Most traffic lights sense the presence of vehicles by means of wires buried in the pavement. Current flowing in these wires sets up a magnetic field that is disturbed by the presence of metal. This is like the metal detectors used by beachcombers, except the loop is buried in the road and the treasure, our bikes, is above the road. In Palo Alto and most surrounding cities, these loops can detect a bicycle if it is placed properly. The figure below shows the most common loop patterns and their sensitive areas. Placing your bike on the sensitive area should get you a green light. If not, contact the city's traffic operations department and file a complaint!

Sometimes the loops cannot be seen easily. Some agencies mark the proper location with a bike logo. If you see one of these, stop with your front wheel just over the top bar. You must stay on the detector until the light turns green -- you will be forgotten if you roll off of it.

The latest technology is video detection. The photo below shows a video camera mounted on a mast arm at an intersection near me. These cameras, if adjusted properly, can reliably detect a bicycle anywhere in the lane. They can even detect you before you get to the intersection and  you can get a green light without having to stop, if traffic is light. The cameras and associated equipment are more expensive to install than the wire loops, but they are easier to maintain and they don't damage the pavement. Unfortunately, some locations have cameras that are used for monitoring traffic but still use loops for triggering the lights. There is, at present, no way for a bicyclist to know what mechanism is in use at a particular intersection. If you see a bike logo, stop on it. If you see a camera and no logo, experiment to see if you can be detected when you use the position that is safe and appropriate for your intended movement.