Wednesday, December 06, 2006

triggering traffic lights on a bike

Ever find yourself stuck in a traffic purgatory, waiting for a light change that never seems to come? Have you ever gotten so frustrated at not being able to activate a left-turn signal, that you eventually decide to blow the light? For cagers, this is usually just a lack of patience. But for cyclists on the road, this can be a real and frustrating problem. We used to have a terrible time turning left at the light at the bottom of Broadway hill.

In Ann Arbor, we use inductive loop sensors to trigger many traffic signals. They are built into the pavement where traffic should wait at a signal. The larger size and greater conductivity of cars makes them much more likely to trigger the light than a bike. However, all the signals should be sensitive enough for bikes. These loops can detect any conductive material, so aluminum frames and wheels work as well as steel. Even if you ride a carbon fiber frame you should be able to trip the signal trigger since the best "signal" comes from the wheels.

For the best chance of detection, try to get both your wheels over the wire. If you are lucky, there will be a saw-cut in the pavement indicating where the wire was placed. If not look for a mark or stop on one side of the lane and lean your bike towards the center of the lane to improve your chances. You can read a more detailed description of the how and why here.

Indestructables has a quick-and-dirty HOWTO on installing magnets in your bike shoes. It's questionable whether this really helps at all since it is conductivity, not magnatism, that the coil reacts to. Certainly it is more effective to find the sweet spot in the loop.

Some communities actually post signs or mark the pavement to assist cyclists in triggering the signals. Unfortunately, the Good People of Ann Arbor are not so lucky. If you do run into a problem tripping the signal trigger on your bike, call the city to report it (734-994-2744). It's possible (at least, for someone who knows what they are doing, like a city employee) to adjust the sensitivity of the coils at the control box. If you find a sweet spot for signal changes, maybe this is an appropriate time for community-minded graffiti. Spray paint two little "x"s with a circle around each (so they look like 4-spoke wheels) to indicate the proper place to rest your wheels.


Bruce Fields said...

During the Broadway Bridge construction I had a left from Fuller to Maiden Lane on my ride home. I emailed the signals people and they came out, adjusted the sensitivity, and spray-painted marks on the pavement in the left-turn lane to show me where it was most sensitive.

As it turned out, that didn't really work for me, and I found my own positioning that seemed to work better. (Though it's hard be sure sometimes if you're only testing it once a day and don't know what else might have an effect.) But anyway, my experience was that they *were* quite willing to go out of their way to help.

Like you I used to have trouble with the left from the bottom of Broadway, but now I don't any more. There's some kind of cut line up the center of the end of the left-turn lane, and I always ride up that; maybe that helps. Also, it takes a certain amount of patience--that light seems a little slow to change.

Anonymous said...

I find the light at the north end of Stone School quite difficult to intentionally trip with my bike. I know it is sensitive enough because I sometimes trip it without trying, but other times I have to wait for a car to arrive. Unfortunately there are no marks or cuts in the road. What is this magic number to have them added?

Bruce Fields said...

Scott gave their phone number above (734-994-2744); or maybe projectmanagement or transportation at would be worth a try.

jlygrnmigt said...

Ooer, I'm going to have to try that at Mills and Packard. I sat there while the crosswalk light made me think the light would change, but it turned around and stayed red for me. Usually I just hop up onto the sidewalk and ride through the crosswalk, but I feel much less badass doing so.