do traffic lights have light sensors?

Question

can traffic lights detect when a car is waiting by the car's headlights. for example when you pull up to traffic lights in the dark and flash your headlights can the traffic lights detect you are there and waiting especially if there are no other cars around.

Answer 1

The detectors are underground, beneath your car.

More sophisticated control systems use electronic sensor loops buried in the pavement to detect the presence of traffic waiting at the light, and thus can avoid giving the green light to an empty road while motorists on a different route are stopped. A timer is frequently used as a backup in case the sensors fail; an additional problem with sensor-based systems is that they may fail to detect vehicles such as motorcycles or bicycles and cause them to wait forever (or at least until a detectable vehicle also comes to wait for the light). The sensor loops typically work in the same fashion as metal detectors; small vehicles or those with low metal content may fail to be detected
~Love, Wikipedia

Answer 2

There is something exotic about the traffic lights that "know" you are 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 is going to be a lot of traffic. In the suburbs and on country roads, however, detectors are common. They 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 cars -- everything from lasers to rubber hoses filled with air! 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 come back and cut a groove in the asphalt with a saw. 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 obvious.

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 what the inductance of the coil is. 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!

A timer is frequently used as a backup in case the sensors fail; an additional problem with sensor-based systems is that they may fail to detect vehicles such as motorcycles or bicycles and cause them to wait forever (or at least until a detectable vehicle also comes to wait for the light). The sensor loops typically work in the same fashion as metal detectors; small vehicles or those with low metal content may fail to be detected.

I hope this helps.

Answer 3

Induction loops embedded in the road surface have already been mentioned and are the most common. Fixed green time for each movement is sometimes used but not much. Even temporary signals (for example, when bridges are being rebuilt/maintained) have some sort of detection system.

Some of the detection systems communicate with the signal controller via a radio/wireless link... and in many places around the world, there are video detection systems - when a camera's field of view is occupied, this puts in a call to the controller, but these units are sometimes ineffective in rain, fog, etc.

Note, too, that not all traffic signals include an operation for "emergency vehicle pre-emption", where police cars, ambulances, etc. can extend the green time or "change the signals" earlier than normal.

Additionally, flashing your lights when you've arrived at some signals is a waste. Once you arrive and your vehicle has put in the call, flashing your lights doesn't make things happen any faster; it's like when you press the pedestrian button or if you press the button for a lift (elevator) - the call has been recorded and pressing the button many times doesn't make the indicator light turn "on" any brighter.

Answer 4

To clarify for all the people who think that the little sensor on top of the light detects traffic, it does not. It is for emergency vehicles. When they respond to call they have a transmitter in their vehicle that they can either turn the lights red or green in their direction with. Stop lights are either timed or have sensors on the ground. If you ever noticed all the rectanglish patterns in the concrete at the light, thats the sensors.

Answer 5

No. Traffic lights are either simply timed or they work by a sensor in the pavement. That being said, most have a sensor that can be triggered by certain lights in police cars and emergency vehicles. I believe that these sensors turn all lights red. That way the emergency vehicle knows that all traffic is stopped and it then proceeds through the red light.

Answer 6

Yes there are light sensors to change the traffic signals.
Many metropolitan areas equip emergency vehicles with a specially timed strobe light that is read by the sensors, affording them a "green" light so that the may move thru traffic more quickly.
The rest of us must rely on underground sensors to tell the signal we are there.
In the past the sensors were pressure activated, now they rely on an interruption of a magnetic field created by a vehicle.

Answer 7

I personally have seen and activated such a thing on a traffic light by flashing my brights at the red light. Its purpose is to detect emergency vehicles that are going to pass thru the light. Their flashing emergency lights make the light go immediately to yellow.
And this is why flashing your bright lights will also do the same, but not all lights have this, in fact very few do.

Answer 8

they have sensors that detect a car by it's weight.

headlights have nothing to do with it. the myth about flashing your headlights to make a light turn green origonated because people believe that is how emergency vehicles change the lights. Emergency vehicles actually use a device called an opticom or some variation. in most cases they utilize a radio signal, but in some rare systems, a high speed strobe it used.... but that strobe moves far faster than you could possibly flash your lights.

hope this helps.

Answer 9

Some do have sensors, but I'm afraid that 'flashing' your headlights is pointless. They usually work on a laser so your headlights wont make any difference.
The lights that 'may' change if you flash them are usually temporary ones, the sort that workmen use when digging a road-up.

Answer 10

No there are no light sensors, but there are sensors embedded under the road which detect that a car is waiting.