Thursday, February 25, 2010

Excuse me, got a light?

As you ride around the city, you will frequently encounter traffic signals that only activate when a vehicle is present. You can often see sensors embedded in the pavement. Urban legend has it that these detect a vehicle's weight - this is false. They actually use electrical currents to detect the presence of an iron-based (ferric, if you want to get all technical) substance, such as the steel frame of a car.
Embedded wire sensor
This means that a steel-framed bicycle has a good chance of triggering one if it's in the right place. It also depends on how your local public works department has set the sensitivity, which can be adjusted from the controller box. Atlanta, I've noticed, sets theirs so it does detect bicycles most of the time. Well, at least, my bicycle. I don't know about the aluminum and carbon fiber folks. There are a few frustrating exceptions, especially where I need to trigger a left-turn signal. And DeKalb County is hopeless. I simply have to run the light.

From what I understand, this is legal - a traffic light that is supposed to change when a vehicle is present, and fails to, is "malfunctioning" and malfunctioning lights should be treated like an all-way stop sign. Of course, the cars on the cross street don't realize it is "malfunctioning" so I wait for them to clear. If traffic is heavy, the light usually will get triggered by a car behind me, as long as I pull forward far enough.

Hopefully, our traffic engineers will become more sensitive to the needs of bicycle traffic and other lightweight vehicles, and start to adjust the sensors accordingly.


  1. We were just discussing this Wednesday night at Bicycle South! One guy, a motorcyclist, tipped us off to a magnet that triggers the coils. This article explains how to position and how they sense bikes, and Instructables, that bastion of DIY knowledge, explains how to construct your own trigger.

  2. The City of Toronto puts three white dots on the pavement in the centre of the right lane at traffic lights. A bicycle on those dots will trigger the sensor.

    It works too!

  3. Atlanta signals seem to be all or nothing - either any part of the sensor will work, or else it doesn't work for bicycles at all.

    That magnet looks heavy!