just to clarify janet, it's not a magnetic sensor. it's an inductive loop that creates an electrical signal when a metal is passed over it. it's the main reason why bikes have a hard time tripping the lights. the aluminum and other non-ferrous alloys do not generate the same induction that steel does. we used to be able to trip "secure community" driveway gates with 6" electrical box covers slid over the sensors. was great when we couldn't get into a gated community to do service work. we ended up having one labelled as the "master key".
another sensor used is a photo sensor. it detects light. you can flash your high beams at it and have the lights change well before you have to brake at the intersection.
I think I recall reading in the article that they state it's an inductive loop. "There are all sorts of technologies for detecting cars - everything from lasers and cameras to rubber hoses filled with air! The most common technique, by far, is the inductive loop. An inductive loop is simply a coil of wire embedded in the road's surface that acts somewhat like a magnet."
Doug, you don't have to explain these traffic systems to me, that's my specialty in electrical. I have designed intersections, built and programmed the traffic signal controller, and installed all the wiring and controller at the site. The intersection at 100 Mile House near Tim Horton's was a project of mine while spending 6 months at the MOTH traffic signal engineering dept.
On the Barnett Hwy at the refinery intersection, MOTH was experimenting with video detection for activation, and I set that up as well. I don't know what is presently there, as it is no longer under the jurisdiction of MOTH.
