Speaker 1: Welcome to brain Stuff from how stuff works dot com
Speaker 1: where smart Happens. Hi, I'm Marshall Brain with today's question,
Speaker 1: how does a traffic light detect that a car is
Speaker 1: pulled up and is waiting for the light to change?
Speaker 1: There's something exotic about the traffic lights that know you're
Speaker 1: there the instant you pull up they change. How do
Speaker 1: they detect your presence? Some lights don't have any sort
Speaker 1: of detectors. For example, in a large city, the traffic
Speaker 1: lights may simply operate on timers. No matter what time
Speaker 1: of day it is, there's going to be traffic, so
Speaker 1: the traffic lights just use their timers. In the suburbs
Speaker 1: and out on country roads, however, detectors are common. They
Speaker 1: may detect when a car arrives in an intersection, when
Speaker 1: too many cars are stacked up at an intersection to
Speaker 1: control the length of the light, or when cars have
Speaker 1: entered to turn lane in order to activate that arrow light.
Speaker 1: There are all sorts of technologies for detecting cars, everything
Speaker 1: from lasers to rubber hoses filled with air, but by
Speaker 1: far the most common technique is the inductive loop. An
Speaker 1: inductive loop is simply a coil of wire embedded in
Speaker 1: the road's surface. To install the loop, they lay the asphalt,
Speaker 1: and then they come back and cut a groove in
Speaker 1: the asphalt with a big circular saw. The wire is
Speaker 1: placed in the groove and sealed with a rubbery compound.
Speaker 1: You can often see these big rectangular loops cut in
Speaker 1: the pavement because the compound is obvious. Inductive loops work
Speaker 1: by detecting a change in inductance. Any coil of wire
Speaker 1: creates an inductor, so the coil of wire in the
Speaker 1: road is a giant inductor. The capacity of an inductor
Speaker 1: is controlled by two things, the number of coils of
Speaker 1: wire and the material that the coils are wrapped around,
Speaker 1: also known as the core. Putting iron in the core
Speaker 1: of an inductor gives it much more inductance than air would.
Speaker 1: There are devices that can measure the inductance of a coil,
Speaker 1: and the standard unit of inductance is the Henry. So
Speaker 1: let's say that you take a coil of wire, perhaps
Speaker 1: five feet in diameter, containing five or six loops of wire.
Speaker 1: You cut some grooves in the road, and you place
Speaker 1: the coil in the grooves. You attach an inductance meter
Speaker 1: to the coil and see what the inductance of the
Speaker 1: coil is. Now, you park a car over that same
Speaker 1: coil and check the inductance again. The inductance will be
Speaker 1: much larger because of the large steel object positioned in
Speaker 1: the loops magnetic field. The car parked over the coil
Speaker 1: is acting like the core of the inductor, and its
Speaker 1: presence changes the inductance of the coil. A traffic light
Speaker 1: sensor uses the loop in that same way. It constantly
Speaker 1: tests the inductance of the loop in the road, and
Speaker 1: when the inductance rises, it no is that there's a
Speaker 1: car waiting. Do you have any ideas or suggestions for
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