Making Traffic Lights Turn Green

by Bill Hoffman, John Forester, Dave Glowacz and John McClun

Traffic signals can be a boon or boondoggle to the bicyclist. They are a boon as they provide a controlled protected intersection for the predictable bicyclist to negotiate. They are a boondoggle when the bicyclist arrives at a red signal which does not appear willing to turn green.

What is a traffic detector?
Many traffic signals are controlled by a piece of wire, called a detector, buried in the street. This detector, similar to a metal detector, will determine the presence of metal objects. It will turn the light green when it detects a sufficient amount of metal disrupting the coils' frequency. Automobiles are large and create a large signal for the detector, therefore the light turns green readily for them. Obviously a bicycle does not have as large a presence.

Can a bicycle trigger a traffic detector?
Absolutely! Traffic detectors have several designs. Unless a road has been repaved since the detector was installed, you will be able to see the shape as you approach the intersection. The placement of your bicycle is critical to being able to successfully trigger the detector. If the cuts form a rectangle or square, stop directly over one that is parallel to your direction of travel. If the rectangle has an additional cut through the center, stop over the center line. If the cuts form a diagonal pattern, stop right over its center. Some detectors have overlap- ping wires in the corners of the rectangle. Stop there.

Bicycle placement is critical
Think for a moment about your bike. If the goal is to provide the largest area of metal to trigger the detector, will your front wheel accomplish that goal? Probably not. Making Traffic Lights turn Green. The amount of metal there is minimal. The frame, bottom bracket, cranks, and pedal areas have a larger quantity of metal. Align it carefully over the visible detector wires. You may be more successful placing that area of your bike over the areas outlined above or those that may contain overlapping wires as in the rectangle corners.

In some areas, motion detectors are now being used to "detect" traffic and trigger signals. These devices may be mounted on the signal light or a pole next to the intersection and work similar to the motion detectors installed with home and business security systems and many automatic garage door openers. They project an invisible beam designed to trigger when they detect the movement of a particular sized object. Your challenge: ride through the invisible beam. A lazy "S" motion by the cyclist in the potential area of the beam should be the most successful at tripping the trigger.

Uncooperative traffic detectors
Detectors of all types are, to varying degrees, effective devices for cyclists. They require careful calibration to detect all possible road users effectively without oversensitivity causing various malfunctions. Bicycles are vehicles. As the driver of that vehicle, you, the cyclist, are responsible for obeying traffic laws. A non-detecting signal prevents you from doing that. Report malfunctioning traffic detectors to the appropriate local traffic official.

When all else fails
You are over the wire. You are patient. The light doesn't turn. Now what? Wait a reasonable amount of time, even at a signal you know may not respond, then treat it as a stop sign and proceed when the crossroad is clear. And, definitely report that signal!

The graphic above is excerpted from Urban Biker,' Tricks & Tips" by Dove Glowacz, due in June with over 700 illustrations.

Reprinted from "Bicycle USA", magazine of the League of American Bicyclists, May/Jun 1997. Effective CyclingTM.
For more information about the League of American Bicyclists, visit their web site,, or e-mail them at

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