how do stop lights work?

Question

we see and use them everyday but how do they work?
is there like alittle man in a box watching the cars and pressing buttons ????

Answer 1

yes. he is a wee leprachaun

Answer 2

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


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 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!



it's amazing what you can do with google+copy & paste

Answer 3

Haha. No, there is no little man running them. There are many ways traffic light work but the main two involve sensors.

The newer ones have what looks like a camera on the top, facing all four directions. It will monitor the traffic flow and change the lights accordingly. Lets say a main street intersects with a smaller street that does not have much traffic. When someone is on the smaller street and pulls up to the main street, the sensor now knows it needs to let that car go sometime soon. So it now starts to watch the main street and see when there are not that many cars coming. Once it finds less traffic, it changes and lets the car turn or go straight.

Answer 4

There is usually a detector that is put out near the intersection to count the number of vehicles that pass through the intersection during a specific period of time. The information is evaluated, and then a decision is made whether or not the volume of traffic warrants a light or stop sign at that particular intersection. They also investigate to see what dangers, the intersection have that need to be addressed. Careful, studying and planning goes into the decision on whether or not a traffic light is placed at the location, it is not as case of a person complaining to city or county officials over traffic lights. However, a stop sign is not as hard to get installed at an intersection than a traffic light, because of cost in a number of cases.

Answer 5

There are load cells under the road close to the intersection which measure the weight of how many cars are waiting, then it determines how long to stay green for...thats why when traffic is quiet, the main road will stay green until someone pulls up at a different entrance to the same intersection...they are still monitored by cameras at the roads department somewhere... the load cells send a signal thru a Telemetry (Wireless system) back to PLC's which process the info & switch the lights...there would still be a order for when traffic is busy...but the time for each green period will differ according to traffic flow...

Answer 6

lol. well first off some of the older ones ....there is still a couple in remote places.... are on a time lapse that just switches (safely) traffic around in a time limit. most of the ones you see today, if you ever notced the white bar where all the cars stop?...thats actually a weght sensor that tells the cpu in the light that theres cars waiting at that light or on the "left turn" spot ...so simply its done by weight sensors that tell the light were all the cars are.

hope that helped

Answer 7

they have them automaticly set in a pattern that is why at some intersections there is a box with a bunch of switches that turn off and on and they try to keep the pattern strait but sometimes it messes up (rarely) but somtimes you see a ,an by it which is replacing the switches because they are getting old and are at risk at messing up. but hose are the lod ones for the newer ones look at the top 2 answers because i am getting tired of typing and if i were to write all that i would only be 1/3 of the way there

Answer 8

Some are done by timers some are done by sensors. At stop lights sometime you see those rectangles outlines cutout in the concrete. Those area of concrete have sensors under them to detect cars above. If your ever in a turn lane those cutouts are often 20 feet back to only turn the arrow on if there are more than one car in line to turn.

So if you want a turn arrow, and you're the only car, stop above the mark in the road.

Answer 9

Most these days are computer controlled based off of time studies performed to determine the length of time between Red and Green lights activated. Some even will have sensors that detect traffic prescence to activate and deactivate the signals for traffic control.

Answer 10

traffic signals can be time generated
and they can also be influence but sensors placed in the
road

My favorite laugh wiwhen people get up to the light - pass
the sensor - THEN STOP when the light turns red.
The sensor is already triggered to think that this car made
it through the intersection - but no - they are past the stop BAR and into the walkway

Soooooooo when the light turns - they do no get their green
arrow to turn in front of traffic - and must wait.

pretty neat - when they work

Answer 11

Wll each light is on a timer. Every minute it changes from red to green. Then from yellow to red is about thirty seconds. then from there the sensors from the ground calculate the weight and tell the lights to go on or off!!