How do stoplights detect a car?

Answer 1

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In most municpalities, its called a magnetic sensing loop, there is one aproximately 1/4 mile from the signal, and 2 or 3 closer to the light in each lane. You may have noticed a black stop sign shaped marking (or in some cities a circle) in the roadway. This is where they have cut into the concrete or asphalt and laid in a magnetic loop (the black stuff is a sealer to keep the rain from rusting it). As your car passes over it, it sends a magnetic pulse (from the metal in your car or motorcycle) thru an underground cableing system to a computer terminal at the intersection (usually housed near the intersection in a large metal box on one corner), signaling that a car is coming from a certain direction. This is how a turn lane signal knows when to activate the turn signal for you. As for the length of a green or yellow or red light is dependant on timers in the computer terminal and some areas are need based. You ever wonder why you fly down the highway and the light turns yellow as soon as you hit the first crosswalk? This is how. As for your headlights, it is a common myth that flashing your headlights will trigger a signal to change for you. This is false. Their is a system used by police, firefighters, ambulances as such called an Opti-Com or Opti-Flash, which is a traffic signal preemption device that sends out a coded digital white strobe flash at a certain flash rate and candlepower, much faster and brighter than any headlight. This light is read by a small sensor usually on the traffic light pole and sent to the computer to trigger a green light for the emergency responders so they dont have to wait in traffic while the other 3 sides have red lights (this is why it seems like your light got skipped when a fire engine went through, its just the computer resetting itself into its normal pattern)

Source(s):

Traffic siganl info at: http://www.dot.state.sc.us/getting/signa…

Info on loop detectors at: http://www.dot.state.sc.us/getting/signa…

Tips on how to trigger a loop better for motorcycles at:
http://www.dot.state.sc.us/getting/signa…

Info on flasher systems used by ems and police at:
http://www.prioritygreen.com/preemption.…

and specifically for 3M Opticom system at: http://www.3m.com/us/safety/tcm/products…

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Highly detailed and leaving not just one source, but plenty of them.

Answer 2

both the wieght of your car and your headlights

Answer 3

There are sensors a few feet back from the light. If you back up and go forward again, the light will probably change for you.

Answer 4

Most likely with a motion sensor..........

Answer 5

magnetic sensors burried under the road surface detect the steel that is in every car.

Answer 6

Magic.

Seriously, the older systems have weight sensors built into the roadway. The newer ones use the same principle as the motion detector lights you sometimes see. The very old systems have a box with a timer, the light is red for X minutes, and green for Y minutes no matter what the traffic is like.

Answer 7

Switches in the road detect the car.

Answer 8

They have radar sensors above the stop lights which determine the car objects on the road.
depending on the length of the line standing on the road the timer is set.

Answer 9

My wife is a signal light tech and if u notice on the asphalt that someone has tared up a square just before the light, it is a loop that sends a signal back to the cabinet that controls the light

Answer 10

There are motion/weight sensors under the roead...if you look at the road it looks like the piecees were just put together, but those are the sensors....

Answer 11

I have heard that there are underground sensors that weigh the street above. So when the weight builds up, that means that traffic is building up so the light changes via a pre-programmed sensor.

Answer 12

BY THE BAD SMELL OF YOUR CAR.....

Answer 13

Now days a lot of cities have a sophisticated sensor that is above the light itself. It can detect the amount of cars that are driving or on the road and change the light accordingly.

I had no clue that they also had weight sensors under the road! I guess we learn something new everyday!

Answer 14

have you ever seen the street department cutting a line across the road near a street light on a perfectly good roak? What they do is cut a line...then they place a wire of sorts in it then they seal it down. When your car crosses this "loop" they have put in the road...it sends a signal to the light system telling it there is a vehicle there and thus it causes the light to change

Answer 15

They have a mind of their own, called a sensor. Once a car passes the sensor, wallah you got green or red depending on which side of the street you are on. Wonderful technology we have!

Answer 16

Some are controlled by sensors ,others by timers.

Answer 17

The lights themselves don't, but today's cameras do. They are mounted so they see you coming, measure your speed, and take a picture of your tag when you pass, then they send you the ticket. They have the evidence and will take you in to pay the ticket if you don't voluntarily show up when notified.

It varies from city to city that's why the variation in answers.

Answer 18

Motion and weights detectors....but nothing to do with lights...

so that urban myth about flashing your high beams will not change the light any faster.

And alot of lights are on a timer system that changes as the day goes by, like in the morning by my house it changes every 3 minute, but at night its every 6

Answer 19

They have sensors, have you ever noticed those grey metal box things on the wires? When a car comes up to a traffic light, it detects a car has come to a stop, thence changing colors for the oppisite trafficwhen it doesn't have as much and giving the oppisite an advantage. It's hard at first, but watch how it gives which traffic direction(s) and /or turns, you'll catch on.They aren't on timers like the old fashioned ones (which some are still in use).

Source(s):

When riding along in a car, I see it at most of the traffic lights.

Answer 20

There are censors. Also, in some, I believe actual people control it.

Answer 21

they dont they are on a timer ive just stood by a stoplight and watched them even when there are no cars they still change.

Answer 22

all traffic lights have sensors but alot of them are old because some streets do not have so much traffic. however they do change them often here in miami and each last about 55 seconds or a minute and a half for busy streets.

Answer 23

there are loops of wire embedded in the pavement that detect the magnetic field of the car as it passes over the loop, or a photo eye is used to detect the presence of the car at the light.

Answer 24

The correct answer is.....some are weight, but the vast majority these days are magnetic.

Answer 25

They put a loop of metal in the asphalt that acts as a rather large metal detector. It isn't a very good metal detector, but then it only has to detect a rather large piece of metal. Motorcyclists are often confounded by this because there isn't enough metal to set them off. So if you see a motorcyclist running a red light that never seems to change, now you know why.

They don't use weight because it would be too complicated to deal with the weighing mechanisms. They don't detect magnetism, because cars aren't magnetic. Always look for the simplest answer. Detecting metal (iron specifically) is a an easy task to perform, electrically speaking, so that is why it is used.

Answer 26

the little gremlins in the stop light tell the magic squirrels to change the lights for the cars. Except they make the red lights longer for you because they don't like you very much.

Answer 27

There are wires embedded in the road that detect vehicles based upon weight. Look on the phone poles in the vicinity and you will see a metal box that regulates the wires beneath the pavement.

Answer 28

they have camera's on them somewhere and every time the light turns red the camera takes picutes until the light turns green again, so smile when u run a red light

Answer 29

sensors in the lights or on the road

Answer 30

well there is this sensor that when it is red and you step on the yellow line well it turns green

Answer 31

people r right.. they do have sensors and also cameras inside the light.. at the station they have a supercomputer that a dude is on and he can change by the length of traffic and time.. so if u ran a red light the camera would follow your license plate to get your number.. hope i said that right..