Traffic engineer Bob Shanteau is an avid cyclist and active cycling advocate. He will present this paper on “Detecting Bicycles and Motor Vehicles Using the Same Loop Detector” next Tuesday at the state capital on the detection of bicycle traffic using inductive loops. Bob has asked for input on his paper; if you have any constructive feedback, please feel free to email him or comment here. He also plans to publish this as an article in WesternITE, the newsletter for the Western Region of the Institute of Transportation Engineers.

The paper is somewhat technical, but describes in great detail how the various inductive loops work, the challenges of various designs with respect to cyclists and where we ride, and his experiments with new designs in the city of Monterey, California.

I’m looking forward to seeing Bob’s recommendations translated into state guidelines for traffic signal actuators that work well with bicycles.

How loop traffic light detectors work

Loops of wires are embedded in the road surface. You can often see the saw cuts where the wires are installed at intersections. An electrical current passes through the loop, creating a magnetic field. When a conducting object — such as a car, motorcycle, or bicycle — intersects this magnetic field, electrical currents are actually created within the metal object. This electricity in turn creates its own magnetic field in the opposite direction from that created by the loop, resulting in decreased magnetism. When the magnetic level drops below a preset threshold, the actuator is tripped.

If the threshold is too low, traffic in adjacent lanes can trip the actuator and incorrectly trigger a signal light change. Unfortunately, this threshold is often too low to detect bicycles and even motorcycles. In his paper, Bob makes recommendations for inductive loop configurations, loop placement, markings, and engineering practices for setting threshold levels.