How does a traffic light detect cars?

Traffic lights detect vehicles using a variety of methods. Check out this podcast from HowStuffWorks.com to find out how timers, inductive loops and other detectors keep traffic running smoothly.

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Episode Transcript

Speaker 1 (00:00):
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?

(00:21):
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

(00:41):
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

(01:05):
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

(01:26):
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

(01:49):
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

(02:12):
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

(02:35):
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

(02:57):
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
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brain stuff blog on the how stuff works dot com

(03:18):
home page.