How do traffic lights work? who monitors it?

Question

I am in Chennai, Tamilnadu. There are more than 1000 traffic intersections in the city. lights work even when there is a massive power cut and all through the day and night. While all our appliances go out of order some time or the other, I am amazed that the lights never go out of order and never behave strangely.

Answer 1

there are sensors in the road so it knows when cars go across them, usually different sets of traffic lights are all synchronised so traffic can flow and doesn't have to stop at every intersection

in aus when the power goes out so does the traffic lights, causes h**l!

Answer 2

there is no one working there. there is little magnetic coils in the floor, covered in a small layer of tar or something, they sense when something metallic goes over them.

,Greg

Answer 3

There probably actually are sensors. Modern sensors can be buried in the pavement (no marks on top) and still work. Some cities use cameras and software for their sensors instead.

While most traffic lights are on a timer, the software is quite intelligent and will modify timings or change when people want to cross, cars approach, emergency vehicles in the area, busses, traffic, or even when cars go too fast or too slow. They get the information for various types of sensors everywhere.

In larger cities, traffic lights are usually networked. Meaning they operate together to get more traffic through. They will combine their sensor information and decide how to run the lights. Some places even used networked signals to make people drive the speed limit by giving extra reds if people drive too fast, or extra greens if people are too slow. If traffic gets stuck, then some will let the other direction through while they wait. That's why in big cities you usually don't stop that often.

Also, many large cities have traffic management divisions. They monitor traffic from cameras and sensors then they can manually adjust traffic signals if needed from their offices. They also know right away when a signal fails.

It also sounds like you get a lot of power failures there. The government likely ran separate power sources to them that don't fail. This keeps them going all the time. In Canada if the power goes out, so does the light. But then again the power goes one one or twice a year if we're unlucky (and it's usually restored within an hour).

Answer 4

No one monitors them directly.  They are set up on timers.  The times may be adjusted by a pedestrian pressing the button to walk across the intersection.  Also, more modern roads have sensors under the road to check for stopped cars waiting for a red light.  If the sensor does not detect any waiting traffic, it will keep the light red ( ask motorcycle riders about this as bikes don't always trigger those sensors so a biker may be sitting at a red light through several cycles ).