Many intersections have inductive loops buried in the concrete or asphalt that sense the presence of vehicles and initiate a signal light change. Perhaps you have seen the cutouts in the pavement that have been filled with tar. In the case of an advanced left turn arrow, they may be set back from the intersection by several car lengths. If you know what to look for, you can stop on one of these loops and force the left turn arrow even if you are alone in the lane.
Loops are also used in counting vehicular traffic to determine pavement wear and tear and when painting of white lines is warranted.
Here is a circuit that is used to detect a vehicle by shifting an oscillator frequency. Typically, the frequency emitted can be measured by a microcontroller and the software can determine presence or absence of a vehicle over the loop.
Another Vehicle Loop Detector Circuit
Detection of large metal objects (cars) by inductive loop sensors.