WEBVTT - How does a traffic light detect cars?

0:00:00.560 --> 0:00:03.600
<v Speaker 1>Welcome to brain Stuff from how stuff works dot com

0:00:03.600 --> 0:00:14.600
<v Speaker 1>where smart Happens. Hi, I'm Marshall Brain with today's question,

0:00:15.000 --> 0:00:17.840
<v Speaker 1>how does a traffic light detect that a car is

0:00:17.880 --> 0:00:20.439
<v Speaker 1>pulled up and is waiting for the light to change?

0:00:21.079 --> 0:00:24.439
<v Speaker 1>There's something exotic about the traffic lights that know you're

0:00:24.520 --> 0:00:27.920
<v Speaker 1>there the instant you pull up they change. How do

0:00:28.000 --> 0:00:32.080
<v Speaker 1>they detect your presence? Some lights don't have any sort

0:00:32.120 --> 0:00:35.080
<v Speaker 1>of detectors. For example, in a large city, the traffic

0:00:35.159 --> 0:00:38.520
<v Speaker 1>lights may simply operate on timers. No matter what time

0:00:38.520 --> 0:00:41.239
<v Speaker 1>of day it is, there's going to be traffic, so

0:00:41.280 --> 0:00:44.640
<v Speaker 1>the traffic lights just use their timers. In the suburbs

0:00:44.640 --> 0:00:48.639
<v Speaker 1>and out on country roads, however, detectors are common. They

0:00:48.680 --> 0:00:51.920
<v Speaker 1>may detect when a car arrives in an intersection, when

0:00:52.000 --> 0:00:54.800
<v Speaker 1>too many cars are stacked up at an intersection to

0:00:54.840 --> 0:00:57.480
<v Speaker 1>control the length of the light, or when cars have

0:00:57.560 --> 0:01:00.560
<v Speaker 1>entered to turn lane in order to activate that arrow light.

0:01:01.240 --> 0:01:05.040
<v Speaker 1>There are all sorts of technologies for detecting cars, everything

0:01:05.120 --> 0:01:08.880
<v Speaker 1>from lasers to rubber hoses filled with air, but by

0:01:08.959 --> 0:01:13.000
<v Speaker 1>far the most common technique is the inductive loop. An

0:01:13.000 --> 0:01:16.440
<v Speaker 1>inductive loop is simply a coil of wire embedded in

0:01:16.480 --> 0:01:20.520
<v Speaker 1>the road's surface. To install the loop, they lay the asphalt,

0:01:20.560 --> 0:01:22.640
<v Speaker 1>and then they come back and cut a groove in

0:01:22.680 --> 0:01:26.120
<v Speaker 1>the asphalt with a big circular saw. The wire is

0:01:26.160 --> 0:01:29.280
<v Speaker 1>placed in the groove and sealed with a rubbery compound.

0:01:29.680 --> 0:01:32.920
<v Speaker 1>You can often see these big rectangular loops cut in

0:01:32.959 --> 0:01:38.039
<v Speaker 1>the pavement because the compound is obvious. Inductive loops work

0:01:38.200 --> 0:01:42.360
<v Speaker 1>by detecting a change in inductance. Any coil of wire

0:01:42.440 --> 0:01:45.320
<v Speaker 1>creates an inductor, so the coil of wire in the

0:01:45.440 --> 0:01:49.520
<v Speaker 1>road is a giant inductor. The capacity of an inductor

0:01:49.600 --> 0:01:52.640
<v Speaker 1>is controlled by two things, the number of coils of

0:01:52.680 --> 0:01:56.520
<v Speaker 1>wire and the material that the coils are wrapped around,

0:01:56.680 --> 0:02:00.440
<v Speaker 1>also known as the core. Putting iron in the core

0:02:00.480 --> 0:02:03.840
<v Speaker 1>of an inductor gives it much more inductance than air would.

0:02:04.400 --> 0:02:07.640
<v Speaker 1>There are devices that can measure the inductance of a coil,

0:02:07.960 --> 0:02:12.040
<v Speaker 1>and the standard unit of inductance is the Henry. So

0:02:12.480 --> 0:02:14.919
<v Speaker 1>let's say that you take a coil of wire, perhaps

0:02:15.000 --> 0:02:18.720
<v Speaker 1>five feet in diameter, containing five or six loops of wire.

0:02:19.200 --> 0:02:21.440
<v Speaker 1>You cut some grooves in the road, and you place

0:02:21.520 --> 0:02:25.160
<v Speaker 1>the coil in the grooves. You attach an inductance meter

0:02:25.320 --> 0:02:28.040
<v Speaker 1>to the coil and see what the inductance of the

0:02:28.080 --> 0:02:31.280
<v Speaker 1>coil is. Now, you park a car over that same

0:02:31.320 --> 0:02:34.960
<v Speaker 1>coil and check the inductance again. The inductance will be

0:02:35.120 --> 0:02:39.080
<v Speaker 1>much larger because of the large steel object positioned in

0:02:39.160 --> 0:02:42.840
<v Speaker 1>the loops magnetic field. The car parked over the coil

0:02:43.280 --> 0:02:46.079
<v Speaker 1>is acting like the core of the inductor, and its

0:02:46.160 --> 0:02:50.239
<v Speaker 1>presence changes the inductance of the coil. A traffic light

0:02:50.320 --> 0:02:54.559
<v Speaker 1>sensor uses the loop in that same way. It constantly

0:02:54.720 --> 0:02:57.600
<v Speaker 1>tests the inductance of the loop in the road, and

0:02:57.639 --> 0:03:00.760
<v Speaker 1>when the inductance rises, it no is that there's a

0:03:00.840 --> 0:03:04.760
<v Speaker 1>car waiting. Do you have any ideas or suggestions for

0:03:04.800 --> 0:03:07.800
<v Speaker 1>this podcast? If so, please send me an email at

0:03:07.880 --> 0:03:11.000
<v Speaker 1>podcast at how stuff works dot com. For more on

0:03:11.040 --> 0:03:13.400
<v Speaker 1>this and thousands of other topics, go to how stuff

0:03:13.400 --> 0:03:16.160
<v Speaker 1>works dot com and be sure to check out the

0:03:16.160 --> 0:03:18.639
<v Speaker 1>brain stuff blog on the how stuff works dot com

0:03:18.680 --> 0:03:19.239
<v Speaker 1>home page.