Traffic Light Syncronism

Long post, all based around how Traffic lights are timed.
For me, in GA. USA it seems that traffic lights are for the most part, an all green or all red affair. There are times when you sneak out and get on the opposite cycle as it where, or perhaps you are in a new system. What determines when the lights switch? Is there any sensor or weight plate (I doubt) that triggers the change? (Other than the Walk/Dont Walk button)
Is it all set up with an algorithim in those control boxes seen next to most intersections? If human input alone determines the times and rates of change, how is this rate calculated, and reviewed?
There is one horrible intersection that I know of where the actual intersection (not the striped lanes) is blocked by intersecting traffic almost every time.

Some locales have electronically controlled systems to keep traffic moving at a fixed rate, I usually see 5 mph under the speed limit. These were common in my native PNW decades ago.

Atlanta is “still looking into” adopting such systems, since they are so high-tech, state-of-the-art, etc. (Yeah, right.)

This differs from systems that have embedded detectors near the stop lines to figure out, e.g., if the left turn light should kick on, etc. (Or keep you waiting forever since your 1969 Beetle doesn’t register.)

These are considered minimally useful systems.

If you’re ever stuck in traffic trying to get onto I-85 from Jimmy Carter Blvd, you can “thank” the traffic engineers who go out every few months trying to “tweak” the timing of the lights. Hardly a sophisticated system.

It’s very much determined by the municipality. In NYC, the one way avenues are timed so that you can drive 25mph and hit all greens. Each block turns green a few seconds later than the one you just passed. This scheme stops being good when it gets very crowded, since you can’t move because the line of cars ahead of you is waiting for a green. Other roads in NYC turn green all at once, or in blocks of 4 lights, that sort of thing, it depends on the needs of the road.

In nearby White Plains, NY, the lights are timed, but not in any scheme I can identify. All I can say is that if you take the same route all the time, you see the same light pattern, though it does get out of whack sometimes.

Other lights might be timed, but triggered by a sensor, or just timed by themselves, not in relation to anything.

The lights that are timed in relation to each other are probably not all that complex, but they DO need to communicate with all the other lights, that might be prohibitively expensive.

In general it’s not possible to time an arbitrary arangement of lights. It’s mathematically impossible. The problem is “overdetermined” and any arrangement that makes, say, 4 adjacent north/south streets flow smoothly will cause traffic mayhem on the 4 east/west cross streets.

in fact, due to left turns, you can’t even be sure that synchronizing for smooth flow say, northbound, won’t create a mess for southboubound traffic on the same street(s).

So in addition to the city-to-city variation others mentioned, even if you’re in a city which has tried to synchronize you’ll see great variation in apparent synchrony depending on which way you’re travelling

I’d wager that (at least in the US) for all but the densest areas (e.g. Manhattan), very few of the lights are syncronized in any meaningful way at all.

I understand that the lights could work alone, but they have to at least ‘respect’ each other. If they didnt, what would be the point to having them installed? Also, I think a huge part to traffic lights working is the human aspect. For most of the time, we all understand and obey, mostly out of safety. What Kindergartener doesn’t know what a red light means? ((Any utterance of that TAXI Line about yellow lights and the postee will be sacked :wink: ))