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Traffic lights around the world use the yellow light to warn people that it will soon turn to red, so that people can stop before. This is perfectly understandable.

However, I found out that in some European countries the yellow light is also shown when going from red to green.

Can somebody tell me what's the purpose of this? From my judgement, it is completely unnecessary. It's not like it is warning drivers of any danger. Do drivers need a sign that tells them to "prepare to go, but don't go yet"? And why would they need it?

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Implying that software user interfaces aren't "real world"...? ;) –  agib Dec 30 '11 at 15:38
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It might be the force of habit. A few decades ago, when less-reliable cars were abundant, a few seconds notice was useful to let the driver rev up the engine and shift into first gear without stalling. Sudden-green creates an unwelcome sense of haste if you're driving a jalopy. –  dbkk Dec 30 '11 at 15:47
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Whatever the reason, I wish we had this in USA. I am tired of looking at the side to look for when the crosstraffic light has turned yellow. Do I need it? No. But if you're impatient it's a nice FYI. –  user606723 Dec 30 '11 at 18:51
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I think it's automatic (us) vs manual (hello europe) transmission. –  mreq Dec 30 '11 at 22:40
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@djeidot: that means you'll have to be fully concentrating at the light the whole time or you have to be familiar with the light timing on the intersection. If you have yellow (amber) between red->green light, you have time to relax for a while with your hands away from the wheel and your breaks engaged; with sudden green you have to be prepared all the time (lest you be caught surprised by the sudden change). –  Lie Ryan Dec 31 '11 at 12:34

14 Answers 14

up vote 43 down vote accepted

Do drivers need a sign that tells them to "prepare to go, but don't go yet"?

That is exactly what this signal is for. It's to stop drivers being surprised when the light turns green and gives the driver time to prepare to move off:

Red and Amber— this combination of bulbs indicates that the lights are about to change to green, and gives drivers time to release their handbrake and prepare to drive off as soon as they are allowed to do so. This phase was first introduced in 1958.

Source

Note the part about releasing the hand brake. Most drivers seem to ignore this, but it is part of the UK driving test that you engage the brake while waiting at lights.

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Most people take the car out of gear, too. –  DisgruntledGoat Dec 30 '11 at 23:59
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Interesting. I've never seen anybody use a hand brake on a light, be it in the U.S. or Europe... –  houbysoft Dec 31 '11 at 3:26
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I've used a handbrake at lights in both Europe and the US. –  DJClayworth Dec 31 '11 at 4:15
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You'd find it tricky in a manual to not use the handbrake if you're on a hill facing upwards. Either that, or your clutch won't last long. –  Jeremy McGee Dec 31 '11 at 10:22
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@Espen - you have to take your foot off the brake to use the accelerator. Unless you are very quick you'll slip back if you try to swap from brake to gas. So you use the handbrake then bring the clutch up to the biting point while "stepping on gas". This holds the car instead of the handbrake. –  ChrisF Feb 6 '13 at 19:46

in some countries you are encouraged to turn off the engine on red (to reduce pollution), so the yellow before green tells you that you can turn it on again and prepare to go.

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That would cause a lot of additional wear and tear on the vehicle's starter! Personally, I would be very annoyed if my vehicle or the one in front of me failed to start again because it had been restarted 10+ times per trip. –  jimp Dec 30 '11 at 21:03
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@jimp I would be annoyed if my car's quality couldn't cope with the additional starts and stops. In fact, most modern (European) cars feature a Start-stop system –  Flow Dec 30 '11 at 21:31
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As long as you are wating for more than about 15 seconds, most modern cars save petrol by turning the engine off. The starter should cope just fine. Older cars - not so much, maybe :-) –  Rory Alsop Dec 31 '11 at 1:52
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I would find such an indication helpful. My car shuts the engine off at stops automatically, and having a bit of advance warning before the light turned green would permit me to restart the engine (by easing off the brake) a bit sooner, making for a much smoother start. –  David Schwartz Dec 31 '11 at 3:38
    
@David: Is this a mechanical problem or an intentional feature? If it's the latter, is this something that has been around or only in certain new cars? –  Lèse majesté Jan 2 '12 at 16:34

As far as I recall, (I need to check in "the real world") here in Denmark at least, it's like this:

  • From Green towards Red: Yellow alone.
  • From Red towards Green: Red + Yellow at the same time

This way the two "modes" can be differentiated while maintaining the "change warning" behaviour.

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yes that's the correct behaviour and it works like this here in Slovenia too. But it doesn't work like this in Italy. In Italy it goes straight from Red to Green and since I just returned from there I can tell you it's quite annoying. It's easier for the eye to register both Red and Yellow turned on, than to register the movement from Red straight to Green. You lose the feeling of how long the Green was already on and it actually happened to me that drivers behind me were beeping. The effect being, I kept my eyes focused on the lights from there on and I don't think that's a good practice. –  mare Dec 30 '11 at 16:17
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How is that the “correct” behavior? FYI, it does not work that way for most of the european population. In fact, there are many variants in traffic light sequence: en.wikipedia.org/wiki/… –  Gala Dec 30 '11 at 18:56
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Since it is a better way it is the correct behavior. Basically saying, "it's great, everyone else catch up already". I think timers would be even better though. –  s_hewitt Dec 30 '11 at 20:47
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It does distinguish between "about to be red" and "about to be green". Which is important if you are approaching them. –  Schroedingers Cat Dec 31 '11 at 11:13
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Even if it was somehow better, breaking expectations is hardly the “correct” behavior. From a UX perspective, consistency is certainly a lot more important than the slight advantages any particular variant might have. –  Gala Dec 31 '11 at 23:29

Efficiency could play a factor.

For example: Assume two cars are stopped at different red lights. The first light goes directly from red to green. The other light goes from red to yellow to green. Also, the driver in front of the second light knows from experience how long the yellow light will last. Finally, both lights will turn green at exactly the same time.

Now, ask yourself the following questions:

  1. On average, which car will go first?
  2. By how much?
  3. How many times do people stop at red lights per day in the world?
  4. What is the world travel-time difference if the world used one method vs. the other?

Now, being American, I see only red-green lights on a daily basis, and so I am used to them. Thus, I treat the cross-street's light turning red as my "yellow before green" cue, and know that it'll take about 3 seconds before I get the green. However, that makes my neck a bit sore, so maybe it is a chiropractor-conspiracy for why countries like mine do not do this. However, lousy jokes aside I'd opt for red-yellow-green scheme if I had the choice.

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+1 - wanted to say the same thing: you get a little bit more cars over during the green phase, which may be good when traffic volume is high. –  Martin Dec 30 '11 at 19:41
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Incidentally, in Europe, most of the time, you can't see the cross-street's lights. –  Gala Jan 8 at 9:21

Here in the UK we have a nice 'relaxed' (ie quite long) yellow phase between Red and Green:

http://www.youtube.com/watch?v=Zn-woMkK9is&NR=1&feature=endscreen

It's a bit like 'Ready, STEADY, Go' - rather than 'Ready - Go'. It gives you a bit more time to get ready to go without feeling stressed about it.

I've found driving in countries where it goes straight from red to green a bit more stressful.

I suspect it reflects 'cultural characteristics (conformity / impatientness / etc)' of the country.

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it's more like 'Halt, Go' vs 'Halt, Ready, Go' –  Lie Ryan Dec 31 '11 at 12:27

To play devils advocate, the Red-Yellow > Green pattern of lights can be more dangerous than a plain Red > Green. With the Red > Green pattern, When you are approaching a red light, unless you know the intersection very well, you must make the assumption you are going to come to a full stop.

With the Red-Yellow > Green pattern, you are given warning that the light is about to turn green, which in turn means that you can anticipate the green light and not brake as hard, or perhaps even realise you have time to speed up or not brake at all, which means you would reach the intersection sooner than you otherwise would.

This is a problem if someone at the crossroad is impatient and runs the red light at the very last second. If a Red>Green scenario, the driver approaching the intersection will typically have a much lower speed, meaning that the impact of any accident is severely deceased, perhaps even avoided. If the approaching driver anticipated the green light, the impact would be much worse for both parties.

As an anecdote, there was a Red-Yellow > Green set of lights near where I used to live in Melbourne, Australia. It was the last one left in the city and was left-over from the 1960's. They operated perfectly, but for some reason the council changed it to a Red > Green cycle in the early 2000's. Don't know why. And for the record, I do what everyone else seems to anyway which is watch the lights on the road to see when they go yellow, and would love the Red-Yellow > Green pattern myself.

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Traffic lights (in Germany, anyway) have an phase were all lights are red, as to reduce this security risk. The duration of this phase depends on the size of the intersection, because it's at least "the time time to leave the intersection when entering it at the moment the light turned red". So the additional Red-Yellow gives some security margin, actually. –  giraff Jan 2 '12 at 15:01

At holiday in Asia I've seen a countdown indicating how long the actual light keeps till next change.

So you see how long it will be green and how long it will be red. I really love that idea. In Germany most people would act like Vettel, though.

enter image description here

Source: http://tripwow.tripadvisor.com/slideshow-photo/count-down-timer-on-traffic-lights-by-travelpod-member-flash101-bangkok-thailand.html?sid=14706062&fid=tp-3

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Also Bulgaria has some counters like this. –  finnw Jan 5 '12 at 22:23

I'm living in germany and we do have the 3 step traffic lights everywhere. As for the first point, I completely disagree with ALoR's answer. It's simply wrong.

in some countries you are encouraged to turn off the engine on red (to reduce pollution), so the yellow before green tells you that you can turn it on again and prepare to go.

Turning off the engine won't save petrol at all, not even on large crossroads. As a direct result of this it also doesn't reduce pollution.

You can think of it "when the (us?) traffic light is still red, the european already has turned into yellow". From my driving licenses' book:

Auf die Weiterfahrt vorbereiten

Which means basically

Prepare to go on driving

The main advantage is: You can start quicker. This may sound a bit silly, but it has a large effect on traffic. Im very curious the 3-state-traffic-light isn't there all around the world.

You can spare time, gas and reduce traffic density. I think it's a fine addition to the red-green-traffic-light.

Information source: My driving instructor & experience :)

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Actually I noticed this in Germany too :) –  djeidot Dec 31 '11 at 1:04
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Turning off the engine does save petrol on any stop over about 15 seconds, for most modern cars. And thus pollution as well! –  Rory Alsop Dec 31 '11 at 1:54
    
@RoryAlsop Even if that would be a point for turning off the engine, it would be a very inefficient one. Spark plug and many other parts are getting very worn if you turn on and off the motor on every crossroad. I don't really want to know the average $/(saved pollution). You probably search on the wrong place there. Additionally, no serious guy really turns off the motor - it's simply too inconvenient and how can't say how long you will actually stop. –  Michael Dec 31 '11 at 9:07
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@Michael - no, actually all the major global manufacturers are starting to develop automatic power down/restart systems because of the savings in fuel and wear. This has been around for some time in Europe and Japan, and is now available on Ford SUVs and other vehicles even in the US. Your spark plugs and other parts may suffer on old inefficient engines, but not on new cars. I can't believe you find the couple of seconds an inconvenience - the yellow before green gives you ample time to start the engine. –  Rory Alsop Dec 31 '11 at 15:04

I think this is simply caused by most European cars using manual transmission, unlike cars in the U.S., which usually have automatic transmission.

When stopping on a light with a manual transmission, you might shift into neutral for the wait. When the light turns yellow, you'd shift into first, and then when it turns green, you'd start going.

With an automatic transmission, you simply hit the brake and then gas, so there's no need for the extra step (shift into first) like in manual transmission cars.

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If the US users would have the yellow light, they could start to press the gas pedal at yellow and release the brake pedal when the red goes off. ;) (Of course this would require the usage of both feet) –  Mike L. Jan 1 '12 at 8:51

Traffic signals sequences often date back at least to the 1960s so it's often difficult to find relevant documentation and to know what the original intent was. Once a particular sequence is established in a country, it tends to stick and for good reasons also (e.g. cost of changing existing traffic lights, training/information need and driving errors created by any potential change, etc.) Even if it is tempting to consider the particular sequence you learned as the “correct” one, the value of consistency and stability should not be underestimated.

Still, there is quite a lot of research on the effect of various signals on traffic, using either models of traffic flow or simulator studies (field studies and especially before-after studies seem somewhat rarer nowadays). The particular sequence you describe is known as “starting-amber” and it has been found to increase crossroad capacity but also to increase the number of people crossing while the light is red.

An overview of this research with some relevant references can be found in this report of the TRL in the UK: http://www.trl.co.uk/online_store/reports_publications/trl_reports/cat_road_user_safety/report_literature_review_of_road_safety_at_traffic_signals_and_signalised_crossings.htm

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A French friend of mine once answered my asking of this question by saying that their light durations tend to be quite long, so the norm is to switch your car's engine off while you wait. The amber warning is then used as an indication to start your engine again and prepare to disembark.

Here in South Africa our standard light durations are relatively short, in the 15 to 30 second range, so it seems odd to us, but it could make sense if your light duration is 30 seconds and longer.

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After talking to my car racing brother, the joint theory is that this order of lights is leftovers from the manual transmission days. The yellow signal gives extra time to regain attention and switch into the first gear from neutral. Unlike automatic transmission, the process is multi-step and can result in a stalled engine if the steps aren't followed.

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When I drive a manual transmission and make a stop, I shift into first so all I have to do to start is release the clutch and hti the gas. Is that not normal? It doesn't seem to be any slower than with an automatic, though it's true that less-experienced drivers may stall and that would be a delay. –  Monica Cellio Dec 30 '11 at 17:31
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You can keep the car in the 1st gear while stopped but you have to keep your foot on the clutch pedal. When the car is in neutral, no action is necessary - some drivers relax their legs at traffic lights. –  dnbrv Dec 30 '11 at 17:38
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Manual tranmission days? LOL Where do you live? My car is still very much manual transmission and I am unlikely to switch to automatic transmission at any time in my life... :) –  Marjan Venema Dec 30 '11 at 18:01
    
I read the same explanation on Wikipedia but I am not entirely convinced. With an automatic transmission, you need either to press the brake (if the road is flat) or switch to N or P (if the road is not flat and you want to relax your leg), so you always need to switch pedals or change gear/mode. Both can be done quickly if you have some experience and both can go wrong if you are not used to it (true you can't stall the engine with an automatic but if you drive for some time in a country where manual transmissions are common, you will notice that this isn't really a common problem). –  Gala Dec 30 '11 at 19:05
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The other argument to switch to neutral is safety -- if you accidentally release the clutch, your car might make a small jump forward, which could be thought of as a bad idea if pedestrians are crossing at the same time. –  Simon Richter Jan 2 '12 at 9:36

Wether by design or accidentally, I think the yellow signal has a greater impact on pedestrians than drivers.

In cities where lights go straight from red to green the crosswalks are full of stragglers when the signal changes. When there's a yellow signal in between drivers are already revving their engines and green really means go. From my personal, wholly unscientific observations, pedestrians quickly learn to respect and follow the crosswalk signals when they see that drivers have the yellow transition light.

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I don't follow that. Pedestrians have flashing green lights warning that it's going to turn to red, and the red light for pedestrians shows up 3-4 seconds before the green light for cars shows. Plus, in most cases pedestrians can't see the traffic lights for cars. –  djeidot Dec 31 '11 at 1:07
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Flashing green lights are not available, for example, in Germany (but e.g. in Austria). –  Mike L. Jan 1 '12 at 8:50

Baking FRESH ALASKA

The purpose of the Red-to-Yellow-to-Green system is to increase throughput, due to FRESH ALASKA!

If you don't know what that is, you shouldn't feel bad, as it's because I just reverse engineered it to fit my answer. I will setup and define both acronyms below. I will prove that in a Red-to-Yellow-to-Green system, more cars will be able to go through the light because more drivers will be prepared to go in according unison.


First, let's look at the opposite to demonstrate its defect:

Red-to-Green

Imagine an infinite number of cars waiting at a red light. We'll label the first car, "Car A", the second car, "Car B", and so on all the way to infinity.

At the instant the light changes from red to green, Car A and Car B have a special relationship that exists for every successive pair:

  • For some time X, Car B must wait for Car A to move, since it is not sure if Car A knows the light is green.

X is normally a fraction of a second, but it must exist. If it did not exist, then Car B would step on the gas as soon as the light turned green, assuming Car A was immediately aware that the light was green.

Again this carries on for the B-C relationship, the C-D relationship and on into infinity.

Let's call this FRESH, or:

  • Fractionally Related Effectual Summation of Hesitation

You can guess that as we move toward infinity, all of these fractions of a second add up and the end result is a particular average number of cars making it through the light.

  • Let's call this average the "Red-to-Green Average Throughput".

FRESH, then contributes negatively to the Red-to-Green Average Throughput.


Now, let's take a look at the competing model:

Red-to-Yellow-to-Green

Reset the infinite number of cars scenario with everything labeled the same way. The special relationship of pairs still exists, as Car B will still not step on the gas until it is sure that Car A is moving.

This time, however, Car B will be more confident that Car A will be moving when the light turns green because of at least three factors:

  1. Car A has a high likeliness to know that green is coming because of the yellow transition stated.
  2. Dependent on the first factor being a success, Car A is more likely to be alert and therefore prepared to move (Important if they are driving multi taskers).
  3. If the driver of Car A is an every day driver at this light, they are more likely to know the pre-red yellow duration due to familiarity of the yellow light time.

For these reasons, ALASKA can be introduced as:

  • Accorded Likeliness Anticipating Surely Known Alertness

For those in the real world Red-to-Green system, how many times have you been in the position of Car B when the light turns green, only to look ahead at Car A and notice they are head down looking at their cell phone or doing their makeup or eating a sandwich? They could be doing any number of things and have no time to prepare to drive. I would be willing to bet that this happens more often in the FRESH driven Red-to-Green system than the ALASKA driven Red-to-Yellow-to-Green system.


Conclusion

Because of the negative effects of FRESH and the positive effects of ALASKA, I now feel comfortable in saying this:

  • Red-to-Yellow-to-Green Average Throughput > Red-to-Green Average Throughput

This explains the purpose of the Red-to-Yellow-to-Green system as a means to move more cars through the light for any given time when compared to the Red-to-Green system.

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-0, for lame abbreviations. –  Lie Ryan Dec 31 '11 at 12:31
    
That was a fun read. Thank! –  Evik James Jan 2 '12 at 17:14

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