Welcome to brain Stuff from how stuff works dot com where smart Happens. Hi, I'm Marshall Brain with today's question, how does a traffic light detect that a car is pulled up and is waiting for the light to change? There's something exotic about the traffic lights that know you're there the instant you pull up they change. How do they detect your presence? Some lights don't have any sort of detectors. For example, in a large city, the traffic
lights may simply operate on timers. No matter what time of day it is, there's going to be traffic, so the traffic lights just use their timers. In the suburbs and out on country roads, however, detectors are common. They may detect when a car arrives in an intersection, when too many cars are stacked up at an intersection to control the length of the light, or when cars have entered to turn lane in order to activate that arrow light.
There are all sorts of technologies for detecting cars, everything from lasers to rubber hoses filled with air, but by far the most common technique is the inductive loop. An inductive loop is simply a coil of wire embedded in the road's surface. To install the loop, they lay the asphalt, and then they come back and cut a groove in the asphalt with a big circular saw. The wire is placed in the groove and sealed with a rubbery compound. You can often see these big rectangular loops cut in
the pavement because the compound is obvious. Inductive loops work by detecting a change in inductance. Any coil of wire creates an inductor, so the coil of wire in the road is a giant inductor. The capacity of an inductor is controlled by two things, the number of coils of wire and the material that the coils are wrapped around, also known as the core. Putting iron in the core of an inductor gives it much more inductance than air would.
There are devices that can measure the inductance of a coil, and the standard unit of inductance is the Henry. So let's say that you take a coil of wire, perhaps five feet in diameter, containing five or six loops of wire. You cut some grooves in the road, and you place the coil in the grooves. You attach an inductance meter to the coil and see what the inductance of the coil is. Now, you park a car over that same
coil and check the inductance again. The inductance will be much larger because of the large steel object positioned in the loops magnetic field. The car parked over the coil is acting like the core of the inductor, and its presence changes the inductance of the coil. A traffic light sensor uses the loop in that same way. It constantly tests the inductance of the loop in the road, and when the inductance rises, it no is that there's a car waiting. Do you have any ideas or suggestions for
this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com and be sure to check out the brain stuff blog on the how stuff works dot com home page.