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Red Light, Green Light
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2012/10/30
2:01pm
San Diego, California
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Hot traffic talk! A caller is looking for a word for the point at which you have to reach in order to make it through a stoplight before it turns red.

Released August 17, 2011.

Download the MP3.

Photo by Joey Parsons. Used under a Creative Commons license.

2012/10/31
12:20am
Ron Draney
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Did you ever get a compelling suggestion for this request?

I listened to the minicast last year when it was first posted, but it occurs to me now that the term is commit point. The idea is that if the light turns amber before you reach that point, you have to abandon your original commitment to proceed through the light and brake instead. Once you're past the commit point, it doesn't matter if the light changes because you're committed and you're going through.

Whenever my momentum is such that I go through the intersection on an amber light, I say "sure, I could have hit the brakes, but I still would have gone through, and probably sideways."

2012/10/31
5:16am
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When the city where I work debated red-light cameras, I copyrighted and tried to get accepted a caution line. It was a yellow line across the lane that traffic beyond it and traveling at posted speed would make it through the intersection before the yellow (caution) light changed to red (stop). It was a passive version of the active "'RED' SIGNAL AHEAD" signs which are posted in some nearby high-speed signal lights ('RED' indicates that it flashes before yellow and throughout red; SIGNAL AHEAD is always displayed). Multiple lines could indicate different speeds.

I would post images of an intersection and accompaning signage, but they are on my hard drive and do not have an URL.

Emmett

2012/10/31
10:01am
New River, AZ, USA
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These are all interesting ideas, but doesn't the average human reaction time, as well as the mechanical response time of the vehicle, complicate any attempt to define where the commit point is effectively located? Same with the caution line suggested by Emmett.

Add to that the fact that many drivers accelerate when they see the light turn amber, and the reaction distances increase proportionally. See:  http://www.visualexpert.com/Resources/reactiontime.html

Been many years since I had to attend traffic class for a "rolling right turn" I did at a red light. If I recall correctly, in Arizona anyway, if the vehicle breaks the plane of the far crosswalk line (thus entering the intersection) before the light turns red, you did not run that light.

2012/10/31
1:15pm
Dick
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Heimhenge said

Been many years since I had to attend traffic class for a "rolling right turn" I did at a red light. If I recall correctly, in Arizona anyway, if the vehicle breaks the plane of the far crosswalk line (thus entering the intersection) before the light turns red, you did not run that light.

Maybe I'm misunderstanding.  I would have said "near crosswalk" to define entering the intersection.  Wait, I just read it again and I see you said, "far crosswalk line" not "far crosswalk".  Anyway, that's the rule in Texas.  If you actually enter the intersection before the light is red, you're okay.

2012/10/31
2:32pm
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Heimhenge said

These are all interesting ideas, but doesn't the average human reaction time, as well as the mechanical response time of the vehicle, complicate any attempt to define where the commit point is effectively located? Same with the caution line suggested by Emmett.

Add to that the fact that many drivers accelerate when they see the light turn amber, and the reaction distances increase proportionally. See:  http://www.visualexpert.com/Resources/reactiontime.html

Yes, the yellow light time should (and does) take into account human reaction time. And, different people would have different commit points, that is not really the definition of the caution line.

The caution line is determined only by the yellow light time and the speed limit. Then, someone who was traveling at the speed limit and already past the caution line when the yellow illuminated would be guaranteed to enter the intersection before the light became red. They would not have to worry about the commit point. Anyone behind the caution line entering the intersection would be guilty of some infraction, i.e. already speeding, speeding up to beat the red, or running red. If the caution line is not far enough away for someone to safely stop, then the yellow light time is too short already. The physics allows nothing else.

Emmett

2012/10/31
4:58pm
New River, AZ, USA
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I see your point. Good physics indeed. The commit point is a matter of psychology, and surely depends on the driver. The caution line is set by simple kinematics. That's really a great idea Emmett. Too bad it didn't catch on.

There must be a TON of studies out there on traffic safety at lights. It occurred to me that, if the change of green to red was delayed by a second or two after the crossing traffic light went red, that might really cut down on accidents caused by red light runners. I tend to delay anyway, even if I get honked at from behind. Of course, some of those scofflaws would no doubt adjust accordingly, figuring they'd have time to get through even on a red light. But that's where the cameras could help.

Just did a search for some stats, and each year there's 800 deaths and 20,000 injuries caused by red light runners. Ironically, it not the red light runner that usually gets hurt. Probably a matter of the efficacy (or prevalence) of side air bags compared to front air bags.

2012/11/01
7:44am
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Heimhenge said

I see your point. Good physics indeed. The commit point is a matter of psychology, and surely depends on the driver. The caution line is set by simple kinematics. That's really a great idea Emmett. Too bad it didn't catch on.

Maybe it could still catch on. If somebody could refer me to a traffic engineer (with clout) in a responsible jurisdiction (city, township, county, or state), progress still could be made.

Emmett

2012/11/01
10:08am
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There is at least one more issue to consider.  Many jurisdictions have two offences related to this discussion. First running a red light which is generally entering the intersection (by whatever demarcation they use) after the light has turned red.  The second is failure to clear the intersection, which means you are actually supposed to be out of the intersection before either the light turns red, or the signals have changed so the next traffic flow starts.  It is the driver's responsibility to handle the vehicle in whatever manor is necessary to clear the intersection. These days it is pretty much a foreign concept that the driver is held responsible for knowing his and the vehicle's capabilities, as well as road and traffic conditions, to properly control the vehicle. Still, whether there are signs, lights, or other warnings or aids about the signals it is the driver's responsibility to be out of the intersection even if he didn't run the red.

2012/11/01
10:50am
New River, AZ, USA
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bluecanu said
The second is failure to clear the intersection, which means you are actually supposed to be out of the intersection before either the light turns red, or the signals have changed so the next traffic flow starts.

Oh yeah … that applies in Arizona too. I was a passenger in a car that was in the left turn lane with one car ahead of us. We were already "in" the intersection, but there was a lot of oncoming traffic. The car ahead of us was slow to start up again, and then decided he didn't want to make the left turn, put on his right turn signal, switched lanes, and proceeded straight through. By the time we got moving again, both lights had changed.

Of course, there was a police cruiser watching all this, and he decided we were in the wrong. Pulled us over for failing to clear the intersection. Even though I was just the passenger, I argued that the car ahead of us is what caused the situation. Since he had seen the whole thing, he let the driver off with a warning. Maybe it was my respectful attitude, calling the guy "officer" and "sir." Or maybe he just realized he had blown the call and (with me as a witness) it wouldn't hold up in court if challenged.

2012/11/06
7:27am
Glenn
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We live near a state border, and the two states have two very distinct philosophies of reaction time, as judged by the duration of the amber signal. One state has a much longer than average amber signal; the other, shorter than average. We live in the "long amber" state. Maybe we all have slow reactions.

This situation has resulted in a few harrowing experiences and many hilarious ones. If you stop too short in my home state, the car behind you will likely rear-end you. In the nearby state, we have unintentionally run more than a few red lights. Now, when my wife and I approach an intersection and see the light turn amber, we shout out the state name as a reminder of where to place the "event horizon" or the "commit point" or whatever you want to call it.

2012/11/06
9:59pm
Robert
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http://wiki.answers.com/Q/When_turning_left_on_a_green_light_and_there_is_oncoming_traffic_do_you_wait_in_the_intersection_or_at_the_stop_line#page1
That's the best I've seen on the topic of clearing the intersection.
There's not rule everywhere that penalizes blocking intersection, even when the driver clearly sees congestion ahead and still enters the intersection just because his light turns green (though in that case the outrage all around will be palpable to say the least).

2012/11/07
7:15am
New River, AZ, USA
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This is yet another example of a thread that started out as a discussion of language, then morphed into something else. And I take at least partial "blame" for that, but it does seem to happen often on AWWW. So anyway …

That Wiki answer cited by Robert explains why my friend and I got pulled over, I suppose. But drivers in Arizona enter the intersection for left turns all the time, and the law is generally ignored. I have tried waiting at the crosswalk for a left turn when congestion was obvious. If there's another car behind me that also wants to turn left, I've often gotten honked at for NOT entering the intersection.

My wife (who's in the insurance business) tells me auto insurance rates are so much higher in Arizona because we have the worst drivers in the country, statistically speaking. We also have one of the highest rates of "road rage." It doesn't take much to get honked at (or worse).

2012/11/19
9:32am
BigK
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In the UK the amber light, after a green, means Stop – unless you have already crossed the Stop line or that you were so close to the Stop line that stopping might cause an accident. See Highway Code http://www.direct.gov.uk/prod_consum_dg/groups/dg_digitalassets/@dg/@en/documents/digitalasset/dg_070561.pdf

 

I've always thought that the latter part of the advice was 'loose' in that the question of whether an accident might have been caused was so subjective and open to interpretation, as to amount to danger. It would only be a reasonable cause for prosecution if there was no vehicle close behind you which might 'rear end' yours.

The alternative to a 'rear ender', in that by stopping you might end up actually stopping in the middle of the crossroads would be an interminable argument between the driver and a police officer which the driver would probably lose because it would be said – and you would actually unwittingly agree with it, that your speed on approach was excessive for the circumstances, even if it was not in excess of the speed limit.

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