Anyone who bicycles in a city has had this problem: You stop for a red light, you wait and wait, but it never turns green. Then a car pulls up behind you, and the light turns green. The light’s control box detected the car, but it failed to detect your bike. It’s a real irritation, and it probably convinces some bicyclists that traffic lights aren’t for them.
The Ohio Bicycle Federation is working to allow us to treat such lights as legally defective. (That’s part of House Bill 145, which would also require three feet of passing clearance by cars.) But the irritation will still be there. We’re operators of legal vehicles! Our vehicles should be reliably detected!
In the meantime, it helps to know how traffic light detectors work, to give you your best chance at getting a green.
First, understand that most lights detect vehicles using loops of wire buried in the pavement. You can usually see these; look for a large (maybe 4 feet wide, 8 feet long) rectangular slit that’s been cut in the pavement, then re-sealed. (See Figure 1, although size, shape and placement vary widely.)
These “loop detectors” don’t respond to weight, and they don’t need magnetic material to function. It’s more complicated. They have a circuit that oscillates the electric current in the loop. This generates an oscillating electromagnetic field that sort of sprays up from inside the rectangle, then falls outside of it like a fountain. That invisible field loops around under the ground, back to the underside of the rectangle. (See Figure 2.)
To be detected, a vehicle needs enough metal to encircle or “capture” enough of that field. (It will then change the detector circuit’s resonant frequency, but don’t worry about technicalities.) Obviously, a car or truck has a lot of metal; but your wheel rims should do the job - IF everything is tuned properly!
Trouble is, if the traffic folks turn the field strength way up to be sure of detecting a bicycle (or motorcycle!), it can spill over into the opposing lane. Then the light gets triggered by cars going the opposite direction, and unnecessarily slows traffic. So lots of times, they set them too low for bikes.
When that happens, I’ve had some success phoning the city or county to get the sensitivity turned up. In fact, I just phoned about three lights that I remember having problems. But these loops usually work fine for me, because I know where to put my wheels! So, where’s that?
If you’re looking at a loop that’s a simple box, put your wheels right on top of the sides of the box that line up with the direction you’re riding. I shaded those sweet spots in Figure 3. That position captures the biggest amount of field, much better than the center of the box.
If you’re stopped on top of those side wires and you don’t get a green, try tilting your bike toward the inside of the box. This lets you capture even more field, and often does the trick.
But there are other designs! Sometimes you’ll see a box with an extra wire down the center (like Figure 4).
Those concentrate the field inside the box (Figure 5)
and are actually better at detecting bikes without false trips from cars. But here’s the weird part: They don’t work if the bike is on the sides of the box! So if the box has an extra center cut, put your wheels right on top of the center wire, not on the sides, like the shaded area in Figure 4. (Wouldn’t it be nice if they marked the sweet spot for you?)
Almost all the loop detectors in our area are one of those two types, but there are others.
Some states use circles. The sweet spot is the same as for a box (Figure 6).
Some use diagonal “D” shapes in pairs (Figure 7). These are supposed to detect bikes almost anywhere within them, but they’re costlier to install, so not used around here.
Although I’ve not tested one, there are now some “powerhead” loops here, that are big rectangles cut with an extra side-to-side line near the stop line. (Figure 8) Supposedly, they’ll detect a bike anywhere in the small box, but it may help to have your wheel at an angle.
And there may be others! I think the best advice is to be at the edge of any single shape (box, circle, diamond, whatever). And be in between any shape that comes in pairs.
There are also digital cameras! These are new, and supposedly better, plus cheaper in the long run. They look for changes in contrast. So if you see a camera on the pole, you can try making yourself look bigger (maybe opening a jacket?) or perhaps shining your headlight at the camera.
But again: As a legal vehicle, your bike should be detected. I’ve had good luck phoning in complaints, to get detectors’ sensitivity turned up. The city, county or state won’t want to be on record, and liable, if you’re injured because a light didn’t change.
And again: Wouldn’t it be nice if they marked the “sweet spot” for every loop detector? There is an official, approved marking for the pavement, plus a sign that goes with it.
Should we make a club project out of getting these markings and signs installed? What do you think? After all, we’re here to make biking better, aren’t we?
Finally: If you do find a traffic light that won’t detect your bicycle or motorcycle, report it to this email address:
and this phone number:
(614) 387 0722
They are supposed to be valid for any light in Ohio, regardless of whether it’s owned by the state, the county, or a city or village.
Please do your part. If you find a defective sensor, report it!
- Frank Krygowski, OSW Safety Chairman