Traffic lights and bicycles: The technical explanation

California Governor Arnold Schwarzenegger recently signed AB 1581 into law, which requires all new traffic actuated signals to detect bicycles and motorcycles after Caltrans adopts uniform standards, specifications and guidelines for these kinds of traffic signals.

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.

Photo Credit: “I’m Going To Be An Actuator” by Hen Waller. Creative Commons “Some Rights Reserved” license.

21 Comments

  • Illinoisfrank
    October 11, 2007 - 7:52 am | Permalink

    I'm sure he can make it work for my 32 pound steel Gary Fisher mtb which I use as a commuter. But how about my friend's 16 pound carbon-frame wonderbike?

  • Illinoisfrank
    October 11, 2007 - 2:52 pm | Permalink

    I'm sure he can make it work for my 32 pound steel Gary Fisher mtb which I use as a commuter. But how about my friend's 16 pound carbon-frame wonderbike?

  • Yokota Fritz
    October 11, 2007 - 10:17 am | Permalink

    I asked Bob-the-traffic-engineer. According to him, the wheel material is the important part, so metal wheels should be fine. If your friend has carbon wheels, though, he's out of luck.

    For carbon wheels, Bob suggests winding a thin insulated copper wire around the rim (under the tire) a couple of times and splicing/soldering the ends together so it makes a continuous circuit.

  • Yokota Fritz
    October 11, 2007 - 5:17 pm | Permalink

    I asked Bob-the-traffic-engineer. According to him, the wheel material is the important part, so metal wheels should be fine. If your friend has carbon wheels, though, he's out of luck.For carbon wheels, Bob suggests winding a thin insulated copper wire around the rim (under the tire) a couple of times and splicing/soldering the ends together so it makes a continuous circuit.

  • Paul Tay
    October 12, 2007 - 2:03 pm | Permalink

    Hey, how 'bout this idea? Lose traffic lights?

    Red lights "CORK" flow. Motorists speed between the lights, just to catch the red.

    Solution: Allow continuous, CONTROLLED flow at the intersections, and REDUCE speed limit within city limits to 25 mph.

  • Paul Tay
    October 12, 2007 - 9:03 pm | Permalink

    Hey, how 'bout this idea? Lose traffic lights? Red lights "CORK" flow. Motorists speed between the lights, just to catch the red.Solution: Allow continuous, CONTROLLED flow at the intersections, and REDUCE speed limit within city limits to 25 mph.

  • Anonymous
    October 13, 2007 - 7:30 am | Permalink

    Until most traffic lights are designed better, you might try the idea in this video to get lights to turn green. It involves attaching a neodymium magnet low on your bike.

    http://www.ecogeek.org/content/view/987/

  • Anonymous
    October 13, 2007 - 2:30 pm | Permalink

    Until most traffic lights are designed better, you might try the idea in this video to get lights to turn green. It involves attaching a neodymium magnet low on your bike.http://www.ecogeek.org/content/view/987/

  • Yokota Fritz
    October 14, 2007 - 11:05 pm | Permalink

    I think most cyclists are aware of these magnets. Since the poster is anonymous, I'm a little suspicious about the spammish nature of the link. Please disclose your relationship to the advertiser in the informercial. I truly don't mind if you ARE the advertiser as long as you're honest about it.

  • Yokota Fritz
    October 15, 2007 - 6:05 am | Permalink

    I think most cyclists are aware of these magnets. Since the poster is anonymous, I'm a little suspicious about the spammish nature of the link. Please disclose your relationship to the advertiser in the informercial. I truly don't mind if you ARE the advertiser as long as you're honest about it.

  • cafiend
    October 15, 2007 - 10:07 am | Permalink

    I wondered if strong magnetic fields on my bike or shoe would frig up anything important, like my personal electronic gadgets, or make me suddenly veer toward steel posts holding up road signs and traffic signals. Okay, maybe I don't worry about the latter, but somewhat about the former. If I toss my magnetized shoe down next to something I don't want to worry about side effects.

    At the few lights I deal with here in the back woods I just wait for a "red light buddy" to drive up or go ahead and run the light (with all due caution).

  • cafiend
    October 15, 2007 - 5:07 pm | Permalink

    I wondered if strong magnetic fields on my bike or shoe would frig up anything important, like my personal electronic gadgets, or make me suddenly veer toward steel posts holding up road signs and traffic signals. Okay, maybe I don't worry about the latter, but somewhat about the former. If I toss my magnetized shoe down next to something I don't want to worry about side effects.At the few lights I deal with here in the back woods I just wait for a "red light buddy" to drive up or go ahead and run the light (with all due caution).

  • Brutus
    October 15, 2007 - 3:20 pm | Permalink

    Logged in this time. I posted the ecogeek video link. I found it during my own searches after hearing about the magnet idea elsewhere. There are several lights along my usual riding routes that don't trip green for me, and I am sick of rolling up onto the curb to push the crosswalk button. To cafiend, you should definitely keep these things away from electronics.

  • Brutus
    October 15, 2007 - 10:20 pm | Permalink

    Logged in this time. I posted the ecogeek video link. I found it during my own searches after hearing about the magnet idea elsewhere. There are several lights along my usual riding routes that don't trip green for me, and I am sick of rolling up onto the curb to push the crosswalk button. To cafiend, you should definitely keep these things away from electronics.

  • Yokota Fritz
    October 15, 2007 - 3:23 pm | Permalink

    Thanks Brutus; sorry about my suspicious nature!

    Someday I intend to try these magnets in a methodical manner.

  • Yokota Fritz
    October 15, 2007 - 10:23 pm | Permalink

    Thanks Brutus; sorry about my suspicious nature!Someday I intend to try these magnets in a methodical manner.

  • Bob Shanteau
    December 18, 2007 - 1:54 am | Permalink

    Brian suggested using a magnet to trip actuated traffic signals.

    Sorry, but a loop detects conducting metal. They work like the metal detectors people use to find coins at the beach. Of course, a loop at a traffic actuated signal needs a larger piece of metal than a coin, but not more massive, such as the bottom of a car.

    A piece of conducting metal causes an decrease in the inductance of the loop and therefore an increase in the resonant frequency of the oscillator circuit in the detector. Sorry, but there is no way that a permanent magnet can cause a change in the inductance of a loop in an oscillator circuit. Just ask any electrical engineer or physicist.

    So you want to try a magnet anyway but can't tell when you have been detected by a loop at a signal? Buy a metal detector at your local outdoor store and see if it detects your magnet. I would be curious to learn what you find out.

    If you really want to increase your chance of being detected at a traffic signal, then wrap several turns (the more turns the better) of insulated magnet wire (try Radio Shack) around your front wheel and splice the ends together, insulating the splice. You can use thin magnet wire (size does not matter) under the rim tape of a clincher rim with electrical tape around the splice. Then stop with your front wheel directly over the slot of a loop, if you can see it. If you cannot see the loop, then stop about 3 feet from the edge of a 12 foot travel lane. Or you can try asking your local friendly traffic engineer to paint a bicycle detector symbol over the loop slot.

    If you cannot see the loop slots, do NOT stop in the middle of the lane!!! That is where most loops are LEAST sensitive to vertical pieces of metal such as a bicycle wheel. (There is an exception with figure-8 shape loops, but if you can't see the loop, how do you know what shape it is?)

  • Bob Shanteau
    December 18, 2007 - 8:54 am | Permalink

    Brian suggested using a magnet to trip actuated traffic signals.Sorry, but a loop detects conducting metal. They work like the metal detectors people use to find coins at the beach. Of course, a loop at a traffic actuated signal needs a larger piece of metal than a coin, but not more massive, such as the bottom of a car.A piece of conducting metal causes an decrease in the inductance of the loop and therefore an increase in the resonant frequency of the oscillator circuit in the detector. Sorry, but there is no way that a permanent magnet can cause a change in the inductance of a loop in an oscillator circuit. Just ask any electrical engineer or physicist.So you want to try a magnet anyway but can't tell when you have been detected by a loop at a signal? Buy a metal detector at your local outdoor store and see if it detects your magnet. I would be curious to learn what you find out.If you really want to increase your chance of being detected at a traffic signal, then wrap several turns (the more turns the better) of insulated magnet wire (try Radio Shack) around your front wheel and splice the ends together, insulating the splice. You can use thin magnet wire (size does not matter) under the rim tape of a clincher rim with electrical tape around the splice. Then stop with your front wheel directly over the slot of a loop, if you can see it. If you cannot see the loop, then stop about 3 feet from the edge of a 12 foot travel lane. Or you can try asking your local friendly traffic engineer to paint a bicycle detector symbol over the loop slot.If you cannot see the loop slots, do NOT stop in the middle of the lane!!! That is where most loops are LEAST sensitive to vertical pieces of metal such as a bicycle wheel. (There is an exception with figure-8 shape loops, but if you can't see the loop, how do you know what shape it is?)

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