How to trigger Vehicle Loop Detectors
After we published our original article on this subject, we received the following from an engineer who seems to know what he’s talking about:
“I read your article on triggering traffic signals with a bicycle and would like to tell you a few facts about vehicle detectors as I am a designer of these units. Firstly, magnets have no effect at all on loop detectors (unless of course the magnet is so big that it acts more like a big mass of metal.) If you want to trigger the traffic lights with 100% certainty, simply get off the bike and lay the front wheel of the cycle flat over the loop towards one corner for a second or two. The rim of a cycle wheel acts like a big short circuited turn of wire and used in this way will produce a bigger signal than most automobiles do. The reason for this is that you can place your wheel flat on the ground so that it is about 10 x closer to the loop than most automobiles can reach.” – Graham Lill, Dept. of Infrastructure, Energy, & Resources, Tasmania
That pretty much makes Barry Stephens’ article below obsolete, but we’ll keep it here anyway for historical purposes. By the way, Graham wrote back in May 2004 to add the following:
Further to my earlier note, I have been alerted to the fact that bicycles are now available that use plastics and carbon fibre etc instead of metal. If anyone has such a bicycle then it would be a simple matter to install a single turn of copper wire around the rim and solder the ends together in order to make the wheel detectable when laid over a loop. If you put this under the inner tube, take care to remove any sharp ends and also cover the join with tape or heat shrink sleaving. The size of wire is not critical but don’t use anything as thin as bell wire, something more like one of the conductors from an appliance cord or even automotive wire would be fine. There is nothing magical about putting the wire loop in the wheel, it could also be attached to the triangular frame of a plastic bike and be quite effective so long as that part of the bike is placed flat over the loop. I don’t know what American practice is regarding loop detectors, passage detectors only require a momentary actuation, but presence detectors may require a longer period of actuation otherwise the signals controller may conclude that the “vehicle” has left the loop and no longer requires a green light. You could best determine this yourself for the local conditions because it is likely that different administrations set the equipment up differently.
Thanks again to Graham for the useful info. We’ve heard of plastic wheels but not plastic frames. Maybe we’re just behind the times…
Colly Kreidler of the City’s Bicycle Program offered this in may 2004:
In conversations with the traffic engineer responsible for loop detectors, it appears to me that department stays pretty much on top of the signal loops and motion detectors. They have experience with a range of loop sensor circuit board manufacturers and stay with the product that offers the greatest sensitivity, longest life, and dependable operation. There is an engineering technician that is very experienced with sensor loops and he personally responds to requests to test or adjust them. Through a donation from the Yellow Bike folks, I provided a small bicycle with chain, crank, and pedals removed. This makes it easier to manuver in and out of the trucks, plus there is less metal to detect when it is placed on a loop detector. When it senses that bike it’ll pick up most any bike. There are lots of them throughout the City, and odds are there will be problems with them on any given day. I trust their efforts at responding appropriately and in a timely manner.
I’ve read reviews of electro-magnet products for motorcycles fastened under the frame which were found to be essentially ineffective. Ultimately, it would seem that proper placement on top of the wire, not inside the loop, is the best bet. If it’s operating correctly it’ll detect the metal. If it isn’t, give a call, I always pass it on.
I’ve seen a wood frame bike parked in front of a store on the west curb of Lamar a half block north of 5th St., and I have found websites of bike builders specializing in bamboo frames. Metal lugs and hollow bamboo frame tubes with mountain bike type bars and drive train. Gotta keep ’em varnished I suppose.
Experimenting with ways to trigger traffic signal detectors
by Barry Stephens, Dec. 2002
Last month I began riding a bicycle regularly for exercise. I ride mainly at night and in the early morning when there are fewer cars on the streets. I was getting frustrated with traffic signals not changing for my bicycle so I made some attempts to solve this problem.
First I found out everything I could about traffic sensors. I found that many of the traffic signals in Austin have sensor loops placed in the pavement. Most, but not all are visible on the surface of the pavement as saw cuts, where a buzz saw was used to cut a groove in the pavement. Later a wire is put into the groove in the shape of a square or rectangular loop that is used as part of a big metal detector. The metal detector/sensor loop is supposed to “tell” the traffic signal when a vehicle is in the lane/over the loop and change the traffic signal to green. The loop cuts are sometimes covered over by later pavement and cannot be seen, but most of the ones I saw in Austin were not covered and were easily visible (see picture). Information obtained from the City of Austin indicated that the best way for a bicycle to be detected by a traffic signal sensor loop is to place both of the metal wheels directly on top of one of the sensor loop cuts. I tried this and it is true. Remember though that if the rider is busy looking for the loop cuts and attempting to place the bicycle wheels on one, (s)he’s not watching traffic, either. Also, loop cuts covered by tar or paint will become very slick when wet.
I also attempted to trigger the signals with magnets. My first trial was with Radio Shack magnets on the bottom bracket of the frame, which did not work. While browsing the internet I came across the Green Light Trigger (GLT) which is a magnet made for motorcycles to make them more “visible” to traffic signals. I purchased a GLT at a local custom motorcycle shop for $10.80. I placed it on the bottom bracket of the frame and thought that it might be working. Later I moved the GLT to the bottom of the left pedal (see photos) where it seemed to work better.
After riding around for a few days it seemed like the magnet was working but I was not sure how well. So I did a test at 43 Austin traffic signal sensor loops. My results indicated that placing the wheels of the bicycle on a loop cut works just as well as a GLT. The magnet sometimes makes the traffic signal trigger a few seconds sooner but that was the only benefit noted. The bicycle used for the test was typical, Steel frame road bike with alloy wheels and plastic/resin ATB pedals. During the test I saw many other people out riding bicycles at night. Half of them had no lights and I did not see any that were thinking about triggering traffic signals.
During the testing I tried very hard to get the traffic signals here in Austin to trigger, but found that they would only change for a bicycle about two thirds of the time. This means that if you are riding a bicycle in Austin, one out of every three traffic signal sensors will not change the light green for you.
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