Traffic Lights: Timers or Sensors?

July 8, 2013 11:47 AM

Is there an easy way to tell if a traffic light is on a timer or has sensors?

My boyfriend is a creeper. As in, he will be at a red light and will creep forward with the car so that "the sensor will sense the car" and trigger the light to turn. This creeping forward drives me up the wall and makes me mildly carsick (all of the starting and stopping). Plus, sometimes he winds up with his car halfway into the intersection from all his creeping forward.

I know that there are traffic lights that have sensors. I know that there are traffic lights that have timers, and no amount of creeping will help. Is there a way to tell which kind of light it is?

I would like to be able to ascertain whether or not a traffic light is a sensor or timer light on the spot, while in the car sitting at the red light. So although I think that the science behind traffic patterns is really cool, it's not what I'm looking for here. I'm looking for a "See this thing? It means that" sort of solution.

Thanks!

posted by Elly Vortex to Travel & Transportation (19 answers total)

Sometimes if there's a sensor, you see square patterns in the concrete where the sensor pad itself is. However, more recently I've encountered sensor lights where that wasn't the case. If you DO see such a square, though, his staying on top of it should help you.
posted by ldthomps at 11:49 AM on July 8, 2013

It's not a pad, it's and inductive coil.

Newer system use cameras mounted on the light poles. They're pretty easy to spot.
posted by Confess, Fletch at 11:55 AM on July 8, 2013 [1 favorite]

Best answer: The most obvious visible sign of a loop-detector is the square pattern in the pavement (the seal over the loop). If an intersection has that, then its timing is most likely taking the presence of vehicles into account. There are smaller detectors on the market that aren't the big traditional loops, though they aren't that common yet.

The other thing to look for is pedestrian actuated signals, which will also affect the timing (when pressed). It really depends on the intersection design.

This might be a bit technical, but here's FHWA's Traffic Detector Handbook for the how's and why's of loop-detectors in signalized intersections.
posted by kendrak at 11:55 AM on July 8, 2013 [5 favorites]

Even on timed lights there may be that rectangular loop in the asphalt indicating the sensor for a red-light camera.
posted by payoto at 11:56 AM on July 8, 2013

I'm also not sure why he thinks the "creeping" will trigger the sensor. The presence of the car (either there or not) is what triggers the sensor - not moving.

The wikipedia page has an explanation of how it works and a picture of the loop.

Just also keep in mind that if the road is freshly paved, the loop will probably have been placed before the paving, so you won't see the cuts in the pavement.
posted by dforemsky at 11:58 AM on July 8, 2013 [2 favorites]

Sometimes the patterns are hexagons or circles, but yeah, usually they look like someone made straight cuts into the pavement with a saw (which is actually what happens), with one cut going back to the corner.

Obviously, there's no reason for them to install sensors that are actually in the intersection past the limit line (unless they're sensors for red light cameras), so going past that point is pretty dumb.

The sensors don't detect motion - they're inductive and detect the car itself.
posted by LionIndex at 11:59 AM on July 8, 2013

Best answer: The sensors (which are inductive coils) that are buried in the asphalt are behind the white lines, as that's where cars are supposed to stop and wait for the light, and so that's where they put the sensor that detects cars. By moving forward into the street, he's moving his car *away* from the sensor, and making it less, rather than more, likely to trip.

Anyway, the sensors detect the presence of cars, not the movement of cars, so his logic is faulty regardless.
posted by tylerkaraszewski at 11:59 AM on July 8, 2013 [7 favorites]

Newer system use cameras mounted on the light poles. They're pretty easy to spot.

The only systems like this that I've ever seen are automatic red-light cameras, which detect if you've entered the intersection too late.
posted by LionIndex at 12:00 PM on July 8, 2013

A lot of the lights in my area are sometimes on a sensor and sometimes a timer, depending on the time of day.
posted by something something at 12:12 PM on July 8, 2013

Newer system use cameras mounted on the light poles. They're pretty easy to spot.

The only systems like this that I've ever seen are automatic red-light cameras, which detect if you've entered the intersection too late

There are camera systems in use for vehicle detection at signals all over the place now - they are cheaper than the in-pavement sensors. This MoDOT page has a pretty good overview of how they work. And they are not based on movement.

I’m not aware of any sensors used at traffic lights that ever have used motion, it really doesn’t make sense to use motion to trigger something at a location where the vehicle is supposed to be stopped.
posted by Sabby at 12:16 PM on July 8, 2013

My town has several lights that use camera based sensors that aren't red light sensors. They're pretty prevalent nowadays, and cheaper and easier to use than loops.

However... the other reason to not creep up is that often there are several loop sensor, or in the case of the camera, sensor regions, and they try to count the length of the cars queued at the light. From experience, many of them are set up to assume that if they can't detect a car at the front but detect a car further back they'll assume the line is longer and give you a faster light turn.

So if your boyfriend wants the light to turn quicker he should be trying to position the engine over the furthest back sensor he can see. In no case does this ever put the front bumper over the white line, often it puts the car two lengths behind it.
posted by straw at 12:21 PM on July 8, 2013

Response by poster: My theory about his creeping is that he thinks the sensor is either in the traffic light above (looking straight down at the road) or somehow connecting to the two lights on the sidewalks on either side of the road, like a winner's ribbon at the end of a race.

We have cameras at some intersections in MN, but not all of them. We also have tiny white lights mounted on over-the-road lights that will flash when there is an emergency vehicle approaching. I sometimes look at the over-the-road lights to try and find the "sensor" but I never knew what I was looking for. I had no idea they were on the pavement. Huzzah!!

I will have to look for the road sensors the next time we're out. I will definitely point out that he needs to stop on the sensor...all his creeping forward probably moves him right off it. Ha!
posted by Elly Vortex at 12:46 PM on July 8, 2013

Creeping is useless, unless he's stopping, like, 25 feet behind the actual stop line (people sometimes do this behind me when I'm on a bike, and it drives me nuts). The sensor, as others have mentioned, can usually be seen as grooves in the pavement, and generally stretches from about 2 feet in from of the stop line back by at least 20 feet. If they're tuned even remotely well, the presence of a car over even half of them should set them off (hell, if they're actually tuned right, a bicycle can set them off).

Inductive loop sensors of this sort are the most common I know of. Camera-based systems are rather rare — and if there are camera-based systems, I can promise they're trained on a point behind the stop line, seeing as how that's where it needs to know if there are cars or not.
posted by jackbishop at 12:58 PM on July 8, 2013 [1 favorite]

Not only does creeping forward not help the sensor register him, it also really annoys the person trying to make a right turn on red because he keeps moving up and blocking the view.
posted by ckape at 2:33 PM on July 8, 2013 [3 favorites]

Another problem with the creeping is that he's risking a ticket from a redlight camera.

You're legally supposed to stop completely behind the fat white line --- if there's a crosswalk at the intersection, you'll come up to that stop line BEFORE entering the crosswalk, so you're not stopped inside the crosswalk and making pedestrians walk around you into the actual intersection. If your BF is pulling up ahead of that stop line, he's risking one of the cameras registering him as inside the intersection, and therefore 'running the redlight'.

So: sensor pads read stopped cars as opposed to 'creeping' cars; BF is probably creeping himself right PAST the sensor pads anyway; and he's risking both ticked-off pedestrians and ticket-producing photos.
posted by easily confused at 4:32 PM on July 8, 2013

the sensors under the pavement are there to detect the engine block, the big dense piece of metal in the front of the car, not the body of the car. if he creeps forward he could over-shoot the sensor and wait longer.
posted by cupcake1337 at 5:47 PM on July 8, 2013

the sensors under the pavement are there to detect the engine block, the big dense piece of metal in the front of the car, not the body of the car.

Some inductive loop sensors are sensitive enough to detect a bicycle frame. Most aren't. I'm quite sure that at least some of the reputation cyclists have among motorists as scofflaw red light runners is because of this.
posted by flabdablet at 1:33 AM on July 9, 2013 [1 favorite]

In some places you will see a small stencil of a bicyclist in the forward right corner of the inductive loop. This is to indicate to a cyclist the most sensitive position for detecting a bicycle. Ideally you want to position your rims parallel to and exactly on top of the line on one side of the loop.
posted by JackFlash at 8:03 AM on July 9, 2013

Looks like Seattle paints little "T" markings on the pavement at the sensor locations.
posted by straw at 9:58 AM on July 9, 2013