Site icon BikePortland

Bike Science: Making sense out of traffic signal sensors

Buffered Bike Lane with a bike symbol and arrow pointing forward

A traffic signal sensor in Portland.

Welcome to our newest column: Bike Science with Shawn Small

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?

To supplement my own understanding of this topic, I got in touch with Peter Koonce, the Division Manager of Signals, Street Lighting, and Intelligent Transportation Systems (ITS) for the City of Portland (learn more about him here).

Electrical inductance.

The word on the street seems to be that the sensors are either weight or magnet based, but Koonce assures me that’s not the case. The truth is, neither weight nor magnets will get you the green; the sensors are actually based upon the principles of electrical inductance. Here’s how it works…

If you ride around Portland, you’ll observe one of following three patterns in the road at each traffic signal;

This graphic by HumanTransport.org shows common inductive loop patterns.

A loop of electrical wire is embedded in these patterns. An electrical current (like your home electricity) runs through these loops at a set frequency and power; this produces an electromagnetic field in the loop of a constant value. When a metal object — like a bicycle wheel — passes over this loop, it disturbs this electromagnetic field. A detector measures this disturbance and sends a message to the traffic signal controller saying that someone is waiting. The most sensitive part of the inductive sensor is marked with a small bike lane icon. By putting your wheels directly on top of this marking you will disturb the electromagnetic field and signal the detector.

For best results, place wheel on icon.

According to sensor manufacturers, bicycle rims and cranks generally provide enough metallic material to trigger the sensor. But if you have a full carbon fiber bike, carbon fiber wheels, and carbon fiber components, you could be out of luck.

These sensors may not be set sensitive enough to measure the disruption in the field caused by a bicycle; this can lead to triggering problems and leave you stuck in the road. So if you are at a familiar traffic signal with your bike correctly placed on the sensor and unable to trigger the sensor in a timely manner, contact your local Department of Transportation to report the sensor (if you’re in Portland, call (503) 823-1700).

Loop sensors aren’t perfect, so there are a lot of interesting new systems being developed. One of the more exciting ones uses a digital camera and image recognition software to detect when someone has pulled up to a traffic signal. The City of Portland has one of these on the west end of the Broadway Bridge to activate the bike signal. Personally, I would love to see a small light bulb turn on when you have triggered the sensor to let you know that you are correctly waiting in line.

So save your time and money and don’t bother gluing magnets to your shoes. The easiest way to trigger the traffic sensor is to observe the markings on the road and safely put your wheels on those marks and wait for your turn.

Want to know more?


— Hope you enjoyed the first Bike Science column. We’re just getting started so stay tuned for more. If you have a question or a topic you think Mr. Small should tackle, drop us a line and we’ll send it to the lab for analysis.

Bike Science is sponsored by: Strava.com – GPS cycling, virtual competitions and detailed ride analysis.