Traffic light sensors are inductive loops in the road surface (sometimes you can see the cuts in the asphalt where it is). An alternating current is sent through the loops, and if metal passes over it, the magnetic field from the loop causes eddy currents in the metal, which will alter the electrical characteristics of the loop. These altered characteristics can be detected. It makes no difference if the metal is magnetic or not, just that it is an electrical conductor. The loops currently installed are not set with the correct sensitivity.