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Many jurisdictions have installed sensors under the pavement that will know when a car is queued up at a signalized intersection. Where you have a local road intersecting with a major arterial for instance, there will be sensors on the local road. The traffic signals are tied to the sensors and the signals will be set to keep to the green light on for the arterial road on a very long cycle unless the sensor picks up a vehicle waiting on the local road. By setting up your road like this, you don't have instances where the traffic light for the arterial goes from green-amber-red yet there are no cars waiting on the local road. That would be infuriating for the drivers on the arterial. Sometimes the sensors are set back from the intersection so that one vehicle waiting on the local road doesn't trigger a signal change. That's an inefficient road transportation network where numerous vehicles on the arterial are stopped to let in one car.

So your more savvy drivers on the local roads may be aware that to trigger a quicker signal cycle change, they can hang back from the intersection about 20 feet and let the sensors detect the presence of a vehicle that far back from the intersection and trigger the signal. Basically, they are shortening their wait time at the intersection at the expense of the busier arterial road drivers.

You see this a lot at intersection left turns too. In that circumstance, on the arterial road the road network does not want the left hand queue length to get too long because it's bad for traffic flow and it triggers bad driver decisions (basically, the longer a driver has to wait to make a left turn the shorter the space between oncoming traffic in which the driver is prepared to attempt to make a left turn) which leads to more accidents. So in the left hand turn lane you'll see a sensor set back maybe 20 feet or more to detect when the left turn queue is getting too long. When it does sense that, it triggers the traffic signals to introduce a "left turn only" sign to empty the queue. So in this instance, your savvy driver who enters the left turn lane and sees there are no cars waiting ahead of him to turn, will stop their car over the sensor back from the intersection to trigger the "left turn only" signal ahead a full green signal so they can make their left turn in priority ahead of every other traffic movement in the intersection.

I may or may not employ this knowledge on a regular basis.