Sometimes in front of a stop light you will see that a thin, circular or diamond-shaped cut was made in the pavement and then filled in with new asphalt. There is a loop of wire under there that detects when a large metal object is over it - I presume it uses magnetic induction.
Those loops are calibrated for cars and often bikes will not trip them. Sometimes leaning your bike over closer to the ground will get more metal closer to the loop and will sometimes cause it to register that you're there. Now if you have some fancy carbon fiber bike you're probably out of luck regardless of how far you lean your bike over to one side.