marking (or in some cities a circle) in the roadway. This is where they have cut into the concrete or asphalt and laid in a magnetic loop (the black stuff is a sealer to keep the rain from rusting it). As your car passes over it, it sends a magnetic pulse (from the metal in your car or motorcycle) thru an underground cableing system to a computer terminal at the intersection (usually housed near the intersection in a large metal box on one corner), signaling that a car is coming from a certain direction. This is how a turn lane signal knows when to activate the turn signal for you. As for the length of a green or yellow or red light is dependant on timers in the computer terminal and some areas are need based. You ever wonder why you fly down the highway and the light turns yellow as soon as you hit the first crosswalk? This is how. As for your headlights, it is a common myth that flashing your headlights will trigger a signal to change for you. This is false. Their is a system used by police, firefighters, ambulances as such called an Opti-Com or Opti-Flash, which is a traffic signal preemption device that sends out a coded digital white strobe flash at a certain flash rate and candlepower, much faster and brighter than any headlight. This light is read by a small sensor usually on the traffic light pole and sent to the computer to trigger a green light for the emergency responders so they dont have to wait in traffic while the other 3 sides have red lights (this is why it seems like your light got skipped when a fire engine went through, its just the computer resetting itself into its normal pattern)

They utilize a system similar to police radars that are used at speed control checkpoints.

An emission of radio waves is sent direction incoming traffic; these get reflected off any approaching vehicles, which are thus detected, and their measured speed and distance are taken into account.

Many entrance doors that open automatically, such as entrances to supermarkets, public buildings, etc, use similar technology to detect an approaching person.

Other devices, such as optical sensors, can be used for the same purpose.

Beyond the sensor system built into the roadway, our city (and some of the cities around us) have installed video cameras on the lights at busy intersections. They say they don't have the resolution to pick up license plate numbers, but they are used, in conjunction with computers, to change the lights depending on how many cars are in line (ie, how busy is the intersection going a certain direction).