How do traffic lights work? Sensors, timing, connection with other traffic lights … the whole shebang.


How do traffic lights work? Sensors, timing, connection with other traffic lights … the whole shebang.

In: Technology

At the most basic level, the traffic lights have a pre-programmed controller. The controller tells the light how long to show each colour to each direction. If the controller is a bit fancier, it will allow the program to be different at different times of the day or different days of the week.

Most traffic lights have detector loops in the pavement (they look like big black circles.) The detector loops are coils of wire that develop an electric current through induction when the metal frame of a car is above them. The controller would adjust the program for when cars are present – for example you might have a green light that only lasts 10 seconds, but it stays green when cars are coming up to a maximum of 30 seconds.

Very fancy traffic lights use cameras instead of coils, and they could be networked with other controllers in the city. That would allow for things like setting up a “green wave” for emergency vehicles.

Telling this from the perspective of dutch traffic lights (or traffic coördination installations (TCI).

We’ve got pre-programmed and detection systems. Both kinds have detector-loops in the roadconstruction and where pedestrians and cyclists use the intersection pushbuttons for them. The difference is in the way the loops are used.

In the pre-programmed TCI the loops are used to determine if a green light sequence can be skipped for a direction, because there isn’t any traffic waiting to enter. These intersections can also be programmed to skip pedestrians and cyclists until they push the button indicating they want to cross.

The TCI’s with a detection system have loops reaching out of the intersection to detect if somebody is comming. Depending on what the TCI/intersection owner wants, this system can have loops for cyclists and detectionsystems for pedestrians added, replacing or aiding the pushbutton.

I’ve also heard about some TCI’s having weathersensors added so pedestrians and cyclists get their turns faster when it’s cold or rains. Thought behind this is that a car/truck-driver can wait a couple of second longer in their warm and dry vehicle while the others get to a warm and dry place faster.

The programming and timing would be based on traffic studies. I had to take a full semester of traffic study in my civil engineering program. Study an intersection. Predict the traffic flow, adding a left turn lanes or arrows. There are equations to predict traffic buildup and if the intersection is quality. I did not like that class. Our final grade was to pick an intersection and make an improvement. The night before it was due i drove to this intersection and found our recommended improvement was already there and my groupmates and i were like oh shit!