Why do traffic lights need that big old hurkin’ control box at every intersection? Seems like a central computer (maybe the size of a PC) should be able to control all the lights for miles around, and at most you’d need a set of light switches to control the actual traffic lights. The lights can’t be more than 250 watts so i can’t imagine needing a huge amount of juice to power them…basically it seems like overkill. What exactly is in those boxes and why do they need to be so big? Are there, like, little people in there fliping switches?
Most of the big ones you see are legacies from the older mechanical switching sytems. Many of them have been upgraded with electronic or computerized switching. Newer ones are often just a smallish box buried flush with the ground with an access panel on top. They aren’t usually centralized because that requires some sort of network infrastructure which doesn’t exist in most places. Additionally, a centralized system has at least one drawback where reliability is concerned. If the central system goes down, then ALL the lights controlled by it go down. This is bad. If one roadside controller goes, then only the affected light is down.
Aren’t there centralized systems in place? Someplace there’s a traffic engineering team that concentrates on optimized flow requiring a large network, surely? I’m genuinely interested in reading about such a thing.
Of course, if the network does go down, then of course there’s a default mode the light could go into.
heh…ALL GREEN!
There are, which is why I said “usually”. Many of them do have a centralized monitoring system, but the lights themselves essentially operate independently, although the timings can be changed remotely. There really isn’t any one thing you can say that “this is how traffic control works”. I’m just trying to speak generally here, s’all.
Nah, all yellow - it’s faster.
As for the big curbside boxes - around here it’s a mix. Some still have mechanical motor-driven switches that you can hear thunking along if you’re standing next to it. Some have been retrofitted with a small PLC which looks funny if the thing’s open - big huge empty cabinet, except for a paperback novel-sized box at the bottom.
What puzzles me is why many controllers raise havoc w/ the car radio. I’ m usually listening to FM so I’m not sure if they affect AM also.
They’re little computers which aren’t well shielded. Take the cover off your PC case and try listening to a radio placed nearby.
Thanks, but that raises the question of how much it would cost to shield them. I can easily make a case that it creates a serious distraction, and therefore a hazard. This at a time when the driver needs to be concentrating on traffic, moving in several different directions, and not be distracted by loud annoying sounds from the radio.
Make that, “the Reverend All Green…”. 
The controller is usually in a metal box which shields it pretty well electrically. I suspect that the radio interference is coming from the loop detectors under the street. What they do is bury a big loop of wire, then send a high frequency signal through the wire so that they can measure the inductance of the loop. When something big and metal (like a car) gets close to the loop, the inductance of the loop is affected and the controller picks up on the change.
AM radios are more affected by electrical noise than FM radios.
That sounds right, I only hear it when I’m close to the stop line, if I’m back three or four cars there’s no effect.
Some large cities tried the central processing approach but it takes MILES or buried cable and large mainframes which pretty much tie one to a technology at the design/bid stage.
With the advances in small computer power, many agencies tend to go with an “On-Street” processor with detection and communication capability. These are tied together for communication to allow co-ordination or signal systems. The system Master is typically located at one of the intersections in the system and directs timing plan implementation for the intersection controllers in that system as well as uploading traffic volume data and status to a computer at the office. Additionally, many, if not most, intersection that are in a system run"free" during off peak or late night hours.
So, essentially, what you have is a controller at an intersection that is capable of running independently that works in coordination with other signals in a system according to those plans deemed appropriate by the Master controller and in a system that reports to/receives direction from and office based computer via phone lines, telemetry or fiber optic.