Yellow light timing

[DataDan]

You're at a particular point approaching an intersection, the signal green. If you were going straight, you could safely and legally cross at the speed limit, so you would not brake for a yellow. But instead you are slowing and positioning yourself for a right turn ahead. Then the light turns yellow. Do you stop or continue?

As it happened, you continued with your turn but crossed the limit line after the light turned red. You were caught by a red-light camera, and a week later bad news arrived in the mail.

That was, more or less, what happened to the wife of Oregon engineer Mats Järlström. The ticket spurred him to look into the standard for yellow light timing. What he found was that, while it accommodates straight-through traffic appropriately, it does not allow enough time for drivers slowing in preparation for a turn. In fact, even the 1960 paper on which the timing formula was based acknowledged this shortcoming.

Järlström went to work revising the formula to allow for that, and it paid off. The Institute of Transportation Engineers has now published an updated standard for yellow light timing. Flip to page 34 of the ITE Journal for more on Mats' formula. You can also see his own short and mostly readable paper here. For a tangential political view of government traffic engineering fascists see here.

 

[mlm]

Longer yellow lights mean longer waits for green lights. Lady ran a red light plain and simple, but now we ALL get to wait longer :thumbdown

 

[rodr]

Here in Oz there are a lot more roundabouts than traffic lights, and almost no stop signs. Many trips I make without stopping at all. It's great.

 

[mean dad]

Longer yellow lights mean longer waits for green lights. Lady ran a red light plain and simple, but now we ALL get to wait longer :thumbdown

Longer yellow lights also mean fewer accidents.

Don't be so impatient. :laughing

 

[ctwo]

Longer yellow lights mean longer waits for green lights. Lady ran a red light plain and simple, but now we ALL get to wait longer :thumbdown

How's that?

 

[mlm]

Longer yellow lights also mean fewer accidents

I wouldn't think so. People won't know the yellow cycle has been extended so I'd expect most will stop the same.

The delay between red light and green (the time when it's red in ALL directions) would be what makes it safer.

Don't be so impatient. :laughing

Not likely :laughing

How's that?

The only benefit is if more traffic passes through a longer yellow, but I doubt many drivers will adjust, so the extra yellow light time means longer wait for the next green (don't forget the extra delay from the cross traffic yellow too)

 

[bikeama]

In many European countries, the lights cycle "Green-Yellow-Red" and Red-Yellow-Green. No one runs the red as cars are moving as soon as the light is green. If you have time to stop for a yellow stop.

 

[mlm]

FWIW, the OP's articles are pretty interesting. And what he didn't mention is that the engineer who filed the complaint got fined (in addition to the red light ticket) for publicly criticizing the yellow light timing. That part is pretty cool (that he fought that and won), but I still disagree with the need for lengthening the time to accommodate turning

 

[CABilly]

It’s crazy that that Oregon board thought they could muzzle him as “misrepresenting” himself as an engineer.

CA department of consumer affairs is probably similar, in that it’s illegal to represent yourself as a MD, RN, etc without the proper licensing. Disappointing they would be so petty about it, though, so I’m glad they got slapped and I hope they had to cover his legal fees at least.

 

[mean dad]

I wouldn't think so. People won't know the yellow cycle has been extended so I'd expect most will stop the same.

The delay between red light and green (the time when it's red in ALL directions) would be what makes it safer.

In a 2004 Texas DOT study, traffic engineers Bonneson and Zimmerman noted that when the yellow interval duration is set one second longer than the “minimum time” based on the 85th percentile speed, violations decreased by 53% and crashes decreased by 40%.

:dunno

 

[ctwo]

Did anyone analyze or read the article to determine how much yellow time we're talking about? I would casually expect a couple/few hundred milliseconds more at most. A whole second is a long time.

I think I'd prefer a green light countdown.

 

[mean dad]

Did anyone analyze or read the article to determine how much yellow time we're talking about? I would casually expect a couple/few hundred milliseconds more at most. A whole second is a long time.

I think I'd prefer a green light countdown.

5 seconds after the last oncoming left-turner crosses their limit line, your light goes green.

Not every time, but damn near.

:burnout

 

[ctwo]

5 seconds after the last oncoming left-turner crosses their limit line, your light goes green.

Not every time, but damn near.

:burnout

You're saying all lights are red for 5 seconds?

 

[mean dad]

You're saying all lights are red for 5 seconds?

No, that is not at all what I'm saying...and I'm not sure how you got there. :dunno


What I said was that if you count about five seconds after the final left-turner has crossed their limit line, enough time will have elapsed so that your light will turn green.




Nowhere in either post of mine do I imply that all red lights last for five seconds...or even any of them.

 

[ctwo]

So, what causes an intersection to be void of traffic for 5 seconds damn near every time? Meaning, what is stopping the oncoming left turner from entering the intersection two or one second before your light turns green?

Quite a lot of times I have to wait for left turners to clear the way after my light turned green. Almost every day...

 

[mean dad]

So, what causes an intersection to be void of traffic for 5 seconds damn near every time? Meaning, what is stopping the oncoming left turner from entering the intersection two or one second before your light turns green?

Quite a lot of times I have to wait for left turners to clear the way after my light turned green. Almost every day...

Again, you're extrapolating my simple comment about timing a green light to several hypothetical scenarios that have absolutely nothing to do with my statement.
Whether or not it is safe to enter the intersection does not impact the timing cycle...whatever that cycle may be.

I think I'd prefer a green light countdown.

If there was a visible countdown timer facing you, that accurately displayed the time remaining until your light turned green, cars would still be able to illegally enter the intersection so I'm not really sure why you're chasing this so far.

 

[Climber]

5 seconds after the last oncoming left-turner crosses their limit line, your light goes green.

Not every time, but damn near.

:burnout

I counted it the same way, throughout the Bay Area.

Down here in Fresno/Clovis, it's 10 seconds. That's not all yellow, but I time the yellow at 4 seconds.

 

[mean dad]

Late at night under the right circumstances, I've run the occasional red light from being a bit too enthusiastic about timing the light and misjudging it completely. :laughing

 

[DataDan]

The original formula in the article for yellow duration (without Järlström's modification) is:

Ymin = Tpr + v0/2amax ​

In words: Minimum yellow time is the time it takes to perceive and react to the light change plus the time it would take to comfortably and safely brake to a stop at the limit line from the vehicle's initial speed.

Let's say it takes 1 second to perceive and react to the light change, that a safe and comfortable deceleration rate is one-fourth of a g or 8 feet per second squared, and that initial speed is the speed limit.

[table=head]---speed---|yellow time (sec)
25 | 3.3
30 | 3.8
35 | 4.2
40 | 4.7
45 | 5.1
50 | 5.6
55 | 6.0
60 | 6.5
65 | 7.0
[/table]

 

[ctwo]

Again, you're extrapolating my simple comment about timing a green light to several hypothetical scenarios that have absolutely nothing to do with my statement.
Whether or not it is safe to enter the intersection does not impact the timing cycle...whatever that cycle may be.



If there was a visible countdown timer facing you, that accurately displayed the time remaining until your light turned green, cars would still be able to illegally enter the intersection so I'm not really sure why you're chasing this so far.

I'm just not understanding what you're saying then. As I indicated, when I'm sitting at a red light waiting for oncoming left turners, they are usually still in the intersection when my light turns green. The intersection is not clear for 5 seconds after the last left turner crossed their limit line, and before my light turns green. That applies to cross traffic too, but less often. It's why I don't jump green lights.

By green light countdown, I mean the green has a countdown and there is no yellow. It's Green 5, 4, 3, 2, 1, Red. Don't some countries do it that way? Sometimes the ped crossing countdown is a good indicator, but may lead the red light (it gets to zero before the light turns red).

Here a scowlery article:
https://www.hindawi.com/journals/jat/2017/5838520/

and science YO!
https://phys.org/news/2017-10-traffic-countdown-timers-driver-responses.html (although that is a countdown to yellow)

Dan's data supports my understanding of how it works.