Traffic signal reference wheel

Most “traffic actuated” traffic signals in California are triggered through electromagnetic detectors embedded in the road surface. They’re simply large metal detectors that detect the presence of a vehicle by inducing a magnetic field in them.

They work well when there’s a large, horizontal metal surface over them, like you have in cars and trucks. In bicycles and motorcycles, these detectors induce a current in the wheel, which is vertical and small, and which leads to problems for many motorcyclists and bicyclists.

California AB 1581 became law in January 2008. This law mandates that any new traffic-actuated signal must detect bicycles and motorcycles after Caltrans publishes standards and procedures on bicycle and motorcycle detection. The Caltrans committee that created these standards determined that small wheels, such as those used on BMX bikes and folding bikes, are the most difficult to detect.

In the old days (like four years ago), a traffic engineer would invite a cyclist to a problem intersection. The cyclist would stand with his bike over the signal actuator while the engineer fiddled with the sensitivity. Since the traffic engineer worked the same hours as your typical bike advocate, it was often difficult for the two to arrange a time to meet.

Behold the “reference wheel.” It’s simply a wheel mounted to a block of wood so the wheel can stand vertically.

The Caltrans suggested “reference wheel” to use for traffic signal calibration is a 20″ BMX wheel. Below is Bob Sutterfield‘s “AB1581 loop detector test rig.” He built it from a castoff 20″ wheel and a scrap piece of wood. The traffic engineer simply places this wheel vertically over the detector in the pavement, adjusts the sensitivity controls, then tosses it in the back of his work truck with his safety cones and tools.

I know several people are having problems getting their local traffic engineers to adjust traffic signals to detect bikes. This PDF doc giving background on California’s AB1581 and the findings of the bike subcommittee might be helpful if you’re fighting that fight. There’s also this old article explaining how loop actuators work.


  1. OK, so how do we get one of these to the road crew of Redwood Shores, and then get them to use it. None of the sensors in Redwood Shores (Twin Dolphin, Holly, Marine Parkway, etc) detect my bike.

  2. Does the detector sensitivity adjustment work to account for the wheel material (aluminum, steel, or carbon)?

  3. As long as it's conductive, Al or steel doesn't really matter. Carbon, OTOH, won't work. The suggested workaround for carbon wheels is a strand of thin copper wire wrapped two or three times around the inside of the rim, with the ends connected so you have conductivity.

  4. I've found wheels to be kind of flaky with the detection. However, I've had 100% success placing my bike with the bottom bracket/crank right over the intersection of the loop.

    My theory is that the bottom bracket (regardless of frame material) is a more dense mass of conductive material to trigger the sensor.

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