How do traffic lights time or decide when to display their colors (red/green/yellow)?


How do traffic lights time or decide when to display their colors (red/green/yellow)?

In: Technology

Depends on how it’s programmed. Some lights go
Through a pre-programmed sequence no matter what. Other lights go through a sequence based on what cars it can sense in what lane. And then there’s lights that default to a particular configuration and only change if there’s a car sensed

Depending location, there’s basically 3 systems.

First, a simple timer is rigged up to the lights of the intersection. Be green for X amount of time in this direction, then be green for X amount of in the other direction. Simple.

For areas like suburbs where a lot of the traffic occurs at certain times (and/or one road needs to be prioritized over the other), they build sensors into the road surface. These can tell when there’s a car waiting for a green light (or several cars, depending on placement) and triggers the lights to change. It’s basically so that the lights only change when someone needs them to, so the flow of traffic on the busier road doesn’t get disrupted as much.

In really busy places like major cities, there’s an entire network that runs the traffic signals from a computer based on minute to minute traffic patterns. This is the optimal way to deal with tons of traffic because they can spread cars out more and help alleviate logjams by preventing more cars from getting into the mix.

as far as I know there’s 2 common methods for it; a simple timer, and pressure plates.

The timer method is just that – there’s a timer set up for how long to have each light be red or green, and that’s pretty easy to do with some programming. They do have to still be programmed (and that needs to be in it’s own weather/water proof box somewhere near the intersection), and how they decide at how long to program each light change usually involves researching how much traffic uses that intersection.

Pressure plates are used when it’s not *that* busy of an intersection, or they want to favor one of the roads more than the other, only changing it’s light when the pressure plate detects the weight of a car above it. Again someone still needs to program how long to have the lights change for – but instead of being on a clockwork timer, it’s only going to change the lights when the pressure plates send a signal.


Some newer camera detection methods exist too, that are pretty much doing the same thing as pressure plates, but instead of needing to install a pressure plate switch under the road, they can use cameras instead, which are a lot cheaper to install, and relatively easy to maintain. They’re still operating on the same kind of basis as the pressure plate method – only change the light when it detects cars – just doing it with a different sensor.

Some are timed.

Several have pressure plates to tell when cars have been waiting and for how long.

Others might have light sensors, but that might also be urban myth (yet you’ll still see some people flashing their brights trying to get their light to change).

It’s almost entirely automated with the safest paths being directed in their order of traffic density and waiting duration, and the pressure plate may be several car spaces back or require a certain weight to work, which might be why some cars never seem to trigger some lights.

It’s like a bicycle opening the gates at an apartment complex exit by magnetic detection, but more complex for gating 4-way traffic instead of 1-away / exit only

I know those here (Ontario, Canada) have camera/sensors that detect any traffic waiting for a green light. Especially during the night, if an intersection is not busy with no one waiting, you can see the pedestrian counter run down 3..2..1..and instead of creating a red light, it just resets to green since nobody is waiting.

At other lights, it’s fixed so even if nobody is there, it goes through the same process regardless; which I find annoying at night.

At night and at non-busy intersections, i feel should be flashing yellow lights on all intersections. Doesn’t waste everyone’s time, and gets the job done more efficiently