Loop Detectors

Traffic signals are a fun topic. Everyone encounters them, and most people have their idea of how they should work (basically: always green on my route). Unfortunately, making it green for all directions all the time would cause some small safety concerns, so we have to deal with how to allot the green time between all the different traffic movements.

A simple way is to set it up so that each street gets the same green time every time. So, the north-south cars get 30 seconds of green, then the east-west cars get 30 seconds of green, then the north-south gets 30 seconds of green, and so on. Pretty easy to set up, but it's rarely the most efficient option. What if sometimes the north-south street has a lot more cars but at other times the east-west street has more? Then you've got people just sitting there while the green time is displayed for the other street - and there's no cars coming on the other street. Cue the angry driver music.

The other way we could do it is to make the signal "smart" - that is, give it the ability to tell when there is more traffic on one street versus the other. Traffic engineers do this by using vehicle detectors. The most common form of this is the loop detector.

Here's the basic principle. When you take a coil of wire (e.g., the loop detector) and run electricity through it, it creates a magnetic field. When you pass a metallic object (e.g., a car) over the coil, it causes a disturbance in the field. By placing these loop detectors in the pavement at a traffic signal and tying them all in to the central control box, the signal can know when there are cars on the various approaches and adjust the green time accordingly.



So, now let's go the mailbox. Let's see, Suzy in Charlotte says, "I want to know how many times I have to go back and forth to trip it." Well, Suzy, technically the signal shouldn't care how many times you trip it. Once it knows you're there (traffic term: registers the call), a well-functioning signal will look for the next possible time to give you the green. I think you're probably referring to the signal at Parks Farm and Rea. My guess it that the loop detector there has some "dead areas" in its range. Maybe they buried it too deep in some places. When you go back and forth over it, you're probably just having to move around until you get to an area where the loop will detect you.

JimBob in Raleigh asks, "Anyway to get a traffic light to pickup a lightweight motorcycle? Especially ones made of high counts of non-magnetic materials i.e. aluminum, plastic, and fiberglass?" Well, JimBob, that one will vary by signal. Some loop detectors are set up to be more sensitive than others. The more sensitive ones have a better chance of picking up what metal is in your bike. The less sensitive ones..well, make sure you check all four directions for cops. I will say that you have a better chance of being detected if you position yourself directly over the cuts. From what I've heard, the loop is more sensitive over the cut than in the middle. You could also carry a big chunk of iron with you.
The good news for you is that traffic engineers are starting to use video cameras to detect when someone is waiting at the signal. They're mostly at the larger intersections for now, but the camera should be able to pick up a motorcycle without a problem.

Ok folks, that's all for the Traffic Hour today.