Had an interesting conversation with one of the transportation engineers in my office today regarding traffic lights and the systems used to trip them. Just thought I'd pass along what he told me since it affects most of us from time to time.
In general there are two types of signal triggers predominately in use. Those are the video type and the inductive coil type.
The video type is somewhat intelligent. It uses a camera and initially takes a photo of the empty intersection for comparison purposes. It then continually monitors the image for "Changes" as would be caused by a vehicle moving into the intersection. When it senses a change that it determines is a vehicle in a traffic lane it then initiates the proper signals based on the direction of travel.
As most of us know, the inductive coil type uses a coil of wire buried beneath the asphalt that senses a mass of metal moving over it. Unfortunately, most types of non-ferrous metal (i.e. aluminum, magnesium, and many alloys) are not detected well. Neither is plastic. Additionally, there are different coil sensitivities that can be used so that they can be laid in adjacent lanes with interfering with each other. Additionally, the coils degrade over time which just makes things worse. They are typically laid about 1½ to 2" deep in the asphalt, but when roads are repaired and additional layers of asphalt overlaid the sensitivity drops even further. Asphalt overlays are why you run across intersections that require a trip signal but you can't see where the coil was laid. There has been a layer of asphalt put over the existing surface and the coil is buried even deeper than it was. To compound that problem, in many cases the coils don't work or have even been scraped out during construction without anyone even knowing it happened. At one point it was estimated that only about 50% of the inductive coil signals here in Nashville even worked at all. I was told that in many cases the signal shop here is unaware of a problem with a coil until someone complains or they happen to drive over it themselves and notice it isn't working.
Most bikes our size just don't have enough mass of ferrous metal in them to be detected by the sensor. The Gold Wings and other bikes that size might, but we don't. Magnets on the bottom of the bike will help in some circumstances. Likewise getting a mass of metal closer to the ground (i.e. putting the kickstand down) can sometimes help. In the cases where the coil is buried deep, the asphalt has several overlays on it, or the coil is only marginally working the only thing that will help is a huge mass of metal which is something we don't have. Most places are going to the video signalization method, however it is approximately 5 to 10 times as expensive to do an intersection that way as opposed to the inductive coils. Over time most of the triggered intersections will be video controlled, but it will not happen in the near future.
Most signals have a pre-timed sequence that they will follow. The triggered signals are generally used to interrupt the pre-timed sequence. In other words, in the absence of a signal from one of the coils or cameras the lights will follow a standard timed sequence. If a vehicle pulls over one of the coils or enters the field of view of a camera the device interrupts the normal sequence, assuming that it won't conflict with the timing of other traffic, and switches the lights. In many cases this is used in left-turn lanes to decide whether or not to enable a left-turn-only sequence or the normal two-way sequence.
In 2003 Tennessee enacted a law that allows motorcycles to run a red light if they have given it a "Reasonable" amount of time to sequence. This was done because they at least realize that there is a problem associated with inductive coil systems being activated by motorcycles. The word "Reasonable" is ambiguous and has been abused by many motorcyclists. To them stopping (or slowing down!) just long enough to glance both ways is "Reasonable".
"Reasonable" varies from one intersection to the next though. I was told that some intersections can have up to eight sequences before everything has gone full-circle. Just because one intersection runs its entire sequence in 40 seconds doesn't mean that all of them will. It's difficult to find a phrase that will catch all circumstances, so they used the term "Reasonable" and left up to us to act like adults. Unfortunately they didn't realize that there are so many children around.
Because of people abusing this privilege there are some groups asking that the law be reworded and rewritten or repealed entirely. I don't think it will be repealed because Tennessee at least recognizes that there is a problem for motorcycles and took this step to alleviate the problem. If bikers here do keep abusing the privilege something will probably be done though, and we probably won't like what that something is.
But if you think we have it bad think about bicyclists. They are not allowed to ride on sidewalks, only on the streets. They stand absolutely zero chance of tripping an inductive coil signal yet they were not included in the law that allows motorcycles to go through a red light (at least that is my understanding). Legally I guess all they can do is to stop, pick up their bike, and pretend to be a pedestrian long enough to get across the intersection. I guess we don't have it quite so bad