# Traffic lights: “yellow” is progress, not state

When thinking about how (real-world) traffic lights work, I wonder if there is a more intuitive way to display the yellow light. The meaning of Green and Red are states:

Green: Ok, you can go!
Red: Not ok, you need to wait!


However, yellow indicates progress:

Yellow: Attention, the light will become red in an instant!


(And depending on the country, it may mean as well:)

Red+Yellow: Just a second, Green (or Red, in other countries) will follow in an instant!


This can be judged as imperfect user experience, as "instant" is somewhat unspecific, and similar methaphors (lamps) are used for different concepts (state vs. progress). (The "user" here is the driver of a vehicle.)

Now, of course, traffic lights are heavily institutionalized, it would be very difficult to establish a different norm. But are there variations, design studies or research of traffic lamps that try to adress this problem?

• Actually I think this is a great question. I love to read on people's different opinions about this. :-) – Bart Gijssens Sep 15 '11 at 11:39
• "Red stop, green go, yellow...go very fast." – LarsTech Sep 15 '11 at 11:43
• The UK definition of Amber is not as you say "If the amber light appears you may go on only if you have already crossed the stop line or are so close to it that to stop might cause a collision." – user151019 Sep 15 '11 at 11:45
• In Birmingham it's, green = go, amber = go, red = just 5 more cars... – jk. Sep 15 '11 at 11:55
• (In London) The shortest known time in the universe is that time between the traffic light changing to green and the car behind you beeping their horn. – Roger Attrill Sep 15 '11 at 13:14

Check out this rotary traffic signal used between 1938 and 1970:

Slightly less directly connected: ramp meters, and there are definitely lots of variations in operation around the world - and variations in the timings too. Good ol' wikipedia!

• Wow, I didn't know there are so many different traffic lights! Like the idea of an countdown - if they are couting the yellow phase as well? – giraff Sep 15 '11 at 8:27
• @giraff This one looks a bit confusing though :-)traffic light tree – Roger Attrill Sep 15 '11 at 8:28
• Today I would add a glowing to the currently active phase, so that it is easier/faster distinguishable from afar. – giraff Sep 15 '11 at 9:39
• I would totally agree that this approach is an improvement over the current system of lights. OK, something would need to be done to make it visible from distance and in the dark but it provides all the additional information a driver needs: what the current state is and how long it will be until it changes. How many times have you been sat on Red for ages thinking the lights have broken? This solves that problem too. – JonW Sep 15 '11 at 10:21
• Today's traffic light use camera's and other means to detect the amount of cars approaching or standing still before the lights to determine when the light changes color. So I don't think such a thing would still work, except if the arrow's speed would vary but then it loses its whole purpose again. – Bart Gijssens Sep 15 '11 at 11:35

In some parts of Thailand the traffic lights have a large second timer which counts down to the next traffic light change. I found this amazingly useful and intuitive to use and wondered why other countries don't adopt it.

So the light with the timer tells you how much longer you have until the current light changes.

• Wait - the red phase is longer than 100 seconds? Maybe that is the reason they added a countdown? I've seen some of them in Rumania, however only at big intersection with long red phases. – giraff Sep 15 '11 at 9:37
• I've seen them for pedestrian lights in Slovenia, and they are a good idea. In some circumstances, I think they could work, but for an interface, I might look at making the indicator the timer - so a green countdown to a red countdown. – Schroedingers Cat Sep 15 '11 at 10:40
• Is there any sort of research done whether this increases or decreases safety? – Bart Gijssens Sep 15 '11 at 11:37
• @giraf: Yes I've seen a red 240 (4 minutes). And I think this is actually dangerous because drivers tend to speed up when time is almost up in green stage. – Benoit Sep 15 '11 at 11:45
• In San Francisco you can almost always use the pedestrian countdown as an indication of how long the traffic light will remain green. In some cases it's also a good cue for when the protected left turn arrow will appear. I seem to be the only person in the city who makes use of this information, judging by how spastically everyone else drives. – fluffy Sep 15 '11 at 20:33

This site has a design for a light that would count down each color in a circle around the light.

Though not in the site's design, having the red light turn yellow when it got close enough to the end of it's countdown would work for colorblind people as long as the green and the red lights were still separate.

Take a look at this, this one I really like: http://www.yankodesign.com/2010/11/18/sands-of-traffic-times/

It adresses a lot of problems at the same time.

• Modern technology does not require us to have separate lights for each color. It could just be integrated into 1 display.
• It shows progress for all states (red, green, orange)
• It can have a dynamic implementation where the amount of traffic plays a role in the timing.
• The amount of color/light shown is always the same
• You don't need to be able to read to understand how it works. I think the concept is really simple actually.
• Wow, I agree, great concept! But I don't see why dynamic implentation should be easier with this: the "sand" would need to flow faster as well, right? – giraff Sep 15 '11 at 13:06
• This concept is interesting as well: yankodesign.com/2010/09/22/traffic-lights-gets-smarter – giraff Sep 15 '11 at 13:09
• @giraff: let's say the light is red. At that point it calculates how long it will stay on green, based on traffic and/or other traffic lights. Then it turns to green. The sand flows with a speed that is based on the calculation. You are right that as soon as it is on green it cannot update the calculation. Let's say a car gets stuck, the light cannot decide to go to red without changing the speed of flow, that is correct. – Bart Gijssens Sep 15 '11 at 13:20
• I am thinking now... What about color blind people... – Bart Gijssens Sep 15 '11 at 13:20
• @BaGi That was my first thought as well. The positions of the lights on the established traffic signals are VERY important cues for color-blind drivers. Another issue with the "sand glass" is that there's no distinction for the yellow that comes between red->green and green->red - and you don't always come up to a light when it's green. If you come across a light that is already yellow, how do you know if you can cross? – fluffy Sep 15 '11 at 20:35

Not an answer but a comment: A green traffic light is the most dangerous traffic lights of all, because it is the most dubious of all. In the country where I live both yellow (orange in fact) and red mean exactly the same thing: STOP!

The green light means go ahead, but can turn into a yellow one at any time without no prior warning. There is no timer indication of when it will turn into yellow.

The yellow light exists only for those rare cases where in fact you were unable to stop safely at the time the green turned into orange. You are still breaking the law except you will probably not be fined for it.

So in fact the thing you mention about the yellow light is what should be mentioned about the green light. If the green light would show some sort of progress, there would be no reason for having the yellow light any more.

• I would still think the yellow light is useful; just because there may be a timer of some sort doesn't remove the use of the middle status. It would provide additional information and visual cues that the status is updating and reduces the risk of you just becoming blind to the red status ticking along before it goes green - turning yellow is that extra 'jolt' to the senses to inform you something is happening. – JonW Sep 15 '11 at 10:26
• Where I am, the Amber light on its own means "don't start, do finish" - in other words, don't start into the junction, but you have time to complete the junction if you cannot stop. – Schroedingers Cat Sep 15 '11 at 10:43
• @Schroedingers Cat: how is that possible? If you're already on the junction, it means you have already passed the traffic light. – Bart Gijssens Sep 15 '11 at 11:27
• @BaGi If you are committed to the junction - and there are usually advance lights so you can see them - than you can continue through. If you are past the lights, then you will continue anyway. I should have been clearer. – Schroedingers Cat Sep 15 '11 at 11:46
• This is a great solution: youtube.com/watch?v=NNaI7Mw8n6M (only the first part) – Bart Gijssens Sep 16 '11 at 14:07

There is something similar as a countdown on the green traffic light in Austria (I think it's the same in some other countries). It's a very simple yet useful concept:

Before turning from green to yellow, the green light blinks several times.

This means there is no sudden change from green to yellow.

• No, because a blinking green light means something completely different - it means the lane facing the blinking green light is the only lane moving, and can turn left, right, or go straight without yielding. – Alain Sep 15 '11 at 19:52
• @Alain - wow, and I thought traffic lights were almost intuitive ... which country are you in? – giraff Sep 16 '11 at 7:02
• Canada. From wiki: "In some parts of Canada, a flashing green (known as Advanced Green) light signals permission for a left turn before the opposing traffic is allowed to enter the intersection (i.e. oncoming traffic is facing a RED light)." As it turns out, Vancouver doesn't observe this meaning. – Alain Sep 16 '11 at 14:26

A quick sketch for a rotary traffic signal (updated for our century):

In contrast to the original, this is LED-based, not mechanical. The dark colors are LEDs that are off (should be even darker so that the contrast is higher) - from far, only the slice of color is seen.

• It keeps the typical "red is on top"-convention.
• It keeps the "red light"-convention.
• As time goes by, the slice of color decreases, displaying that it will end soon.

EDIT: 2nd version so that red remains dominant even if it will change soon:

• I don't think this will work, when you are near to a transition almost none of the light will be lit. – jk. Sep 15 '11 at 11:57
• Agree with JK, You will almost see nothing of the red light when it is about to turn green. – Bart Gijssens Sep 15 '11 at 12:03
• Also, what about dynamic traffic light that base their timing on the traffic? Whill you suddenly make it run faster, or run slower, or make "jumps"? – Bart Gijssens Sep 15 '11 at 12:03
• i'd go one step further and have the whole of the middle show the current state, with the border showing the transition, otherwise small timed sections will have less visibility (might be critical in fog) – jk. Sep 15 '11 at 14:19
• @jk: Like this: yankodesign.com/2009/11/30/a-better-understanding-of-stoplights ? – giraff Sep 15 '11 at 14:58

I suppose the answer depends on what you are using traffic lights for. If they are indicating - say - the level of orders that have come in compared to last year, then amber indicates "Warning", which does match with the real meaning of the Amber.

So it indicates that there is a potential change, that this is something that may need to be looked at, that there is a cause for concern in this area. In other words, that something is in the process of changing. Green indicates that everything is currently OK, and red that there is some cause for concern.

The actual real traffic lights show amber as an indication that something is changing. You need to know where it has been to understand the complete meaning, but it is an indicator of something changing.

I think there is a good argument - I have seen this argued somewhere - that the simple colour indicators are probably only reasonable for very blunt displays - where something red needs to be sorted, and something amber needs to be looked at. In the same way, a red light means you have to stop, and an amber means you should be prepared to stop or start.

• I was talking about the real traffic lights - will make this more clear in the question. – giraff Sep 15 '11 at 11:23

Personally, I think Yellow is a state just like Red and Green, if you define "state" as in an FSM. It is a "transitional" state between the "resting" states Red and Green, indicating that the light will change to the other "resting" state in the very near-term. The task at that state is to wait 2-3 seconds, then transition to either Red or Green.

I generally dislike the idea of countdown timers in general, even as I find myself using the pedestrian timer where it's available in areas of D/FW. People who don't notice that the light has turned green are generally so distracted they wouldn't pay attention to a countdown, and those who do pay attention would be encouraged to go, go faster, or stop based more on the timer than the light. If you had a timer in red counting down "10,9,8...", you'd hit the gas right as the timer hit zero. Admit it. However, the laws in many states say that whomever is in the intersection who didn't run a red light to get in and isn't blocking it by entering it without having a clear exit has the right of way. Similarly, having a green light with 3 seconds left would encourage you to goose it to catch that light, much like a yellow currently does for most drivers. That puts others around you at risk as your speed increases and your concentration is centered on the light and not your surroundings.

As much as I enjoy rethinking things, the conventional traffic light design has too many things going for it, despite the unfortunate color choices.

While changing the paradigm to a two state FSM may make more logical "sense," there are a few reasons the three state system is in place and why it works.

Which color the stop light is currently is a legal matter. You can pass legally or you can pass illegally; the simple states provide immediate and absolute information regarding whether you are in accordance or violation of the law.

As has been mentioned there are certain fears that a countdown may cause some people to intentionally speed up (much as the yellow light does), whether that's the case or not would require research. However I would never want my users "eyeball" legality; it's legal to do X or it's not. I don't want just anyone to assume "Yes, I can probably legally cross that intersection". It's obviously a problem with yellow lights as well, but this is a case where giving users more information might cause them to make riskier or more poorly informed decisions. While we can clearly indicate exactly how much time remains on the light the matter of whether one will cross the light while red or green is too complex of an equation to ask people to perform accurately.

Secondly, the colors of light is seen as a state, even if it is an indication of progress. All solutions I've seen that make an effort to indicate progress rather than state are significantly more complicated than the simple 3 colors, 3 positions set up, and users are likely to still think of the system in terms of 3 states.

The progress/timer concept also seems to exacerbate, not alleviate the problem of the 3 states of the standard traffic light. The third "yellow" state wavers between "stop" or "I should speed up" depending on the user, giving them raw data to support this claim seems questionable, especially when you're murkying the legal matter of "do I stop or do I go". And as I said before, it's not raw data that's easily and objectively operated on. In traffic things should be as direct and objective as possible.

• Btw, passing on yellow is legally ok in Germany, but not in France. This and all the other country inconsistencies mentioned in the comments make me doubt that something a "simple, everybody in the whole world can understand"-traffic light is a myth. – giraff Sep 16 '11 at 7:06