It's go time. How do traffic light sensors work?

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We face them almost everyday. Traffic lights. You hate them on the days you’re in a hurry because you’re running late and you only seem to get all reds. But you bless them on other days when they all seem to change to green as you pull up, as if the “Italian Job’s” Seth Green has hacked into the traffic light mainframe, making all the lights green just for you. (Thanks, Seth. Now go work on putting your career back in Go.)

What gives? How does a traffic light detect that you’ve pulled up to a red light, and know to change it in a few seconds so you’re not waiting there ALL DAY? Well it depends on a few factors such as where you live and how much traffic travels through that intersection. Every traffic light has some sort of timer or sensor in it to dictate the flow up traffic.

First, some lights don't even have any sort of detectors. They simply, and always, operate on timers. You’ll find these more often in large cities, where cars are traveling around the clock.

In the suburbs and on country roads, however, detectors are common. Because traffic flow is less consistent, timing is less important than just letting the fewer cars through in less time. These lights may detect when a car arrives at an intersection, when too many cars are stacked up at an intersection (to control the length of the light), or when cars have entered a turn lane (in order to activate the arrow light).

There are all sorts of technologies for detecting when a car has approached the intersection -- everything from lasers to rubber hoses filled with air. (Watch where you point that thing!) By far the most common technique is the inductive loop. What’s an inductive loop? Glad you asked. An inductive loop is simply a coil of wire embedded in the road's surface. To install the loop, they (the “they” here are the guys and gals who pave our streets, most likely the ones that are the shortest routes on our way home) lay the asphalt and then come back and cut a groove in the asphalt with a saw. (Speaking of saw, how many more of those movies are they gonna make?) 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 visually obvious. But don’t look too close. You ARE driving after all.

Inductive loops work by detecting a change of inductance. To understand the process, let's first look at what inductance is. This figure is helpful:

What you see here is a battery, a light bulb, a coil of wire around a piece of iron (yellow), and a switch. The coil of wire is an inductor. If you have read How Electromagnets Work, you will also recognize that the inductor is an electromagnet.

If you were to take the inductor out of this circuit, then what you have is a normal flashlight. You close the switch and the bulb lights up. With the inductor in the circuit as shown, the behavior is completely different. The light bulb is a resistor (the resistance creates heat to make the filament in the bulb glow). The wire in the coil has much lower resistance (it's just wire), so what you would expect when you turn on the switch is for the bulb to glow very dimly. Most of the current should follow the low-resistance path through the loop. What happens instead is that when you close the switch, the bulb burns brightly and then gets dimmer. When you open the switch, the bulb burns very brightly and then quickly goes out.

The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, then current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This current keeps the bulb lit for a period of time even though the switch is open.

The capacity of an inductor is controlled by two factors:

* The number of coils
* The material that the coils are wrapped around (the core)

Putting iron in the core of an inductor gives it much more inductance than air or any other non-magnetic core would. There are devices that can measure the inductance of a coil, and the standard unit of measure is the henry.

So... Let's say you take a coil of wire perhaps 5 feet in diameter, containing five or six loops of wire. You cut some grooves in a road and place the coil in the grooves. You attach an inductance meter to the coil and see the inductance of the coil. Now you park a car over the coil and check the inductance again. The inductance will be much larger because of the large steel object positioned in the loop's 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 knows there is a car waiting! Or rather, a person in the car is waiting.

So next time you’re stopped at a red light and are in a hurry, try putting your car in reverse (make sure no one is behind you. Unless your over 80. Then it’s OK, apparently) and try tripping up the detector below you. Or if you’re at a traffic light that has a headlight sensor, try flicking your high beams on and off to trip the light sensor. You never know… you may speed up the process to getting the light to change green. [via howstuffworks]

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