How do stoplights detect a car?

Answer 1

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)

Answer 2

If you have time to read all the others, here is one more: Some are simply on a timer. Most have sensors buried in the pavement. Magnetic or weight is irrelevant. If you look up on the light support and see a small shiny box, it is a device to detect strobes on ambulances and fire engines. When an emergency vehicle approaches, the device detects the strobes from a long ways back to change the light. (Don't always work in daylight, maybe my strobes aren't bright enough.) And another method nobody has mentioned. The lights are computer controlled from a central location. Ever been on a street with a sign that says"Stop lights are set for "X" mph?" If you drive that speed or a couple of mph faster you will make every light.

Answer 3

Wow. There's a LOT of misinformation out there. Okay, first, the old style lights used thyristors as timers. They were often synchronized with lights a block or more away to make traffic flow at a particular speed.
Now, however, most systems use inductive sensors buried in the streets to tell the computer that you are there. That would be those square or round black lines of tar that ppl have mentioned here.
Those weight sensors though, are obsolete. There may be a few HUNDRED of them still in the US, but they are almost certainly extinct for the most part. Those consisted of a rectangular steel box buried in the street, maybe 5 ft by 8 inches wide with a steel switch plate exposed on the surface. I remember seeing them when I was a kid. But that was over 40 years ago.
The only light sensors I know about were experimental, and designed to allow emergency vehicles through intersections while their strobes were working. Your headights don't affect them.

Answer 4

This Site Might Help You.

RE:
How do stoplights detect a car?

Answer 5

Most stoplights don't, they are on a timer.

Some like the ones at roadworks at night have a light sensor so if they detect a set of lights coming from one direction and none from another they will activate the sequence and give the oncoming car the green light.

Answer 6

most stoplights are programmed to the heaviness of the traffic at each specific time of the day/night. But there are certain stoplights in certain countries like Japan and cities like San Francisco, that are wired to change lights when sensors embedded underneath the road detect a vehicle being idle for a certain amount of time.

Answer 7

Some stoplights are on timers, and just go through a regular timed cycle regardless of whether there is a car waiting. (This is mostly used in busy intersections.)

Others work off metal sensors in the pavement just before the white line where the car stops.

Answer 8

Depends upon the type of sensor. One method employs a coil placed into a rectangular cut in the pavement that detects the metal in the vehicle, a principle similar to a metal detector. Another method is the use of a camera. The camera senses the vehicle approaching and stopping at an intersection. It senses traffic based on the percent of change in the video signal from the camera to the control unit. When no cars are detected, the lights change on a timer.

Answer 9

There is a magneto Loop running round in a square or rectangle cuts into the road surface, it is meant to detect the drop in the magnetic current that flows through the wire, there are usually one or two magnetic loops more back further to detect the flow of traffic whether it gets heavier especially in peak hour times then the traffic lights switch to a timer to prolong the time for the extra vechiles to cross, its funny with motor bikes i've seen some just sit there at night when its not so busy and they wait for the lights to change. then get pissed off and go thru a red light cause the magnetic loop didn't detect it.even though the motorcyclist knew how to activate the loop he jumped up and down on the bike to try and trip the loop but to no avail.LOL. even some cars do the same, i even roll back and then forward to set the loop off, if it doesn't detect me at times, these type of loops are too old and need to be replaced.

Answer 10

There is a little truth in many of your answers. I don't think people control them, they are too small for today's sized people. :-) I don't think they are on timers anymore although some small towns and city's may still be.
For the most part, traffic lights are run by road sensors that have magnets and weigh vehicles. I ride a motorcycle and often find myself sitting through 2-3 red light cycles watching the other sides of traffic get to go and mine stays red, because the bike doesn't weigh enough or I may not be on a magnetic field, In some states they allow a rider to go through the light if it isn't changing and the traffic is clear. So when you are sitting at a traffic light behind a motorcycle and they are the first ones in the line of traffic and motioning you to move up closer to them, you'll know why.