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Alicorn comments on Calibration fail - Less Wrong
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People are just bad at guessing distances and sizes of things that aren't medium-sized physical objects at roughly eye level. Guess how big traffic lights are?
*thinks*
I've walked under traffic lights and looked up at them - from that angle, I'd guess each lens might be ... 0.2 m to 0.5 m diameter, 95% confidence? Which is larger than they feel (note scare-italics) when you're sitting in the car across the intersection.
Apparently they vary in size, but one source claims they can get to be seven feet tall.
I've seen them taken down for repairs, and they're much larger than I'd thought, but not 7 feet tall. Maybe 4' IIRC?
The lenses are either 8" or 12" in diameter in the US: http://mutcd.fhwa.dot.gov/htm/2003r1r2/part4/part4d.htm
So if you're looking at 3 8" lenses in a row vertically, the whole thing is at least 3' tall, probably more.
I just looked through hundreds of photos of traffic lights on google, and not one was of the standard yellow US kind that hangs from a wire.
Inference: your neighborhood (or your belief about it) is not representative and yellow and/or hanging from a wire is not standard. I was surprised by the diversity of results on google. On the first page, there were no yellow-on-a-pole and just two cantilevered yellow ones. Bing put a cantilevered one first and had a couple on wires[dead] in the top 20. But they weren't yellow! They do seem to be in the US. Maybe traffic lights aren't yellow?
ETA: Bing's "similar images" produced a few yellow lights on wires, but it was still dominated by the familiar green and gray.
Ah, that's more authoritative than Wikipedia - thanks for the link!
7' traffic signals, if they in fact exist, are probably reserved for intersections where the lights are hung up unusually high or something. I'm skeptical that the kind you see all the time are really seven feet.
Some traffic signals have several (I've seen up to 6) lights for different directions arranged in a vertical column, I'd guess this is what your source was referring to. For some reason these are really hard to find on google images, but here's one.