How do traffic lights know a car is there waiting to go?

Some sort of motion detection either with a camera or lasers (not sure about that one).

Most are based around a timer though, and the people that press buttons to walk across the streets.

Look at the roadway right in front of the stop line. You will notice a rectangular cut into the ground with some black tar poured into it to make it match somewhat. Those lines are where they cut out a section of the asphalt to put in a pressure sensor. Once the car rolls over the sensor it sends a signal to the traffic control system that a vehicle is waiting and the light will need to be changed.

There needs to be a certain tonnage in order to trigger a pressure sensor, which is an annoyance to motorcyclists. Where I am located the pressure sensors and the motion detectors are used in tandem.

There is also a sensor on some that picks up RF signals. These are semi-helpful to motorcyclists without the needed weight for the pressure sensor. Basically on a motorcycle, the magneto turning can set off the sensor, so you might need to shut your bike down and start it back up a couple of times.

On the pressure sensor ones(in cars or 4 wheeled vehicles only), try slowing down and right before you stop, really jam on your brakes over the sensor. That will transfer the cars weight forward and down and hopefully trip it sooner.

I heard flicking your high beams at a red light when no one is around changes it but I'm sure it's just an urban legend.