I’ve heard it said that some traffic lights are set to trigger a green when a second vehicle trips the sensor.
I know guys (don’t know that I’ve ever seen a woman do it) who’ll pull up to a red light at an odd hour, back up behind the pressure plate, then pull up again, to make the light turn.
Seems to me, it’s just as likely that the time they kill while at this procedure is responsible for the change.
Anydoper out there design, build, or install these devices?
Nothing concrete to add on this except that I tried a similar thing once early in the morning when I pulled up on a minor road trying to get into a major Sydney arterial highway, and the lights were famously slow. There were two lanes, so with a reverse I took a shot at both, hoping the sensor would think both lanes were loaded. It didn’t seem to make any difference.
Flashing your brights doesn’t either, though Sydney taxi drivers insist on doing it.
Most people don’t consider how these things work. One night going home from work, I had to use a turning slip lane with an arrow on the lights. This lane normally had about five or six cars waiting in it, but on this particular night there were about thirty of them spilling back into the through lane. A woman’s car had broken down in the slip lane, and people were going around her. This was okay, but the arrow was NEVER turning green. It seemed I was the only person who worked out that the broken down car (two car lengths behind the line) was imediately behind the road sensor (one car length behind the line), and to drive around her you’d be coming in at a forty-five degree angle right at the line. So no car could ever cover the sensor. I went straight, and did a U-turn, but I’m sure the rest of 'em were there all night.
I’ve had a not-dissimilar experience, with a temporary set of lights set up around some roadworks on a minor-ish road. One end of the lights was at a junction with a very minor road, which had its own light. But they hadn’t bothered to put a sensor on this one, obviously thinking that there’s enough traffic on the major-ish road to provide the necessary triggers.
They didn’t consider that at the far end of the very-minor road is an 800-seat concert hall, which emptied at 10pm, and every single car was queued, with zero traffic on the major-ish road to trigger the lights. Eventually some guy at the back worked out what had happened, overtook the entire queue, and drove around to trigger it.
Another option would be to outfit your vehicle’s trunk with a 3,000 lb barbell, attached to an industrial boom and hoist assembly.
Google for “loading dock.” 
IANA traffic engineer but I think that most of the modern devices are designed to detect a stationary magnetic field generated by large metallic masses. What happens is that some people drive right over the sensor and come to a stop too far away to register the magnetic field. If they back up, they pass over the sensor at a slow enough speed to actuate the switch. They wouldn’t need to pull forward at this point. This is basically the traffic signal’s indirect way of telling you that you stopped too close to the intersection.
I find it hard to believe that they would design traffic lights that could conceivably keep a single car waiting for hours.
In our area (New Jersey), you can see exactly where the coils are located – the use a saw to cut four slots in a diamond-shaped pattern in the asphalt about four feet across. They then cut a straight line from the diamond to the street corner where the electronics are located. Presumably, they drop in a special sensor coil, stretched in the diamond shape, and run its cord down the slit cut to the street corner.
They simply fill in the slots with asphalt, so they are almost always obvious.
In major intersections, I can see seven or eight of these diamond patterns, with their connecting lines all converging on the street corner where there is a metal box.
I make sure my vehicle stops directly over one of these diamonds so that I don’t sit and wait forever for the stupid light.
They certainly design traffic lights that can keep a light motorcycle waiting. There simply isn’t enough metal in them to trigger the sensors (which are really just a type of simple buried metal detector - not pressure plates). Many is the scooter rider who has had to wait for a car to come along for the lights to change.
Not sure about the US ones, but here in Australia the sensor is usually located a car length back (ie. under the second car waiting). I don’t think it matters what speed you go over the thing as even if they can’t register a car at speed (which I think they can), at high speed the light would be green anyway, and a red light would mean you’d be going at a very low speed that close to the line. I think the sensor is under the second car, so it can register and remember the first (even after you’ve passed it, if you’re a lone vehicle, you’ll still get your green), and then register the second directly above it. This gives it a more precise prioritising control: “no car”, “one car”, “two or more cars” rather than just “no cars” and “cars”.
I know one of my cities more dull claims to fame is its traffic light technology. The NSW Roads and Traffic authority developed what it claims are the world’s best traffic lights about ten years ago, and has exported the tech to dozens of cities worldwide (so it says), so you may well have Sydney lights where you live. Of course, the busiest intersections here are still good old timed lights because there’s little point with sensors if all the roads are always full. It’s only ever noticeable driving at two o’clock in the morning - pull up at a little intersection and you’ll see the cross street lights turn yellow instantly, drive along a major highway though, and you might get a red for phantom cross-traffic.
I do this all the time. There are definitely situations where I have been sitting at a traffic light that will not change for a few minutes with no traffic from any direction, backed up, pulled up again, and watched the light immediately change. As somebody said, it only seems to happen when I’ve pulled too far into the intersection.
–p
Around here, there are often several (as many as 4 or 5) sensors in a single lane. One at the stop line, one half a car length back (some people don’t pull all the way forward in their lane), one about a car length behind that and possibly one or two more several car lengths further back. The actual number depends on the amount of traffic that lane gets. Backing up may trip some of these other sensors and fool the traffic light into thinking there are more cars waiting. Depending on how the lights have been programmed, that may or may not give you a green sooner than otherwise.
The sensors around here used to be the diamond minor7flat5 described above and there’s still a few like that in use. Most, though, are a 2 meter circle. But you can’t always see them. Sometimes they are put down before the street is paved, so there is no circle to indicate their location.
However, when you can see the circle, it’s possible to trip the sensor without having a ton of steel (i.e. a car) above it. A bicycle stopped above it will not work, but if you lay the bike on its side directly on top of the circle, it works, even if the bike frame is aluminum. I do this occasionally when there is no car behind me in a left turn lane.[sup]1[/sup] I’ve heard that dropping the kickstand on a motorcycle will also do the trick. So apparently it doesn’t take much iron to trip them if you can get it close enough to the sensor.
[sup]1[/sup] You have to closely watch the crossing traffic’s lights and pick your bike up when they get their yellow. Also you get funny looks from drivers, but that never bothered me.
True. It can also force a cyclist to become a pedestrian briefly while crossing the road.
I never had any problems with these and my scooter or my motorcycle, as long as I was careful to stop directly over the left edge of the coil.
You don’t see women doing it, cause we’re smarter.
It isn’t the weight of the vehicle, at least, not on the west coast that activates the light. Reving your engine, or turning on your A/C creates a magneticly active zone . It works for me every time.
By the way, police and fire have a thingy called an Opticom that changes the light for them. Before you, ask, they are illegal to have in a non-emergency vehicle.
In Toronto a few sesnsor-equiped intersections have three white dots over part of the sensor. Cyclists are supposed to halt their bikes over those dots and the sensor will detect the bile.
In Bellevue, WA, there are little white Xs to mark where cyclists are supposed to stop. I’ve also noticed evidence of sensors in the bike lanes too.
Is anyone else seeing these sensors replaced by cameras? (Info from MoDOT here.) I’ve found that right when the sun is setting, the cameras obviously can’t see some traffic because they’re blinded, and they never see me trying to turn left at rush hour into my development. Grr…
You can also buy a big magnetic thingum to put on the bottom of your motorcycle or scooter, to help you trip the sensors. infamousmom has one on the bottom of her Vespa, and is able to trip all but one or two of the sensors in the area.
Eee-yup. For such situations was the Green Light Trigger invented. http://www.greenlightstuff.com/
It doesn’t work on all lights (there’s one that I keep forgetting about when I go riding my got-a-magnet-on-the-bottom Vespa) but it certainly does work with most.
It’s not about the iron, it’s about the conductivity. Any conductor at all, regardless of whether it’s ferrous or even metallic, will be detected. What the devices do is set up a changing magnetic field, which will induce a current (and therefore a secondary magnetic field) in any conducter. The same coils then detect the secondary magnetic field. So aluminum (which is a better conducter than iron) will work just fine, and one of those fancy-schmancy superconducting ceramics would work better than either (if you don’t mind carrying a liquid nitrogen system on your bike).
Laying your bike down on the side will have two effects: It’ll bring the metal closer to the detector, and (more importantly), it’ll make a nice big horizontal circuit loop for the eddy currents to flow through. I don’t know why a kickstand on a motorcycle would help, aside from the fact that that would lean the bike over a bit, bringing it a little closer to a favorable aspect.
Incidentally, I have tried the laying the bike down trick once, without success, but that may just have been because I wasn’t sure where the sensor was, and missed it. There were no indications on the pavement, incidental or explicit, and I ended up just having to trundle over to the “Walk” button.