Many traffic lights change when a car rolls up to the intersection. How does the light sense the presence of the car?

The most common way is through the use of inductance which
is related to electromagnetism.  When you pass a current through a wire a magnetic field
is generated at right angles to the flow of current in the wire. If a metal object -
such as a car - enters the magnetic field  it acts like the core of a solenoid and
changes the magnetic field and increass the inductance of the electric circuit.  This
change in inductance is used to send a signal to the trafficl light that a car is at the
intersection.  Then, depending on the logic systems of the light, it either stays green
longer, or changes the light from red to green so the car can proceed instead of waiting
for a timed light to change.

You will usually see this type
of sensor as a series of lines cut into the surface of the road. Wires are put in these
cuts, then sealed with a waterproof material and put into
action.