How do traffic sensors work?

The other day, I was wondering how traffic lights monitor when cars are coming. (I think very few traffic lights are on simple timers anymore.) I’ve always assumed there was some sort of inductance loop under the street, which would register a big piece of metal moving overhead. However, just about everyone I asked said they thought it was some sort of pressure sensor. (Which seems really improbable to me.)

If it’s an inductance loop, does anyone know the threshold? (I know my bicycle can’t cause the light to change. Can a motorcycle? What about newer economy cars? Is there enough metal in a GEO to change a traffic light?)

Just curious…

The ones I come across in Los Hideous are induction loops. My motorcycle triggers some, but not all, of them. Shutting off the engine, then restarting it seems to help. Possibly something to do with the current going through the starter? Either that, or I’ll get off and push the crosswalk button. If it’s in the wee hours, I’ll stop for the light and if it doesn’t look as if I’ve been detected, I’ll just ride through the red.

Giraffe - some of the inductive ones can be set off with the trick of laying your bike flat on the ground. I used this in San Diego a lot as you were required to put a foot down at stops anyway. It works with steel frames but I’v never owned alloy and pretty sure it won’t work with composite.

The very first sensors were pressure sensors, but loop detectors have been in use for over 30 years. Their sensitivity can vary, but they’re supposed to be set up to detect a motorcycle. Sometimes they don’t, tho.

They’re using cameras and image processing to detects cars in my town. This must be cheaper on an existing road than going back and installing an inductive loop. The cameras are mounted near the stop-light.


They can also be triggered by a camera, though this isn’t used very often. and if you find lights that don’t trigger your bicycle, or motorcycle, call the city and tell them to make it that way because it is supposed to pick up even bikes. for my bike, which is 700 pounds, a number around where I live don’t pick me up. some people say to use the centerstand and put it on the ground, others say to sit near the middle of the lane to have it pick you up.

When I worked at McDonald’s I used to like to take the trash bins over the sensor, it only had one piece of metal on it, the axle, but it would still pick it up.

I’m convinced that Traffic Lights are the cause for road rage in my area (Tampa). They are not synchronized on any roads outside of downtown, where you proceed on green only to stop 1/4 down the road for the next red.

Should I sue the City/County/State for not fixing this problem, which leads to, 1) increased stop & go traffic, 2) increased drive time (and increased emissions), 3) increased frustration (leading to road rage), 4) increased speed (whenever possible, to make the next light), 5) increased red light runners (tired of stopping all the time)…

One of our roads is in the top 10 in the USA for death. One wonders…


Most of them are loop detectors. The work on a principle similar to electromagnetism. They bury a large loop of wire in the road and when a big hunk of iron enters the field it causes a detectable change in the circuit performance (usually the frequency of an oscillator changes).

There’s really no reason for the systems to be insensitive to motorcycles, though I know they sometimes are. The problem is low iron content, especially in more expensive bikes that are trying to keep the weight low. However, it’s possible and usually desirable to make the circuits sensitive enough to detect bicycles.

First, cameras are definitely NOT cheaper than loop detectors - not by a long shot. The loop detectors are very easy and inexpensive to retrofit. Simply cut a grove in the pavement with a saw, lay in the loop, cover with tar. Cameras are more expensive because… well, a camera cost more than a loop of wire, plus a camera system requires intelligence to recognize the difference between a vehicle and a piece of debris that just blew into the street and changes in lighting conditions from clouds rolling by. This gets even more difficult and more expensive in low light conditions (night). Also, cameras are not very robust, so they would be subject to frequent failure, which would piss off drivers waiting at the light and would add high maintenance costs. I suspect that the cameras are there for less mission critical and more lucrative reasons… to catch people running red lights and going too fast. That’s what most cities are using the cameras for, anyway. Look in the road and I’ll bet you’ll still find the tale-tell rectangular outline of the loop detectors…

I forgot to mention. I remember reading a story a while back about a city that had installed a number of cameras to capture traffic trangressions… The city finally had to abandon their program because all their camers kept walking away…

answers it extensively…

Love that site…:slight_smile:

I went to High School in St Louis.

The lights there were set to turn green when Police cars approached - I guess they sensed the flashing lights.

I know that when I approached lights at night, I could cause them to turn green by flashing my brights a few times, even if it was turning red as I pulled up.

Nice link, Handy. That’s the sort of thing I was hoping to find when I searched the web, but all I got were engineering manuals for Traffic Sensor #1923B-131, left arrow actuator.

ricepad: they used pressure sensors at one time? Just out of curiosity, how do you know? It seems like a pretty unwieldly way to detect a car, if you ask me…

sdimbert: I knew fire trucks and police cars could change the light, but I thought they had a device (like a garage door opener). I’m gonna try that flashing light trick the next chance I get.

Very interesting stuff. I had an idea how sensors worked, but never knew for sure. What I can’t understand is people who stop 10 ft from the corner for a light. Don’t you know there’s a sensor in the road? You know, that big rectangle that’s in front of your car, instead of underneath it? Cripes, that annoys me.

Actually the light on an ambulance/police car/fire truck that causes the sensor on a stop light to trip is a high frequency strobe light. If you ever see the front of one of the emergency vehicles you’ll see a single white strobe that is flasing at an amazing rate. This is the strobe that causes the light to trip

Its most likely just happenstance that you flashed your brights and the light changes, but I could understand how it would be possible for a sensor to be set for too low of a frequency

From kinoons:

That would be the 3M Opticom System. Nice toy, that is.


Ricepad may be referring to those tubes that stretch across the road. When you run over them, they send a jet of air pressure to a sensor. They are more often used as counters, but some cities used to use them to control traffic lights, expecially for temporary installations.

The priority control systems that I’m familiar with use an encoded infrared beam or radio control system. Some cities have a central traffic controller that is in radio contact with the emergency vehicles and controlling traffic signals manually. Flashing your lights will not effect any of these systems…

Inductive systems will actually detect any conductive material, not just iron. you can magnetically induce a current in a conductor, and that induced current will then produce its own magnetic field, which you can detect.

No, ricepad is not talking about those tubes that stretch across the road. ricepad is talking about metal plates (pairs, actually) that used to run across the lane. Each plate was about 5 feet long (total length of 10 feet), and about a foot across. When a car rolled over them, it triggered the signal to cycle.

ricepad knows about these things because ricepad’s father is a retired traffic engineer for a major state highway department, and one of his (ricepad’s father, not ricepad) principle responsibilities was to advise local agencies about how to get the most out of their existing equipment and facilities, including these sensors. Basically, he told them to dump the pressure plates and go with loop detectors.

Yep, its magnetic inductance. The steel and iron in the car body induces a magnetic field in the loop.
As an interesting data point, back in the early days when these sensors were first being used, I had a friend with a Cosworth Vega, which had an aluminum engine block. The car didn’t have enough metal to trigger the sensors, so he always got stuck at traffic lights.

Aluminum will show up more strongly on an inductance detector than iron or steel. See my above post. It’s possible that there wasn’t enough of any sort of metal to trigger it, but that would be due to size, not composition.