How do stoplights work?


I don’t understand how they work, what they’re hooked up to and how they know when to turn from green, to yellow, to red. Are they timed? Is there a sensor, perhaps a small man hiding in the pole controlling it?

In: 7

Depends on the light. Some are on timers, some have pressure plates in the road to sense when a car stops on it, I’ve heard some use metal detectors but I’m not positive on that

Theres multiple different ways they function. They tend to either work on a pressure sensor (the lines in the ground in front of the light), on a timer, or on a motion sensor. Emergency vehicles have a device that can override them and force them to change. Most major cities also have a traffic control center that can change the function of each light throughout the day depending on traffic conditions.

It varies wildly from city to city and intersection to intersection.

For starters, there’s almost always either a preferred direction or a timer. Let’s say you’re on a small rarely used 2 lane country road and you hit a 4 lane road down a rural town. It makes more sense for the people who travel on the main road to go through by default unless someone’s trying to make a left turn right? Well there will be some kind of sensor monitoring the smaller road if anyone gets there, otherwise it defaults to the main road.

What sensor you asked? Again, different types. There are pressure sensors for the weight of the car but these are rare. There are cameras that can detect a car in a specific lane, those are getting more common but not the most widespread yet. The most common right now is a loop of wire that sits under the road and detects a magnetic signal from your car driving over it. Any of those attaches to some control box.

Now in a major city, it’s more likely to have something timed in a way that the city decides it’s “fair” OR they connect the boxes and have some kind of algorithm decide which light to turn on when to get the most people as possible to their destination ASAP, and avoid anyone waiting for too long. This is a thing, but isn’t super common outside of major cities.

It’s a combination of sensors, timers and direct control.

Major intersections generally have sensors at the first and sometimes second queue position to detect if cars are there and need to change the lights.

In some peak tines, the intersection is generally set to specific timing, or can be controlled by the city wide traffic control inordrr to give more time to certain directions.

In off peak times (generally late at night) the intersection lights stay in the direction of major travel, and only change if there is a vehicle detected, or the pedestrian crossing button is pressed.

In emergency situations, traffic control for whole regions can be manually overridden.

At most lights, there will be a seemingly out of place metal box on/near one of the four corners. This is the control box. Once either the timer expires, or the magnetic sensors (thought this was pressure for forever lol) that others have described say it is time to change, the light will change from green to yellow, hold for 3-6 seconds, then go to red. After all lights sit red for a few seconds, the other direction turns green. The sensors/timer for the other direction will let the controller know exactly which lights need to turn green. Sometimes opposite direction left turns only go green, other times all lights in one direction change, sometimes there is no dedicated left turn signal.

Having the control box right at the light allows authorized personnel to override the base programming depending on conditions. For example, the streets/exits leading away from a concert venue normally wouldn’t stay green for very long since traffic is normally pretty light. But whenever there’s an event, the basic programming can be overriden to allow a much longer green to keep traffic moving away from the venue.