how does a red light know when to turn?

Question

ive often wonder how to lights know when to change? its there a timer? a sensor? cause there are times ill flash my brights at the light and it will change. or when i roll up to the line it will change. sometimes the lights stay red forever

Answer 1

If it is the cheapest possible one, then it just has a timer in it. The timer is set to be red on for a number of seconds then green for a number and then yellow for a number.

If it is fancier, it may have a detector loop that realizes that one or more cars are on the side street and may turn the main street to its red light for a timed interval. Even fancier will have loops on a bunch of connecting streets to keep traffic moving smoother by changing light times when side street traffic is heavy.

If it is very modern it may have a radio that firetrucks activate that turns everything to red so no traffic moves in their path until they are gone.

The fanciest ones have computer control of cycle times and cycle starts from a central traffic control office with TV cameras showing where there are problems.

Anything else where you think that the lights are changing because of your headlights or stuff like that is just coincidence.

Answer 2

There are two ways that they work they have a sensor in the road we call it a loop it works off of magnetics so when a vehicle pulls up it senses it and then the timer will switch the signal and some intersections usually the busiest ones work off of a timer and the times are calculated by doing traffic studies to get optimal traffic flow that is one of the reasons that there are places that have a sign that says stop here on red because if you don't pull up to the loop it wont recognize that a car is there some times we install the loop two or three car lengths back for better timing.

Answer 3

some intersections are timed. Here in Jacksonville we have idiotic intersections that take 3 1/2 minutes to cycle back to a green from a red...Other lights around here use embedded sensors to change the light when a car approaches from a low use side road Those are called "demand Lights"

Answer 4

some intersections have weight plates place before the crosswalk, when you car hits these plates, it tells the light controller that there is a vehicle there waiting to cross. This can trigger the light. Same thing or kind of thing they use for red light violation cameras

Answer 5

There are lights with timers & there are lights that have sensors in the roadway...I am not sure of one you can flash your lights at, that may have been a cooincidence.

Answer 6

there is a sensor in the ground and knows when a car pulls up