Solving the mystery behind the stoplight sensor
 

On my usual routes, I encounter three stoplights at which other vehicles may or may not be present. A few months ago, on a mission to undermine the oppressive stoplight hierarchy, I began turning my bicycle ninety degrees to the right. [hmm, never tried left... or sideways] I keep my front wheel turned to anticipate reengagement. Of the three stoplights, I can say that one probably does respond to this technique and one other probably doesn't.
Tonight, at one such stoplight, a car was parked entirely ahead of the stop line. Rather than take a sensor, I let the glorified chunk of metal reign in magnetism. But something strange was going on. After a few minutes of waiting, I pulled up beside the car and made some hand gestures. The driver lowered his window. I told him that he should reverse onto the sensors. In an ironic twist, he complied. With his helplessness couched in a voice of iron, he told me that he had lived across the street for twelve years, yet he could never figure this stoplight out. At first, I thought he was thanking me for the advice, but he made sure to correct this. He rephrased the language to communicate that he had already tried everything to no avail. But I still sat there with the expectation of a light change. After about a minute of waiting, I acted to cease his despair. I began to reorient into the special position as I explained the purpose to him. About five seconds later, the cross traffic's light turned yellow. He sped off, spewing exhaust into my face. [Okay, he didn't spew exhaust into my face. That was Hollywood.]

The sensor doesn't care which direction you approached from; it only knows if there is metal on top of it. I've had very good luck with stoplights by lining my bike up perfectly with the sensor in the ground.

I stop right above the cut mark on the right side of the rectangle, with one wheel over where the wire meets itself again. This always works for me.

Can someone take a picture? I havent seen any such sensor in my neck of the woods, and would like to recognize one when I see one.

I'm amazed at how many drivers are clueless to the sensors and will stop beyond the stop line, thus never triggering them. Or well before the stop line at some intersections.

I faced this often at one intersection I encountered daily while driving. At the bottom of a hill, many drivers would stop too soon and never trigger the light. Which would otherwise stay red forever. In a car, I'd ultimately have to go around them and just run the red light. Otherwise, I'd sit there forever.

http://en.wikipedia.org/wiki/Induction_loop

Anywhere you see sawcut lines in the road forming a loop, that's it. You also see them in places with drive-thru windows like fast food restaurants, banks, etc.

You have to understand these sensors. People say they react to metal interfering with a low frequency electromagnetic field, and you need to find the sweet spot, put the heaviest metalic spot on your bike over the sweet spot of the sensor, etc.

They're all wrong.

These things are actually set up to sense a bicyclist put his foot down. If you can do a track stand for 3, 17, or 70x7 seconds, and then put your foot down, the light's going to change two seconds later. It's a conspiracy to make you clip out!

Put your wheels down the center line of a 3-line loop. They're usually more sensitive there than on the edges, do reduce the chance of false positives from vehicles in adjacent lanes.

There's a pretty good article at humantransport.org

http://bandtcrowd.blogspot.com/2010/...k-sensors.htmlhttp://bikeportland.org/2010/09/27/b...-sensors-39517
http://www.hbl.org/content/whats-sensor-loop


http://motorcycleinfo.org/2009/08/traffic-lights/

http://www.transitboston.com/tag/traffic-signals/

http://www.meas-spec.com/product/t_product.aspx?id=4725

http://www.plan-bravo.com/lefty.html

http://motorcycleinfo.org/2009/08/traffic-lights/

http://ops.fhwa.dot.gov/freewaymgmt/...apter15_01.htm

I'll drink to that! :D

I have a light on my commute particularly on the way in to work. That darn thing never turns green. What's really messed up is me trying to do all kinds of stomping, moving my bike around, etc. From a distance I probably look like Costner dancing around the fire in Dances With Wolves.

So a couple of weeks ago I was on my way home when I happened to see another guy at that same intersection. Just before I made my right turn, he actually laid his bike down and I'll be, the darn thing turned green.

The next day, I tried the same thing. That darn light hates me. I guess his bike was steel. My aluminum frame didn't trigger it.

Thanks for the suggestions. I'll try them out.

There is one red light I often encounter that has the sensor 2 car-lengths behind the stop line, I guess so it only triggers where there is an accumulation of cars. It certainly seems to take forever when I am the only car there and I am at the stop line. So on my bike I either stop 2 car lengths back, on the sensor, or if it is a busy time and I know cars are coming I go in front of the stop line because if I stop at the stop line, often cars give me so much following space at the light that they are behind the sensor, which I guess is kind of like the OP's case of stopping ahead of the stop line and sensor and getting enough following space that cars are behind the sensor.

The article on humantrasport said that the wheels are the most important part of the bike for detection by the sensors. That would explain why I got detected when I positioned my bike horizontally. I had one wheel over each sawcut. By keeping the front wheel turned forward, I was aligning it with the sawcut on the right side.

Most of the lights in Minneapolis are on timers. I'm very glad I don't have to play this song and dance on my current commute.

Some of the sensors here have a painted picture of a bike on the right side of the circle.

Some of the lights around here use timers at certain times of the day, and use the loops the other part of the day. e.g. before 6am the light at the entrance to my housing addition gives me a green light so quickly that I don't even need to stop; Just slow down as I roll up to it, then accelerate as the light turns green. After 6am: Fuggedaboutit; I can either sit there for 3 minutes, or just run the damned thing when there is no cross traffic.

How do these things work with carbon fiber bikes and wheels? Do they work at all?

In NYC we don't have many of these so it isn't much of a worry.

I am a traffic engineer by trade - detector loops work by inductance, i.e. ferrous metals cause a change in the inductance and "tell" the signal controller that there is a vehicle there. If you are riding an all-carbon bike, you are SOL as far as ever getting a loop to detect you.

I'm pretty sure they test them around here to make sure that a bike isn't going to set them off. There is a bike path that crosses all the ones I want to trigger right at the end of the loop. It's annoying

Is there a good way to find the loops if there is no visible cut? I recently went on a trip and could not trip a single light in town because I could not find the marks I'm used to seeing. I end up having to press the pedestrian button at every light if there wasn't a car there.

In SF,I just line up the wheels on the cuts of the rectangular ones. The circle ones are the most difficult. The dual-trapezoid one you just have to have the bike over it anywhere.

Inductive loops only need conductive material to work, not ferrous metals. Maybe you are thinking of magnetic sensor or magnetometer? Most bicycle rims and many frames are are aluminum, which is non-ferrous, but aluminum is a conductor. Carbon fiber is also a conductor so my guess would be that a CF wheel and frame might be able to trigger a light, but I don't know the conductivity of carbon fiber.

I personally wouldn't worry about it and go through the light if it's clear. I think many states' laws allow bicycles to do that - proceed if clear after coming to a complete stop.

That is known as the "Idaho Stop", and AFAIK, only Idaho has that law.

I think most places have that law if you can't trip the sensor.

The Idaho Stop means that a *cyclist* can stop, then can immediately proceed.

Most states do have a law stating that *any* vehicle that doesn't that get a green after a certain amount of time, (eg 3 minutes), can proceed, based on the principle that the signal is faulty. Of course the catch here is that unless you are in the habit of pulling out a stopwatch at every light, how do you know when 3 minutes have elapsed?