Microwaves, bikes, and traffic lights

Posted on October 13, 2010 by .
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East Bay city experiments with traffic light radar detection for bicycles.

Actuated traffic signals are those traffic lights that change when a car, truck, motorcycle or (sometimes) bicycle approach an intersection. The vehicle is detected by various electronic technologies. The most common, by far, are inductive loop detectors. These are loops of wires embedded in the road surface; a hunk of metal over the wires (i.e. a dump truck) causes a change in the current passing through this wire. A computer inside of a metal cabinet near the intersection notes the dump truck’s presence and schedules a green light so he can proceed.

Inductive loops are common because it’s a mature and very inexpensive technology. Each sensor and the electronics that go with it are a few hundred dollars. Many cyclists are very familiar with the inductive loop’s primary shortcoming: they don’t always recognize very small vehicles.

Some transportation departments are moving to video detection systems, but these are very expensive — up to the tens of thousands of dollars per intersection, compared to a few hundred dollars for inductive loops. Some problems with video detection include low performance in inclement weather, during twilight hours, and even in high wind (from camera motion).

Microwave radar is a less common technology, and falls in between inductive loop and video in price. The city of Pleasanton, California is experimenting with microwave radar detection specifically to detect bicycles.

The microwave system in Pleasanton is unique in that it can identify and differentiate different vehicle types, including bicycles. The traffic signal’s green phase is longer when a bicycle is detected, to give sufficient time for a cyclist to cross an intersection. If you’ve ever been caught halfway across a wide intersection with a fast light, you can appreciate this feature.

For the test, Pleasonton has installed a microwave detector for northbound Foothill Rd at Stoneridge Drive. If the test proves successful, the city will consider using more microwave vehicle detectors at other intersections.

This video shows the microwave actuator in action, and shows how it reacts to bicycles and other, larger vehicles.





See also:

  1. Caltrans response to questions about bicycle detection
  2. Traffic lights and bicycles: The technical explanation
  3. Pleasanton’s radar love
  4. Traffic signal reference wheel


  • http://scorcher.org/ Jym Dyer

    =v= Alameda, another East Bay city, is trying these out (on Willie Stargell Avenue, near Webster). They don’t detect bikes.

  • http://ccorlew.blogspot.com Curtis

    I sure like this idea. I get so tired of sitting at lights that don’t change, or getting what I call a “fake green” that lasts only 3 seconds. Antioch, CA, are you listening?

  • http://twitter.com/twwilliams Tommy Williams

    There’s apparently an avid cyclist among the traffic engineers in my city (Bellevue, WA) and all traffic signal detectors are tested and marked where they’re most sensitive to bikes. And from what I have heard, the city has been responsive when there are loops that don’t detect bicycles. Almost every intersection with traffic lights in the city has these induction loops (maybe they all do).

  • http://twitter.com/twwilliams Tommy Williams

    There’s apparently an avid cyclist among the traffic engineers in my city (Bellevue, WA) and all traffic signal detectors are tested and marked where they’re most sensitive to bikes. And from what I have heard, the city has been responsive when there are loops that don’t detect bicycles. Almost every intersection with traffic lights in the city has these induction loops (maybe they all do).

  • thielges

    Properly designed in-pavement loops are just fine for detecting bicycles and you don’t need a lot of metal to trip them. All you need is to position a hoop of metal (i.e. your metal wheel rim) above the right location. And that’s where the problem arises : often cyclists don’t know where to stop in order to trip the detector. Pavement markings are the answer.

  • http://www.cyclelicio.us/ Cyclelicious

    @thielges: In California, new signal light standards require sufficient crossing time for cyclists. There’s a *lot* of pressure from some California cities against this standard, saying changing the green phase like this will severely disrupt traffic. Actuators like the microwave detector here that can detect the difference between a bike and a car and give longer times only when a bike is present are kind of a compromise solution.

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