Triggering Green Lights

When I wrote my last post about getting stuck at red lights, a couple of commenters noted that the bike should be able to trip the green light.  The problem is, a lot of us don’t seem to know exactly how to do it – and I guess I’ll have to count myself as part of that group, since I’ve never been able to trip a green light while on my bike.

Andrew in New Hampshire says bikes just don’t have the weight required to trip the signal.  That’s true, Andrew, and it’s too bad if that’s the way they work in New Hampshire.  In Austin, Texas, they’re supposed to be tripped by metal – the metal on a car, and even the metal on a bike.  But some of us Austinites can ‘t make that work, either.  Steve says he sat through a red light cycle three times before finally waving a car ahead to trip the sensor.

Alice in Louisville has the solution – at least in her town.  “The sensor plates under the pavement show a “T” shaped seam in the road,” she writes. “By placing your front bike wheel at the top part of the T, that’s supposed to trigger the sensor as I understand it. Seems to work most of the time, although there are some intersections where it just doesn’t work.”

Yeah, that’s the way it’s supposed to work here, too.  In fact, Nadia Barrera, the project coordinator for the city’s pedestrian and bicycle program, posted an item on the Austin Cycling Association’s web site explaining how to trip the light:

“To trigger a traffic signal, simply place your bicycle over or within three inches of the ‘saw cut’ of thedetector loop (pictured). The signal detects metal, so your pedal or even the metal in your shoes should trigger the signal. Once you have situated yourself over the loop, stay put. The signal works in such a way that if you leave, it thinks that you were a right-turning vehicle, and the trigger process to change the light is aborted.”

I’ve tried. Lord knows, I’ve tried. An intersection I cross on three out of four bike rides stymies me. I think I can figure out where the cut is that she refers to, and I park the bike right over it. I have never triggered the light. I assume I’m doing something wrong.

And looking at the diagram supplied with her information … uh, I don’t know what it’s supposed to show. What’s that diagonal off to the upper right? Part of the loop? I assume the T at the top center of the top diagram is the part that Alice uses to get her green light. I will continue to try for the magic solution.

In the meantime, I’ll just inch forward into the pedestrian crosswalk so that cars behind me in the left turn lane will be able to get far enough forward that their metal content will trigger the green and get us going.


  1. Janice in GA says

    I think the diagonal part going off to the upper right is supposed to indicate the wires that go to the control box, but I could be wrong about that.

    There is one light on my regular routes that I can reliably trip with my bike. For the others, I either hit the pedestrian signal (when I can), or I wave cars up behind me. The cars can always trip the signal. And many people stop WAY too far back as it is. While I appreciate their courtesy, if they’re not over the sensor, we’re BOTH going to be sitting there too long.

  2. Tim says

    LOL. Depends on what your bike is made of and the sensitivity of the particular sensor. Aluminum and carbon do not interfere with the magnetic field as much as steel and iron and thus induction sensors have a really hard time “seeing” them.

    Even with some steel in your bike, some sensors will be less sensitive than others.

  3. says

    I’ve never had any issues triggering the light sensors. Even the gate in my apartment complex opens with a magnetic sensor and I can easily trigger it. Just put your tires on any of the “lines” in the pavement that have been cut out for the sensor placement and you should be good. Also get a steel bike :-)


  4. says

    Any one of those lines in the diagram will trigger the light. It didn’t sit well with some drivers when I have done it, but I’m sure they have all gotten over it by now.

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