While driving around town today, I noticed something I’d never paid attention to before: each intersection has two traffic lights, and I can’t see why.
The streets I was on are normal, two lane streets (one lane in each direction) and the intersecting street are the same. The two traffic lights are identical-- just ordinary, three-lights-on-each-side with no turn arrows, or anything like that. They are hung side-by-side on the same line, with maybe a foot or two of space between them.
So, why the need for two? Each light is independantly readable-- they’re not obscured in any way from any of the lanes on either street. I’d wondered if it was because if one burned out, there’d still be the other one working, but down the street, there was a malfunction in one intersection’s lights, and both were flashing red.
I’d imagine these things have to be somewhat expensive, not only to buy, but to power and maintain. The logical side of me argues that there has to be a reason for the redundant light, but the practical side notes that sometimes logic has nothing to do with what the city spends its money on.
Huh? Maybe that would be true if the lights are vertical aligned, but the OP said “They are hung side-by-side on the same line, with maybe a foot or two of space between them.” The redundancy has to be for backup or visibility purposes, I’d guess.
No, the idea is that if the truck blocks the one straight ahead of you, you can still see the one that’s off at an angle.
I can guarantee that they’re not hung a foot or two apart. Those things are huge, much larger than they appear. A strong wind would destroy them if they were that close together.
The change to blinking-red-all-directions isn’t from a bulb burning out. It’s from some other kind of electrical problem. I have seen the bulbs burn out, before, but all the rest still work just fine.
A couple of years ago, I found myself cruising the streets of Niterói, across the bay from Rio de Janeiro, and I noticed how important those extra lights really are.
In that city, the intersections have a single traffic light hanging almost directly over the stop line. It was quite awkward to be in the position of the first car – there was no way to see the light unless you twisted your neck in funny ways. Often, one had to simply observe the cross traffic and perhaps wait for a polite toot of someone’s horn (they are very nice with their horns there).
I figured that the light had to be hanging over the first car for clarity (i.e. stop HERE!), as opposed to the more visible option of placing it on the other side of the intersection.
Actually everybody is right – Dual signal heads is for safety redundancy in case a bulb burns out and for increased visibility. The redundancy is the more critical element. Picture if there is only one signal head, the red bulb is burned out and there is a slight glinting of sunlight off the green lens that can be confused with it being lit – potentially very dangerous. Not being able to see the light until another vehicle moves out if the way is nice, but not a life threatening situation, at least not in most cities.
As TJdude825 mentioned, the red four-way flashing happens if there is a problem for other than bulb reasons. It is a default setting if the power surges or is momentarily cut off, or if the internal signal computer (timing system) has a fault. The big control boxes are very robust, and manufacturers like it that way. I’ve thought that the large control boxes (about 4’ tall, 3’ wide near the intersection) could be replaced with something the size of a cigarette pack, but I was told by a manufacturer that they prefer the proven safety and experience of the existing systems.
The single bulb lenses are, in many areas, being replaced by a multiple LED systems. LEDs are less likely to fail, and if they do, a number of the individual LEDs can fail before it degrades the visibility of the signal.
I’m not sure the construction cost at present, but about 5 years ago on some projects I used cost estimates of $100,000-$120,000 for a full four-way system. Most of this is for the poles and system controller and the addition of four extra signal heads (The grouping of three lenses) for the four approaches is a relatively minor extra cost for most road authorities. This isn’t to say there aren’t lots of arrangements like Zsofia mentioned, but there is increased risk and they shouldn’t be used in areas with heavy traffic.
Well, yes and no. Certainly, they are more than a “foot or two” apart, for the reasons you explained. On the other hand, coming from SE Michigan (where my driver’s ed instructor pointed out that we should always glance at the light that is hung over the oncoming lane in case we were behind a high truck) to NE Ohio (where they are not only bunched together like asparagus directly in the center of the lane at which they are aimed, but hung directly above the crosswalk so that the first two drivers at the intersection cannot even see the light without craning their necks below the steering wheel and staring up), they definitely appear to be just a foot or two apart.
As others have answered, visibility when one is blocked by a large vehicle and redundancy in case of bulb burnout are the practical reasons.
In the USA, the federal highway administration requires all traffic control devices to meet certain minimum standards, though in practice, these standards can be varied by states. The feds fund a large percentage of new traffic signals in some areas so “them that has the gold make the rules”.
All (and more than) you will want to know about signal placement and other traffic devices can be found in and near