Has this ever happened to you? You pull up to a red traffic light and it changes to green in a few seconds! How did it detect your presence? Or maybe you've had the opposite experience: You sit at a traffic light for what seems an enormous length of time with no change.
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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).
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Traffic lights commonly detect vehicles using digital sensors mounted on the lights themselves, or through an inductive loop embedded in the surface of the road. Both methods allow the traffic system to keep tabs on stopped vehicles occupying the intersection and help traffic to flow smoothly. However, they achieve this in very different ways.
To install an inductive loop, workers 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. The illustration on this page is helpful.
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Here you see 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. The inductor is an electromagnet.
If you were to take the inductor out of this circuit, then what you have is a normal flashlight. 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 you'd expect when you turn on the switch that the bulb would 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.
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.
Inductive loop systems are commonly used thanks to their simplistic nature. There's much less chance of a breakdown compared to expensive and complex digital sensors, but this simplicity can also be a drawback. All the induction coil "knows" is whether or not a car is currently parked on top of it. This is the main reason that the light may fail to change in a timely manner if a car doesn't pull all the way up to a stop.
Lighter vehicles like motorcycles may also fail to trigger the inductor with their weight alone, making them a hassle for bikers during low-traffic hours. Digital sensor systems do away with these problems, and they allow transportation authorities to log countless hours of traffic data that can be used for future planning of routes and city projects.
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In addition to induction methods, you may see traffic lights employing a variety of advanced sensors for vehicle detection. These are typically installed near the lights, and don't require any laying of metal inside the road. Sensing methods you may encounter include infrared sensors, microwave beam emitters, and video cameras.
Infrared sensors come in "active" and "passive" varieties. An active system shoots out a beam of infrared light that stops just where a car might be during a red light. When a vehicle pulls up, the beam is broken and the sensor is able to detect that the space is occupied. (Emergency vehicles like ambulances and police cars have active sensors that can request a traffic light be changed, usually when their lights or sirens are on.)
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Passive sensors instead use their infrared sensing to detect the heat that comes off a car's engine. However, other intense heat sources like direct sunlight can cause the system to see false positives.
A microwave sensor works similarly to active infrared, but instead generates a magnetic field around itself. When vehicles enter the field, they disturb it and cause changes in the waves. The sensor is then able to detect these changes and see the vehicle. Microwave emitters are relatively expensive, and don't have the heat contamination issues that you may see in infrared.
Video camera systems are the most complex we see in traffic lights, but also have the potential to be the most effective. Installed on the lights are multiple cameras, similar to those would see on a CCTV installation. These cameras allow multiple, maybe dozens, of traffic stops to be networked together.
They all talk to a server running software designed to identify vehicles, and count the number of them at a stop in real time. It may also be able to distinguish cars from pedestrians. The network takes in all this data and then attempts to run the series of lights at maximized efficiency. Long-term analytics can then be used to improve the system further or figure out where roads need to be modified. However, weather conditions like fog can seriously hinder the cameras' vision.
In order to ensure that traffic stays moving, lights are often installed with two types of sensors, or a sensor plus an inductive loop. If one type of detection becomes infeasible due to outside conditions or a hardware failure, the system is able to fall back on another and avoid a major traffic nightmare.
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Traffic lights tell drivers and pedestrians what they must do at intersections and along roads. They tell road users when to stop and go, when and how to turn and when to drive with extra caution.
A green light means you may turn left, go straight or turn right after yielding to vehicles and pedestrians already in the intersection. When turning left or right you must yield the right-of-way to pedestrians crossing the intersection.
If you are looking for more details, kindly visit red yellow green lights.
A yellow - or amber - light means the red light is about to appear. You must stop if you can do so safely; otherwise, go with caution.
A red light means you must stop. Bring your vehicle to a complete stop at the stop line if it is marked on the pavement. If there is no stop line, stop at the crosswalk, marked or not. If there is no crosswalk, stop at the edge of the sidewalk. If there is no sidewalk, stop at the edge of the intersection.
Wait until the light changes to green and the intersection is clear before moving through it.
Unless a sign tells you not to, you may turn right on a red light only after coming to a complete stop and waiting until the way is clear. You may also turn left on a red light if you are moving from a one-way road onto a one-way road, but you must come to a complete stop first and wait until the way is clear.
Flashing green lights and green arrows direct drivers who are turning.
When you face a flashing green light or a left-pointing green arrow and a green light, you may turn left, go straight ahead or turn right from the proper lane. This is called an advanced green light because oncoming traffic still faces a red light.
Pedestrians must not cross on a flashing green light unless a pedestrian signal tells them to.
When a left-turn green arrow is shown with a red light, you may turn left from the left-turn lane. Vehicles turning left from the opposite direction may also be making left turns because they too face a left-turn green arrow.
After the left-turn green arrow, a yellow arrow may appear. This means the green light is about to appear for traffic in one or both directions. Do not start your left turn. Stop if you can do so safely; otherwise, complete your turn with caution.
You can still turn left when the light is green, but only when the way is clear of traffic and pedestrians. If the light turns red when you are in the intersection, complete your turn when it is safe.
Pedestrians must not cross on a left-turn green arrow unless a pedestrian signal tells them to.
Traffic and pedestrians must yield to public transit vehicles at a transit priority signal. The round signal is on top of a regular traffic signal and shows a white vertical bar on a dark background. This allows transit vehicles to go through, turn right or left, while all conflicting traffic faces a red light.
Some intersections have separate traffic lights for left-turning traffic and for traffic going through the intersection or turning right.
When a left-turn green arrow appears for traffic in the left-turn lane, traffic going straight ahead or turning right will usually see a red light. You may turn left from the left-turn lane when you face a green arrow. Vehicles from the opposite direction may also be turning left.
After the left-turn green arrow, a yellow light appears for left-turning vehicles only.
After the yellow light, a red light appears for left-turning vehicles only. Traffic going straight ahead or turning right will face a green light or green arrows pointing straight ahead and to the right.
In these intersections, you may not begin turning left after the green light appears for traffic going straight ahead or turning right. If the light turns yellow while you are in the intersection, complete your turn with caution.
You must come to a complete stop at a flashing red light. Move through the intersection only when it is safe.
A flashing yellow light means you should drive with caution when approaching and moving through the intersection.
During an electrical power loss, traffic lights at intersections will not work. Yield the right-of-way to vehicles in the intersection and to vehicles entering the intersection from your right. Go cautiously and use the intersection the same way you would use an intersection with all-way stop signs.
A traffic beacon is a single flashing light hung over an intersection or placed over signs or on obstacles in the road.
A flashing red beacon above an intersection or stop sign means you must come to a complete stop. Move through the intersection only when it is safe to do so.
A flashing yellow beacon above an intersection, above a warning sign or on an obstruction in the road, warns you to drive with caution.
By the end of this section you should know:
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