Trigger Happy

Lined up on the wire

There’s nothing more frustrating than getting stuck at an on-demand signal and having to wait for a car to come up from behind to trigger the light. In some jurisdictions, if you’re unable to trigger the light, it’s legal to proceed after stopping, but that doesn’t help when you’re at a cross street with heavy traffic moving in both directions. The good news is that in many cases it’s possible to trigger a light with your bicycle.

On-demand signals use what they call “induction loop vehicle detectors” to sense when a vehicle is waiting at a light. These detectors are essentially metal detectors embedded into the pavement. They work by sensing changes in an electromagnetic field and have nothing to do with the weight of the vehicle. You can often see evidence of loop detectors as lines cut into the road surface just behind the crosswalk. Wire sensors are embedded in these cut lines, and it’s possible to trigger a light by placing your bicycle wheels precisely on top of one of the wires to disrupt the magnetic field. Some sensors seem to be more sensitive than others; in those cases where the light isn’t initially triggered, I’ve had some success by leaning my bike over toward the inside of the detector loop. In cases where there are two side-by-side loops, lining up over the center where the two loops meet doubles your chances of triggering the light. Once I understood exactly how loop detectors work, my rate of success at triggering lights considerably improved; I’m currently getting somewhere approaching a 90% success rate on the detectors where I live.

How are the detectors where you ride? Are you able to trigger a majority, or are you stuck running red lights or waiting for cars to help you out?

31 Responses to “Trigger Happy”

  • Val says:

    I find that I can trigger almost all the sensors that I encounter since I put some small rare ear the magnets on the bottom of my bike. There are two on the bottom bracket shell, and one on the end of the left pedal spindle. They disrupt the magnetic feilds much better than plain steel.

  • Paul Gower says:

    Here is another thing you can do that also works very well.

  • CTP says:

    as an ex-motorcyclist, i recall seeing ads in the back of magazines for a device that would trigger the lights, since even motorcycles struggle with this. sounds like the device is just a big lump of metal, mind you…

  • bongobike says:

    The detectors in Austin vary quite a bit. Some are very sensitive, others will never change the light for you on a bike. There is a phone number you can call to report lights that aren’t triggered by bicycles. The city will send someone to adjust the sensitivity.

    I thought the best thing to do was to place my tires right over the cuts in the pavement, but now that you mention these other ways, I will try them.

    BTW, if you took that picture today, we’ve had a strange extra sensory communication of some kind. I rode in today in jeans and the same exact shoes you are wearing!!! Of course, I have stated here that It’s impossible to ride in street clothes right now and arrive presentable, but today I decided to try starting early and riding very sloooooowly… I still had some big sweat patches on my shirt! :(

  • David says:

    Motorcyclists and scooter owners have the same problem. Attach a neodymium magnet or two to the bottom of your bike. There are commercial products but you can order the magnets cheaper. There’s an instructable about it too.

  • Lucas says:

    EVERY lighted intersection should have a detector of some sort… here in New England, much of the infrastructure is so outdated that there could be a roadway with (for example) 10 lighted intersections, and of these 10, anywhere from 3-8 of the sets of lights are on timers rather than sensors. This is especially frustrating when riding along in light traffic and the lights change to red for a car/pedestrian/bike that is not there! I have this frustration whether on my bike or on the rare occasions I drive.

  • brad says:

    In Toronto, they now have bicycle-activated light triggers: you position your bike directly over three white dots in the road at an intersection, and the light changes for you. No need to trudge over to a pedestrian walk button or jockey around trying to trigger the car detector. I imagine other cities are starting to implement these too.

  • Mark K. says:

    Sensors are pretty good here in Seattle. Some of the sensors are even marked with the best place to line your bike up (the streetfilms video with Seattle Mayor Mike McGinn shows this:

    I’ve had the best success with placing my bottom bracket above an intersection of the cuts, where there’s usually a collection of the sensor wires. For some reason, I’ve found that wheels don’t trigger as well as the mass of steel and other metals in the bottom bracket – Even if it’s a bit higher than the rims on my wheels!

  • Tali says:

    I think I’m spoiled on the roads around here (Gloucester, UK). When the roads are quiet in the late evening/ early morning, I can trigger the loops set some way back from the intersection and have the lights change as I approach, without stopping.

  • Larry Guevara says:

    One intersection on the way to my work has newly installed sensors that work very well for bicycles, but they detect my bike so far back from the intersection that I have to be traveling over 15 mph to reach the intersection before the light switches back to red.

  • Alan says:


    That photo was actually taken yesterday evening – we’re out of sync by 12 hours… :-)

  • Alan says:


    Short lights are a real problem around here. So while I may be able to trigger lights, I frequently find myself rushing across to clear the intersection before the light changes back.


  • Bob G says:

    Hi Alan,

    here’s a link I learned of a few years ago that helped me understand how signal detectors work. It’s helped me quite a bit since then.

    Bob G
    Granite Bay, CA

  • Charlie says:

    AS I understand it, the idea that strong magnets can trigger the detector is a myth and/or a scam by those selling them. The sensors work on a high-frequency ac field. The magnets produce a dc field. You can attach the magnets and then trigger a light and think they work, but as Alan explained, you can trigger a light without them too, so that’s no proof that they work.

  • Opus the Poet says:

    Where I live there is only one light I can consistently trigger, because there is only one through lane on that side of the intersection and a very short detector loop that If I’m on it there isn’t room for a car to pull up behind me and trigger the light. People in cars would get tired of waiting behind me for the light to trigger and called the light in as defective. Had I just ran the light when traffic cleared, besides confirming that cyclists never obey red lights, then there would have been no incentive for my local DOT to fix the light so I could trigger it with my bike.

  • Roland Smith says:

    Most intersections here (Netherlands) have a pushbutton mounted on a pole next to the cyclepath in easy reach for cyclists. Simple, and it works. Pushing the button alerts the controller hard/software that someone wants to cross.

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  • cycler says:

    Another reason to ride an all steel bike! Try changing the EM fields with a carbon frame!

    They put a couple of those in on one of my routes, and painted a little bike symbol over them so that bikes would know where to stop, but they put them in the stupidest places-both in right turn only lanes, where you’re just asking to get right hooked, so I ignore them. I’m mostly riding when there’s enough vehicle traffic to trigger the normal sensors, and if there wasn’t ANY traffic, I’d just eventually run the light.

  • Neil O says:

    Happens to me all the time in Los Angeles and the SF Valley. One intersection has sensors with no obvious cuts or lines in the pavement? Impossible to line up the wheels on that one.

  • Alan says:

    @Neil O

    When there are no cut lines, try dividing the lane in half, then dividing in half again and lining up on that mark. In other words, line up your bike at 25% from either the center line or the lane edge. It’s a shot in the dark, but I’ve had decent success using this method.


  • Pamela says:

    What drives me nuts is when I’m at the head of the line for a light which I can’t trigger, and the driver behind me is so far back that he/she can’t trigger it either, and despite my motioning them to move closer, to trigger the light, they refuse, apparently out of concern for my safety!

  • Doug says:

    What Pamela describes is something I’ve experienced many times.

    But no more here in Minnesota. This April our state legislature passed an “unchanging red light law” for bicyclists. The Gov signed it into law. It was already in place for motorcyclists. If a light does not change, it allows a bicyclist to proceed through an intersection on a red light once you have determined it is safe.

  • Val says:

    Charlie: I don’t know about any scams; I paid $4.00 or so for three magnets at my local hardware store, put them on my bike, and immediately noticed that I had no trouble triggering lights that had consistently ignored me before. Seems pretty conclusive to me.

  • lyle says:

    Here in Chico, they all seem to work just fine. Most intersections on bike routes have a small white bicycle painted in the middle of the lane with a white line in front and behind with a small T at the front. I always put my front tire right on the T and it works every time.

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  • Samanhtha says:

    Thank you for the tip! This always drive me crazy, and depending on the traffic, I end up blowing the light anyway. I may have to re-post your tip with full credit of course, on my blog. I used to assume it was based on on weight, but then someone told me that was untrue. Since I ride steel bikes I should have a good shot at making this work!

  • Ladia says:

    Here in prague (czech republic europe) we use simple trick, lay the bike down on the ground, then there is a lot more metal near the loop which can be detected and lights go on.

  • bongobike says:


    How and on what part of the bike did you install those magnets? How big are they?

  • EcoVelo’s Tip on How to Trigger a Stoplight on a Bike | Ding Ding Let's Ride says:

    […] “On-demand signals use what they call “induction loop vehicle detectors” to sense when a vehicle is waiting at a light. These detectors are essentially metal detectors embedded into the pavement. They work by sensing changes in an electromagnetic field and have nothing to do with the weight of the vehicle. You can often see evidence of loop detectors as lines cut into the road surface just behind the crosswalk.  Wire sensors are embedded in these cut lines, and it’s possible to trigger a light by placing your bicycle wheels precisely on top of one of the wires to disrupt the magnetic field…” You can read the rest of the article here. […]

  • Val says:

    bongobike: They are rare earth magnets, about the size of a nickel. I found them at the hardware store, three to a package. I stuck one on the inboard end of the left pedal spindle, where it is flush with the inside surface of the crank arm, and the other two on the bottom of the bottom bracket shell. As I say, there are sensors that I know from experience will not trigger for that bike alone that work every time with the magnets. AC/DC incompatibility? As far as I know, magnets will interact with magnetic fields, no matter what. Works for me.

  • Charlie says:

    This is pretty interesting–if you read descriptions of how the loop detectors work, it seems like a magnet really shouldn’t work. I’ve linked one good description from my name.

    But it could be that the sensitive input circuits of the systems used get messed up when a strong moving magnet moves across the edge of the loop, and take a moment to recover. And during that recovery, they might send a detection signal. My guess is that that’s what’s happening.

    If you google , you’ll find a page with a report from him that he tried magnets, and he thought they were working, but it was really just his bike tripping the detector. So that could be it as well.

    It would be interesting to try a strong magnet as a pedestrian, so that if it trips, it has to be the magnet. I might try duct taping a magnet to my shoe, and then trying a light during low-traffic hours when nothing else is around to trip it.

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