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Old 12-22-2007, 10:21 AM   #1 (permalink)
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Technique quandary: gliding toward a stale green light - to pulse or not to pulse?



A hypothetical, hair-splitting, nit-picking technique question, inspired by Newton's thread - Idea for government: Traffic Light Timers

Here's the hypothetical situation (OK, this actually came up for me last week). What would you do - assuming your goal is maximum fuel economy?

The scenario:

You're alone in your lane (nobody close behind), in a med/low speed glide approaching a stale green light which does NOT have a pedestrian count-down timer, or even a walk/don't walk signal.

You think the light is controlled by sensors for cross traffic. There's a vehicle on the sensor. In other words, the light may change at any second, but you can't predict when with any certainty.

Your options:

1) throw in another pulse as you approach the light, and hope you make it through on green;

2) continue gliding down, considering that if you don't pulse, you'll more or less roll to a stop at the light (which may still be green when you get there);

3) some other option?

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Old 12-22-2007, 11:02 AM   #2 (permalink)
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Well, I would go for it and run the light with one or 2 seconds of red if I had to, but I am usually in a bit of a hurry.

Gliding down probably would be better for the mpg.
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Old 12-22-2007, 01:51 PM   #3 (permalink)
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I would prolly do a glide down and not worry too much about it, Usually sensor activated lights change pretty quickly around here.
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Old 12-22-2007, 05:47 PM   #4 (permalink)
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I'd continue coasting to the light if no one is behind me. Bummer that the scenario has a car already at the light. At nighttime, without any cars at a camera-sensored intersection, I usually flash my high-beams on and off a few times as I approach the traffic light as that often makes the light turn green in my city.

EDIT: oh! you mean the light is green already for me and the car ahead is cross traffic. Sorry my bad.

Yeah, I'd definitely pulse to try and beat the light.

Last edited by Peakster; 12-22-2007 at 05:51 PM..
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Old 12-22-2007, 05:49 PM   #5 (permalink)
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That's a tough one... the ratio in fuel wasted is:

Vspeed up^2:Voriginal^2

Often you are going 50kph or so, and you might have the choice of speeding up to 60kph. So that's 36:25 ratio, not a lot of difference, but if you gamble and lose, you pay even more. And if your car is anemically powered, the gamble is not a good one at the best of times.

If you could only hear me curse in the instances where I have gambled on a speed up and lost!
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Old 12-23-2007, 02:25 AM   #6 (permalink)
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On stale green lights, I always assume they will be red and keep gliding. I won't pulse again until I reach a point where it is certain that I will make it through on the green, ie, as I'm entering the intersection.
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Old 12-23-2007, 02:12 PM   #7 (permalink)
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Quote:
Originally Posted by newtonsfirstlaw View Post
but if you gamble and lose, you pay even more
There's the key point.

If you throw in another pulse, and the light does in fact change to red, you're forced to brake (or run it, if you're igo ) - and probably brake pretty hard. Gas in --> brake dust out = FE nightmare.

If you keep gliding and the light does in fact change to red, you're far better off than if you had pulsed & braked (since in this scenario, I mentioned you would pretty much coast to a stop at the intersection anyway).

I really did face this situation a few drives back. As it turned out, the light *didn't* change to red on the approach, so I ended up re-applying power just as I got to the intersection (having almost coasted down to zero).

It's a completely bizarre, non-intuitive tactic to use. Green means go, right? Yet there I was, coasting nearly to a stop facing of a green light (NOTE! Not affecting any other drivers.) But if max mpg is the goal, only a predictable green means go.
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Old 12-23-2007, 10:03 PM   #8 (permalink)
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I know this has nothing to do with this pulse and glide thing, but it does have to do with traffic lights, Does anyone have trouble seeing the new LED lights?(i think they're LED). I am red/green color blind, which makes red and green colors kinda slewed and discolored, so I occasionally think that darker greens are browns and darker reds are browns, reds look pinkish and greens look yellowed. I can still see green and red, I'm just missing cones in my eyes, which makes colors not what they normally are.

When the wind is blowing the new traffic lights, I can't see them until I'm right under them because they concentrate the light in a straight line, which is bad when it's busy or dark and busy. They might think that it is new and helpful, but one of these days I'm either gonna get a ticket for running a red light, or I'm gonna smash into another car.
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Old 12-24-2007, 12:49 AM   #9 (permalink)
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Where I am there are a lot of lights that are "lensed" so that you can't see the green light until you're 50-80 meters from the intersection. I have noticed that in most cases the red light is not "lensed" so you at least have something to judge the color of the light.
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Old 12-29-2007, 02:01 PM   #10 (permalink)
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A fellow color blind driver, kudos to you for keeping you license. I just look at the location of the light. Green always on bottom. Red always on top. And I coast to the lights as much as possible. Safety first and sometimes heavy traffic won't let me P&G. S.

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