Making sense out of traffic signal sensors

Here's an excellent article from BikePortland.org about traffic light sensors and biking. How do you trip traffic lights when riding a bicycle?

 

"One of the more frustrating aspects of cycling in an urban environment is not being able to trigger a signal and being stranded at an intersection.There you are, riding happily along, when you come to a red light and — after an entire signal cycle — you get the sinking feeling that the light is ignoring you. Fortunately, in many cities (including Portland) traffic lights are triggered by the presence of a bicycle. But how does it work? Is there a special technique for getting the green"

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