Ever wondered why some traffic lights always change to green as soon as you pull up? The answer comes down to a marvellous invention called an inductive loop traffic detector. Essentially, the inductive loop detector is just a few loops of wire under the road, connected to a computer unit. When a vehicle passes over the top, the alternating electric current running through the coil induces magnetic fields in the vehicle (eddy currents). These magnetic fields oppose the force of the original magnetic field (the inductive loop), thus decreasing its magnetic flux density and its inductance. The computer unit recognises that the inductance has dropped, and changes the lights.
For the full explanation, see my report, which discusses the exact detail of how cars change the inductance of loops, and explanation of eddy currents, the layout of inductive loop systems, as well as their history and alternatives.