Explain xkcd: It's 'cause you're dumb.
Title text: There's an intersection I drive through sometimes that has a forward green arrow, a red light, and a 'no turns' sign all on one pole. I honestly have no idea what it's telling me to do.
This comics is an animated gif which features an array of traffic lights which are lighted signals posted at intersections to control vehicular traffic. The standard North American traffic light has three solid lights: red, yellow and green (meaning, in simplistic terms, "stop", "stop if you can, the light will be red shortly" and "go" respectively). More complicated lights sometimes have additional signals such as arrows indicating go or stop for a specific direction such as left or right turns.
However, as in the strip, sometimes an intersection has multiple lights with certain of them designated to apply solely to a specific lane or specific direction of travel. A common one is a left-turn light that allows the lights to stop or allow left-turn traffic independent of the rest of the traffic. Another common example is a light that applies only to transit like streetcars which run on tracks on the city streets.
There are also other rules and features that tend to be unique to different localities as noted in The Wikipedia article for Traffic-light signalling and operation.
In this comic, Randall is commenting on the confusion that can be caused by having too many lights with multiple rules attached by creating an over-exaggerated example.
In this strip, the right-hand light has a sign indicating that the light and the right-hand lane are for left turners, while the 3rd-from-left is a straight or right turn lane and the 2nd from left is right turn only. In normal course, right turns would be permitted from the right-hand lane and left turns from the lefthand lanes. The system in this comic would have turning traffic crossing each other, as well as the straight-ahead traffic and would cause chaos (and require very complicated traffic light phases to control). The left-most light on the post has a sign indicating that left, right and straight travel are all prohibited, which is even more confusing.
The comic, as an animated gif, cycles through various phases, at first appearing somewhat normal, but then adding unusual phases. The animated gif takes about 90 seconds to cycle through the 32 discrete panels before repeating. The left-hand post light has (unusually) left and right arrows, later becoming up and down arrows. At times the light completely shuts off, and at other times, has conflicting signals.
The third-from-left light has red and yellow, and later all three lights come on at the same time, then all three lights go yellow, and then reverse with green at top and red at bottom. The bottom light then becomes an arrow.
The fourth-from-left traffic light switches from a green light to a purple light at times.
The right-hand light only lights red in each position.
The second-from-left light and second-from-right lights do not appear to have any quirks other than changing phases in unusual patterns.
Frame by frame breakdown
The following is a breakdown of all of the frames of the animated gif comic.
Closeups on the Lights
The following is a closeup on the lights, and the bird. The lights from left to right are here shown top to bottom. Time advances to the right.
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I doubt that this comic carries any deeper meaning. 126.96.36.199 16:29, 4 October 2012 (UTC)
Traffic signals really are one of the most inscrutable inventions to ever be made. I mean, red means stop, yellow means caution, green means go. Unless you're in a turn lane, which means you have to watch for the green arrow, if there is one, or wait for an opening. Unless you're turning right, which is permitted to turn at any time providing there aren't any cars. Unless there's a sign that says "No Right Turn On Red". There's also the crazy cities that have special right turn lights. Then there's the intersections that have a sign for each lane telling what can and cannot be done. Or, if you're really lucky one of those intersections that has the LED screen that dynamically changes what the lane can and cannot do. And to top it all off, the Colorado Department of Transportation (as well as a few other states I'm sure) are testing out a 4-stage left turn light to increase the safety of drivers.  Yes, you read that pamphlet correctly. There is a special 4th light, just to blink yellow, because you couldn't just make the yellow light blink, like it does anyway after 9pm. No. There has to be a whole special light that indicates when a left-turn-er must use special caution to turn.
- Note that New York City doesn't allow right turns on red, unless a sign is posted that says otherwise. I suppose this helps reduce the incidence of pedestrians being run into/over. You might also think it helps keep vehicles out of crosswalks, but it doesn't. CityZen (talk) 01:13, 6 October 2012 (UTC)
- What about turning left on red from a one way street onto another one way street? I don't know how prevalent that is but it's fairly common in downtown Columbus Ohio (my locale) and to a lesser extent some smaller towns around here. 188.8.131.52 12:30, 9 August 2013 (UTC)
- First time commenting. Not sure about the reply syntax. But @ Ohio person: Up here in Michigan, we have supposedly special U-turns on divided highways (not to be confused with expressways; I'm just referring to the main business roads.) If there is a light, you can treat it like turning right, even though it is an apparent left-turn and an actual u-turn. "So long as you don't cross a lane of traffic, it is legal to turn if the lane is clear unless posted otherwise." From my driver's ed instructor. This does not apply to the second turn lane. I cannot tell you how many times I've been honked at for obeying the law and not turning right from the left lane. Sorry for any terrible typing or messups. I'm doing this on my phone. 184.108.40.206 08:42, 20 November 2013 (UTC)Dartania
Nope. I'm done driving. Bring on the self-driving cars, people are officially idiots. lcarsos (talk) 16:52, 4 October 2012 (UTC)
- There is a problem with a blinking yellow arrow. Blinking yellow is already used. It means you have the right of way, but the other direction may proceed as well. What is wanted here is blinking red, which means that you stop, but may proceed, as someone else has the right of way. As for adding a fourth light, this just confuses things even more, particularly with respect to color blind individuals. Having said that, Virginia's variant is to have lights with both left arrows and solid greens. If it is green arrow, you have the right of way, while solid green means the other direction also has a green. There is almost always a sign reading "left turn yield on <solid-green-circle>" Divad27182 (talk) 18:19, 4 October 2012 (UTC)
- so the blinking yellow left arrow generally means the same as a round green light - you can go when it's clear, but the opposite traffic has a green light too. I love this idea because a lot of the time where there is a separate left-turn signal, there are advanced lefts for both ways, and then left turners get a red left arrow and have to stop while people going straight get a green - so even where there's no oncoming traffic, you can't turn. this way, you just lose your "advance" status and go back to the regular rule of "turn if you can, otherwise you end up turning as the lights go yellow"
- I think what Lcarsos is pointing out if you check the PDF link, is that the new left-turn lane has 4 lights, all of which are left-arrow shaped. Thus, his point is why couldn't they make the 2nd light (yellow left arrow) blink, instead of installing a third light which is also a yellow left arrow whose job is to blink?
- I can think of a few possible reasons - first, perhaps an all-blinking light has different bulbs which are more efficient for blinking and won't burn out as much; second, because they want to distinguish between the two lights more strongly (i.e. if you glance over between blinks, you don't have to wait a moment to see if it will blink - if you see the 3rd light up, you know it's going to blink without waiting for it to actually blink); similarly, I suppose there could be colour blindness issues where they want to make it clear which light is which.
- PS: Right on Red is more common in north america than other places, but even here there are a few exceptions that do not allow right turns on red lights. New York City is one. Montreal is another. Most of Mexio is a third. TheHYPO (talk) 21:19, 4 October 2012 (UTC)
- You can also white a white 'cigar' light that is inteded for busses only.
- From the first traffic light in London 1868 until standardization in the 1920s people tried out many crazy lights (see http://dx.doi.org/10.1109/T-VT.1970.23426) including---literally---bells and whistles to announce changes. It seems this phase is still not really over.220.127.116.11 10:33, 5 October 2012 (UTC)
Hmm... not one to do the actual analysis, I still wonder whether there could be some message encoded in the pattern of lights -- in binary ASCII, baudot, Morse, or something. Hmm... 18.104.22.168 19:12, 4 October 2012 (UTC)
This explanation mentions a lot of hands where no hands exist. "The right-hand lane," for example. This is a pet peeve of mine. People just adding the word, "hand" randomly into a sentence. It's superfluous and it's annoying. It should be removed before someone slaps whoever did it with their right-hand hand.22.214.171.124 14:35, 14 June 2013 (UTC)
- I believe "right-hand" is used to differentiate "the opposite from the left" from "the opposite of wrong". If I say "get in the right lane", there is a chance for confusion and/or a cliche joke. If I say "get in the right-hand lane", my meaning is clearer. -- Tryc (talk) (please sign your comments with ~~~~)
- Red and yellow
In Poland you can sometimes stumble upon red and yellow -- while yellow alone means that there shortly would be a red, and you can proceed if you are at or almost at crossing, but stop otherwise, red and yellow is to mean that there shortly would be green (go), and to prepare oneself. But it is quite rare. JakubNarebski (talk) 06:45, 5 October 2012 (UTC)
- In Germany all traffic lights behave this way. Joha.ma (talk) 07:45, 5 October 2012 (UTC)
- At least some lights in Czech behave this way too. -- 126.96.36.199 08:04, 5 October 2012 (UTC)
This is the way ALL UK lights behave.
Some lights in the US use red and yellow to mean that a pedestrian is crossing, although I've only seen this when the light is blinking red/yellow to start with (yes, I've seen full 3-bulb traffic lights that are only used to flash red/yellow). Zer0keefie (talk) 11:42, 27 March 2013 (UTC)
OK, here is the difference between the US and Europe:
Originally I believe all the traffic lights went through the sequence (1) green in one direction, red in another, (2) yellow in both directions, meaning "clear off the intersection" for the first direction and "get your car into gear" in the second direction, (3) red in one direction, green in another.
There are two problems though: first, when the green changes to yellow, people try to squeeze through (and it may even be difficult to stop if you're traveling at speed), second, when the red changes to yellow, people who are not stopped but carrying speed are trying to squeeze through earlier while the light is still yellow (this gets worse if the yellow is long and the incoming drivers don't know if it's after red or after green), and collisions ensue.
The solutions for these problems taken in US and Europe are different. In Europe the yellow-after-red is shown together with the red still on, strongly suggesting "no, you may not enter", and in the other direction the green often blinks once 10 seconds before it switches to yellow. In US there is no yellow-after-red, the red changes directly to green, and yellow always goes only after green (the automatic transmissions being prevalent, there is no need to shift into gear). The yellow is often long, to let the traffic on the fast roads to clear off, Also, there is usually a period of red in all directions which lets the stragglers clear off the intersection for sure before the other side goes green. This is why slipping on red just after yellow had ended is considered no big deal in US and a major no-no in Europe. 188.8.131.52 23:15, 23 January 2014 (UTC)
- Where I live, in Santa Cruz do Sul - Rio Grande do Sul - Brazil, we have no yellow before green, but very few traffic lights blink red once right before green. Here, slipping on red is also common, but it's also common to stretch it a bit more and confusion ensues. Continuing from what has been said below, some traffic lights in Porto Alegre even have a numeric countdown.184.108.40.206 20:06, 9 February 2014 (UTC)
In Brazil some cities have greens and red in a row, and they decrease to indicate how long the green or red will last. something like http://www.guiasjp.com/fotos_noticias/foto_1165344648.8822.jpg 220.127.116.11 20:22, 5 October 2012 (UTC)
In BRazil too, in Goiânia there's a nice one here for pedestrians, with leds, that show an animated pedestrian walking and when the time is running out HE RUNS FASTER! Like, it's about to go green for the cars, but feel free to cross... IF YOU'RE FAST!
In Tianjin China they have a traffic signal that is a single bar of light. If it is green it starts subtracting bar length segments. When there is about a quarter left it turns yellow and then red. It then start subtracting bar length segments from the other end until it gets to about a quarter length and then turns green again. http://www.flickr.com/photos/bob406/3428844012
It's how traffic engineers troll you when you are going the wrong way on a one-way. Learn to read signs!
- Reference to previous comic
This comic has some tongue in cheek self reference to 277: Long Light. #Meta And definite trolling, by Randall
- What about 781: Ahead Stop?
- Turing Machine?
Any chance it is one?
I was almost expected to see the Konami code in the left light, though I'm not sure how "B A Start" would have been shown. CityZen (talk) 01:13, 6 October 2012 (UTC)
I am no musician, but I can't help but wonder if there could be a hidden music chart in there somehow.
Also, if you want confusing, try understanding parking signs in Montreal.
- Might be, but depending on what pitches or sounds you assign to the lights, you could get pretty much every degree of harmony or cacophony you want. - Another possible music reference: Anybody reminded of Hendrix' "The Wind Cries Mary"? "The traffic lights, they turn, uh, blue tomorrow." 18.104.22.168 08:14, 10 September 2013 (UTC)
- And You thought 4 stage was bad enough
Here In BC, in older installations, and I'm not sure whether any still exist, there used to be lights with 5 stages. It wasn't really any different than the standard red-yellow-green-turn combo you generally see where the turning light may come on, it just had the yellow arrow shown when the turning arrow is about to expire in it's own lamp. It behaves pretty expectantly, but it looks very imposing.
- Race cars
The second light has quite a resemblance to the "christmas tree" that governs the start of a drag race, where the lights change colors according to a pattern http://www dot nhra dot com/nhra101/basics.aspx 22.214.171.124 04:32, 27 December 2012 (UTC)
I sat and watched this increasingly surreal show for about three minutes, then I broke down laughing when one of the lights turned purple. 126.96.36.199 06:09, 11 September 2013 (UTC)
In UK, ALL lights go Red -> red AND yellow -> green -> yellow on it's own -> red again. Red and yellow means it's about to go green and yellow on it's own means it's about to go red. 188.8.131.52 21:43, 8 November 2013 (UTC)
What is the bird doing? 184.108.40.206
19:12, 18 August 2015 (UTC)