TheDarkCutlass said:
Some swear by them, some say its just another form of snake oil. Not sure if I read it here, or on another forum, but one person apperantly worked on these traffic devices and he said that a magnet would have no effect whatsoever on the sensor.
97.5% of the time I have no problem tripping the sensor with my burgman, I just aim for the when I pull up to the light as 95% of the time there are grooves where they were placed, and it always trips the light. The other 5% of the time I guess where the sensor is at, 80% of the time I guessed right, 20% I guessed wrong and try to reposition. If it goes through 2 or 3 changes, or about 5-7 minutes and it doesn't change (usually by then a car behind me pulls up and triggers it) I will catiously and carefully run the light, or proceed on and do a u-turn, just depending on the situation.
For the most part that's what I did/do when I encountered a recalcitrant traffic sensor. I will wait a full light change to see it it notices me. An observant cop won't ticket you because it's not your fault the sensor did not pick you up, and provided you execute the maneuver with no safety issues.
I'm sure you know this but for benefit of the new reader or inquisitive mind: The sensor is basically just a coil of wire, inserted into the pavement (they are not generally buried beneath it as that moves the sensor too far below the level of what it has to detect). It's basically the same thing as a run-of-the-mill metal detector, except that instead of being "above" what you are detecting, you are beneath it. It operates on the process of 'induction' - that is, the metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. As the direction of the current changes, the magnetic field is created around the coil and then it collapses as the current is reversed and the field gets 'rebuilt' in opposite polarity. If a piece of electrically conductive metal is close to the coil, eddy currents will be induced in the metal, and this produces an alternating magnetic field of its own. If another coil is used to measure the magnetic field (acting as a magnetometer), the change in the magnetic field due to the metallic object can be detected. The traffic sensor is just a 'reverse' of this process - instead of carrying the coil around and waving it or passing it over metal, the buried loop waits until a large chunk of metal (engine, body, other parts) passes over it. When the change is sensed, it trips the lights.
On our metal detector we have a knob for sensitivity, and it's adjusted for how deep I want to look and what I want to look for. The traffic sensor is adjusted the same way, though we are talking about HUGE chunks of metal here. So it only needs ONE wire, basically, or a series of them connected so it can detect how far back the traffic is backed up, etc. If it's not adjusted correctly then smaller chunks of metal can be missed (ie a motorcycle, ATV, etc). All you are looking for is a disturbance in the field. Now the person who said that magnets don't work is incorrect. Any magnet of sufficient strength will be quite capable of disturbing the electromagnetic field around the coil. The key is that the metal or magnet must be MOVED sufficiently to induce eddy currents in the field. Essentially, the magnet 'simulates' a chunk of metal that has been moved into proximity to the sensor. So if if you have one turned off, and place a metal chunk near it and turn it on, and do not move it, it will not alert. Same principal - if the magnet is not of sufficient strength to induce a current or is not moved enough, the sensor will ignore it. That is why rare earth magnets are used - they create an very strong field for their size, and provided that the field or 'end' of the magnet is pointed in the right direction, it will disturb all but the numbest of sensors. The one thing that defeats these $20 devices is the distance above the pavement being too much, and the way the field is oriented off the magnets.
I discovered that one traffic sensor in my experiments that was and still is, 'numb' to everything but cars, trucks and my boot-magnet.
The loop detectors, are starting to disappear as cities move to computerized camera controls (much more accurate system). Out in rural areas, though, the loop detectors will still probably be used, and if it does not detect you, it's no real issue to 'run the light'. But as for snake oil, yes and NO. Yes in that without careful consideration of the distance, strength and mounting position of the magnets it's snake oil. No in that they CAN work if properly used.
If you want to get down and dirty, you could calculate it, depending on the size and shape of your magnet. However, in the interest of not having to place a lot of algebraic mumbo-jumbo here, basically as you move away from the magnet, the 'strength' of that field is inversely proportional to the distance. But actually the magnetic field drops off much faster as you move away from the source. And this all depends on structure of the magnet itself. Placing magnets under the motorcycle will distort the magnet's field as well, and that distortion could, in effect, render the magnet useless as a traffic sensor tripper.
Wikipedia has a good discussion of it:
http://en.wikipedia.org/wiki/Magnetic_field
Bottom line, though: For $20, it's worth a try and if it works, it works. If not, get your money back. There are a lot of things to have controversy over, but magnets ain't one of 'em!
:cheers: