How does a traffic light detect that a car has pulled up and is waiting for the light to change?
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.
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 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!
More sophisticated control systems use electronic detector loops, which are sensors 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.
It is also commonplace to alter the control strategy of a traffic light based on the time of day and day of the week, or for other special circumstances (such as a major event causing unusual demand at an intersection).
haha, i actually did read the wikipedia thing a lil while ago.. apparently it works like a metal detector and does sense the metal in the vehicle... I'll read the first thing you quoted up now.
That all makes sense and sounds perfectly legit... I know some lights used to not sense Clay's pickup well and some don't like my Dodge. I would conclude that becomes a height issue.. The farther the "core" from the coil the less the inductance increases..
Also we know if there's not a large enough mass of iron something won't be detected.. Guess you're screwed if you bought that gay alumi-tub car with aluminum chassis from Boyd Coddington's crew of retards..
I wonder how much magnetic material is typically needed.. Say we got something like a new Ford GT.. There's a heavy (no pun intended) lack of steel in that thing. Probably not much more than a motorcycle.. I'm trying to think of other iron-deficient cars now.
jac6695 wrote:C6 Corvette uses a lot of carbon fiber, many Audi's use aluminum in the body structure.
I think you are right about the height, but we all know Dodge doesn't use as much steel as GM.
But doesn't the Corvette and Audi still have a fair amount of steel in the chassis?? The GT has none in the chassis itself or the body that I know of.. You don't find steel til you go inside the engine or look at a shock shaft or spring.
loops are actually an outdated method of vehicle censoring still used by many authorities such as VDOT. They are overly expensive and must be replaced each time a roadway is resurfaced, if they last that long anyway. More modern cameras and sonar detectors are replacing loops, especially in areas were the authorities use an above average level of intelligence, which would be why VDOT still mostly uses loops. The newer detectors mount on the stop light posts, can be used for a variety of purposes such as measuring traffic flow, and do a better job of sensing motorcycles and other smaller/lighter vehicles. Most intersections that use loops switch over to a timed system at night so that a single vehicle that may not set off the detector will eventually get a green light based on the timer. Basically the timer is a catch all bandaid.
It's really fun pulling a nest of loop wires out of the drum and teeth on a milling machine. Then watching as they slice up a freshly paved road to install new loops. Atleast now they have gotten a little smarter over the years and make you mill out the old surface asphalt, install the loops into the intermediate layer and then lay the fresh topping. This puts the loops under 1.5-2" of asphalt, but at least the new roadway isn't all sliced up.
I havn't had any traffic light issues in the C6 yet, but riding the bike when there isn't much other traffic around is a major pain in the ass. If it's real late and no one else is around, I usually end up having to run the light.
<------------ Hates traffic light sensors
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When I was studying for my motorcycle license back when I was 16, it said that if a motorcycle is not detected at a stoplight, revving the engine might get it to trigger. And... failing that, running it is ok.
So how do they detect emergency vehicles? I've heard its a strobe detector and if you flash your brights at the right interval all the lights will turn green for you.
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tsmall07 wrote:So how do they detect emergency vehicles? I've heard its a strobe detector and if you flash your brights at the right interval all the lights will turn green for you.
probably more of a radio transmitter to the little box with a light on it that changes the lights.
tsmall07 wrote:So how do they detect emergency vehicles? I've heard its a strobe detector and if you flash your brights at the right interval all the lights will turn green for you.
strobe sensors are mentioned in one of the articles james posted.. Of course, not all stoplights have this feature. I'm not banking on flashing my brights being able to turn the light green, but I won't deny it has probably occurred.
I believe that strobe light censors are used at some intersections. Sometimes when we are out plowing some main roads for VDOT the lights turn green right away for our truck, but this is because we have strobe beacons mounted on the top of the spreaders. The strobes are mounted high like on an emergency vehicle, flashing your head lights should not have the same effect, you would need a fairly bright strobe light, which under normal circumstances would be illegal.
the opticoms that trigger the light for emergency vehicles pick up a high intensity strbe on the light bar, next time you see an ambulance or fire truck, look in the center of the main light bar, you will see a single white strobe flashin really fast. Of course not all vehicles are equiped with them. here in roanoke city, all our trucks have them, but not all the traffic lights do. There is a new technology out, that uses infared sensors on the light poles and a transmitter mounted on the vehicles, that way other vehicles such as work trucks like Mike said, wont trip the light.
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Last edited by 4runner on Mon Feb 07, 2011 5:50 pm, edited 1 time in total.
