Have you ever noticed black circles in the pavement near signalized intersections? They are often circular, sometimes octagonal, and approximately six feet in diameter. Well, while you’re looking and wondering about those black circles, I’m wondering how long before you notice the light turned green…
Those black circles are actually a black sealant protecting several wires buried just below the surface of the pavement. And as you know, when a metal object (such as a car for instance) passes over circles of wire, inductance happens.
Unfortunately, no one quite understands inductance — it is a mystery. And the mystery is further complicated by the fact it’s measured in something called: MicroHenrys (I am not making this up).
Some believe inductance has to do with magnetic fields — others believe it has something to do with what they had for lunch. In any case, the traffic signal controller understands inductance and knows exactly what to do. When inductance is detected, a series of events begin that are simply magical.
At the risk of losing my train of thought, let me mention the two types of detection circles/loops: 1) presence loops are near the traffic signal, and 2) advance loops are several hundred feet in advance of the signal. And these two types of detection loops serve two very different purposes…
The presence loop indicates to the traffic signal controller that a car is waiting for a green light. This could be in the through lane but more often in the turn lane (to trigger a green arrow for instance). Whereas advance loops extend the green time a few seconds. This allows vehicles to safely proceed through the intersection. Otherwise, approaching vehicles may get a yellow light a few hundred feet in advance of a signal and must make an instant decision — do I slow down and stop or do I keep going? Thus, the primary purpose of advance detection is to extend the green time to reduce the chances of getting caught in the dreaded ‘do-I-stop or do-I-go’ dilemma zone.
Green times cannot extend indefinitely — after a predetermined maximum, the green light ends and cross-street traffic gets their turn. If it seems you are not getting your extended green time, either the maximum green time was reached or the signal is not yet equipped with this feature (or, although unlikely, you are not driving the posted speed limit).
Chet Skwarcan (traffic engineer, author, unique insights) with over 25 years of traffic engineering experience, solving (or preventing) traffic problems every chance he gets. He can be reached at Chet@TrafficEngineering.com