Traffic Loop Sensor Activators for Bicyclists

The Loop Sensor Activator is a device for bicyclists that allows a low-weight velocipede to trigger traffic loop sensors. I had no idea there was a need for such a thing, but apparently, the problem's bad enough that a bicyclist can find themselves stranded at an intersection at a perpetual red short of flinging their bike violently to the ground:
A common problem for bicyclists, and in particular for bicycle commuters, is the traffic loop sensor. These are the devices which detect the presence of vehicles to control the sequencing of traffic lights at many intersections. Often, they are installed in figure-eight slits cut into the pavement near a traffic light. The way they work is to detect the slight change in inductance of the loop of wire buried in the slit caused by a large metallic object (such as a motor vehicle) above the loop. The problem for bicyclists is that traffic departments often feel compelled to set the sensitivity of the electronics behind the loop rather low, so that false detections are avoided. This can make it difficult for a bicycle (a relatively small metal object) to make its presence known to the sequencing circuit. The result is a lot of frustration for a bicyclist trying to get a green light or left-turn arrow. This can be especially frustrating at streets with low traffic (often where these sensors are used in the first place), as one waits for a motor vehicle to come by and trip the sensor. Sometimes one must resort to laying the bicycle down on the road surface above the loop with the hope that the extra metal presented to the loop will cause detection. Sometimes it doesn't work, and sometimes frames aren't made of metal. Various devices have been patented in order to address this problem. Some are a little impractical, and some don't work with modern loop sensor circuitry. One patented solution (see USP 5,652,577) has the bicyclist laying down a big piece of metallic foil or screen right on the road surface!.
Christ, what you American bicyclists have to put up with. Here in Zembla, every street has bike paths, bicyclists are respected and feared in equal measure and there are even traffic lights for bikes only. Do I amaze you? Loop Sensor Activator [Plan Bravo via Core77]
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32 Responses to Traffic Loop Sensor Activators for Bicyclists

  1. frijole says:

    All you folks and your magnets are “doin’ it wrong” as we say around here.

    If you pull up and the light doesn’t look like its going to change, just hop off and tilt the bike towards the side.

    The wheel rims act like giant closed-loop coils and distort the field more than a Mack Truck.

  2. Darren Garrison says:

    Random trivia of the day: Microsoft was birthed by traffic sensors:

  3. Anonymous says:

    Aluminum rims are actually better than steel. Doesn’t mention the rims as having the effect of a Mack truck, though. Laying your bike down seems to be the most effective if the detector isn’t sensitive enough to detect it standing.

    Does anyone know if a rare earth magnet would make a measurable difference in inductance? A bicycle with aluminum rims will cause about 1/100 the change in inductance of a car, so even a strong small magnet probably won’t make much difference.

  4. Anonymous says:

    This isn’t a problem. Look both ways, go across the street, works when I’m on feet.

    Also, something I’ve learned as a bike courier is to pay more attention to the cars than the lights, because I have yet to hear of someone getting hit by a traffic light.

  5. SamSam says:

    @19: The vast majority of wheel rims these days are made out of aluminum alloy, which distort no magnetic field.

  6. dragonfrog says:

    Chris S @ 8

    Not a bad idea, except it makes an assumption that I find many drivers do too – that bicycles are incapable of turning left…

  7. Chris S says:

    dragonfrog @21:

    > Chris S @ 8
    > Not a bad idea, except it makes an assumption
    > that I find many drivers do too – that bicycles
    > are incapable of turning left…

    Of course, what wouldn’t be clear from my description is where on the roadway the dots are usually found — typically in line with the driver of a vehicle.

    If you’re going to trip the sensor because you’re on the dots, you will have to be solidly in the same lane as the traffic — which, if you’re turning left, is exactly where you should be.

  8. sawatzky says:

    If there’s no traffic around to trip the signals, there’s NO WAY I’m waiting around for a green light! I’ve never actually heard of bicycles being required to obey traffic signals. Most cyclists here spend half their time on the sidewalk anyways.

  9. Anonymous says:

    or just hit the pedestrian walk signal. that makes lights change pretty quickly.

  10. inserttitle says:

    Re Matt J, #9

    *Some* sensors are optical and work for cyclists (no they aren’t for detecting ambulances etc. Doesn’t seem to be done here, they just go through the red lights).

    A lot of junctions in the UK do use loops like this. I notice them particularly at night when they seem to change mode to use the sensor (or maybe there are just fewer cars).

    And as I do obey Red lights (a very sensitive subject in the uk for cyclists and motorists – try asking about it on uk.rec.cycling.) I sometimes get caught by one or two. I usually get off and walk. Maybe I’ll take a magent out and give it a try though….

  11. dculberson says:

    My understanding is that a large magnet (*not* “rare earth” but regular ..umm.. magnetium maximus) works well. If you have a cromoly frame, stick it (with some wire ties for backup) to the lowest point.

    I’ve seen it used successfully with motorcycles.

  12. wmbozarth says:

    I have this same problem with my scooter.

  13. Anonymous says:

    Hey Brownlee! Nice Nabokov reference. My bike is aluminum (or aluminium) and doesn’t work on sensors at all. I’ll have to try the magnet thing…

  14. zuzu says:

    Sucks to live in a state where bicycles are considered vehicles equal to cars and subject to the same traffic regulations.

  15. w000t says:

    “Christ, what you American bicyclists have to put up with. Here in Zembla, every street has bike paths, bicyclists are respected and feared in equal measure and there are even traffic lights for bikes only.”

    Here in America, all of these problems–plus the loop sensor–can be solved by cycling with a sawed-off shotgun.

  16. The Unusual Suspect says:

    John, as the article says, traffic loop sensors are not activated by weight, but rather by inductance.

    (Think of a metal detector.) The problem for bicycles is that they don’t have the requisite metal mass.

    Matt, are you sure those optical sensors are not there to allow emergency vehicles to commandeer right-of-way?

  17. Anonymous says:

    We have the same thing here in The Netherlands (loads of bikes) and they always work not matter how little metal the bike contains. I don’t know about 100% aluminium bike though…

  18. Will_Tingle says:

    Mmm, despite what #1 said, I’ve seen this work with Rare Earth Magnets – and they don’t have to be so large…

  19. toxonix says:

    Who actually obeys traffic signals? If there aint no cops around, traffic lights are more of a suggestion than a rule.

  20. Geonz says:

    Drift Marlo is prob’ly among those drivers who see what they expect to see…

    A good commuter bike has enough steel to trip the signals, in my experience. It really is a pretty major bunbite to be trying to get home from work and be stuck at a red light. (Going to work, there are cars.)

    Of course, if it’s one of the lights that get the green when emergency vehicles come, then all I have to do is get my blinkies out :)

    Dare you to try cycle-commuting.

  21. Alan says:

    Me too, WMBozarth. Once I was so frustrated at a light that I got off my scooter and went over and pushed the pedestrian walk button. Then a car pulled up behind me.

  22. SamSam says:

    Yes, just buy a $3 magnet. You can buy a pretty powerful magnet for $3, and it doesn’t require any circuitry, buttons, LEDS or extra wires snaking around your bike making it look ugly.

    All this is doing is activating a little coil electromagnet anyway, with some fancy syncing up with the frequencies.

    If they are able to show that this works on a higher percentage of traffic sensors than a magnet does, it will be more interesting, but I don’t see any citations of evidence for that.

    (PS Rare Earth magnets are perfect — they’re more powerful than ferrous magnets, and it’s not as if the magnetic field is any different. A field is a field is a field.)

  23. drift marlo says:

    Here in Milwaukee, I have never seen a bicyclist (myself included) waiting for a red light. Although scooter people would definitely find this useful. Tip #2, make a right turn.

  24. Chris S says:

    Bike Actuated Sensors…

    …recent article…

    The city paints the dots to mark the sweet spot in the detector. If you pull up on top of the dots, then your bike will trigger the light change.

    Of course, it doesn’t work when the ground is covered by ten centimetres of snow, but you can’t see the lines either at that point – and only the hardiest of cyclists is out in that weather anyway.

    The biggest downside is that the city has to do this – so I can see that some cyclists might be in places where they would benefit from having an activator on their bike.

  25. Milo Minderbender says:

    Three words: Hard Drive Magnet

    Works for motorcycles.

  26. dculberson says:

    I could well be wrong, but I thought a regular magnet’s field, if it’s the same strength as a physically smaller neodymium magnet, would be effective over a larger distance. I based that on the fact that the strength of a dipole field field falls off inversely with the cube of the distance from the magnet’s center. But I now realize that rule says nothing about the distance from the center of the magnet to the edge of the magnet – just the distance from the center to the point being measured. So perhaps I’m remembering physics class incorrectly, and it certainly wouldn’t be the first time.

    Regardless, by all means, use a rare earth magnet if it works. That would save weight. A dab of epoxy should keep it in place if you’ve got an aluminum frame.

  27. The Unusual Suspect says:

    Toxonix @#10: “Who actually obeys traffic signals? If there aint no cops around, traffic lights are more of a suggestion than a rule.”

    So Toxonix is driving me to the liquor store one day, when we come to a red light, and ol’ Tox, he sails right through it.

    “Tox!” I yell, “Why did you run that red light?” “Don’t worry.” he says. “My brother does it all the time.”

    We come up to the next light, which is also red, and sure enough Tox speeds through this one too.


    “I told you, my brother does this all the time!”

    Up we come to the next light, which is green – and Tox screeches to a halt.

    “Why did you stop for THAT light?” I shout, and Tox says, “Well, my brother might be coming the other way.”

  28. Anonymous says:

    just what I want, a 10 pound magnet on my expensive light weight bike. How bout fixing the auto-centric laws rather than struggling to adapt to them.

  29. Matt J says:

    In Britain, sensors to detect vehicles are optical. Lots of traffic lights have a device which looks like a camera on top of them.

  30. SamSam says:

    @16: Um, a rare-earth magnet strong enough to trigger the sensor is probably about a quarter of an ounce (looking at a pack of four online which is two ounces total).

    If your thighs aren’t up to lugging an extra quarter of an ounce, you may have more trouble that red lights.

    I’m all for changing auto-centric laws, of course, but I’m not going to wait at a red light from now until the changes happen, though.

  31. Cpt. Tim says:

    i waited like 8 minutes in the A.M. at a traffic light on my motorcylce. Just as a cop came into view. got pulled over. before i even gave him my stuff I explained about the sensors and I had no other choice than either run the light when the coast was clear, or sit there forever.

    he just let me go after that, didn’t even check my license.

  32. Anonymous says:

    City of Berkeley has sensors specifically for bikes at stoplights along designed Bicycle Boulevards (and maybe some other streets too). It is awesome – I have definitely been stuck waiting for a light at a busy intersection forever in places I have lived in the past (especially on a smaller street crossing a bigger street).

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