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?

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.

I would prolly do a glide down and not worry too much about it, :p Usually sensor activated lights change pretty quickly around here.

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.

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!

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.

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 :o) - 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.

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.

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.

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.

I know that, I can see red and green, but these new lights are weird, unlike the old ones where you could see the light from all angles, the light can only be seen when looking at it directly. So these are called "lensed" lights now? They are nice when it's dark, not so much during the day. So the red one is normal and the rest are lensed? I'll take note of that. But I'll be blaming you all when I get pulled over or smash into another vehicle when I blow a red light because it's the new type :P Just kidding btw.

They have some of them around me, mostly on roads where you can see the other light changing and it tempts you to jump the green, :p

Spare me the rants, because what I am going to say will seem theoretical. I adhere to it religiously.

One principle in hypermiling is 'plan ahead'. Which means peak traffic conditions, number, nature and locations of traffic lights and sufficient time margin for any sort of crap happening enroute.

On a light that is not in your regular territory, I would play safe, and coast on. If I pass the stop line before the light turns amber, I am allowed to accelerate and clear the intersection before the other morons gun their vehicles. If you are not that fortunate, just kill the residual momentum (would not be much anyways), to minimize losses.

If this is a familiar signal, I would have my strategy of getting through it without coming to a standstill. That involves noting the distance to the signal, noting it's status, noting which side is passing a green, and that gives you an idea of when the signal is going to turn. All this calculation does not get into the system all of a sudden, you have to practice it, and it is tough sh*t to conquer. And that is where the challenge lies.

Jumping the signal or gunning for the green are mindlessly are far inferior options. Choosing between them is sorta asking "would you like a lump of clay or a bit of gravel will suit you better?" No offense intended, it is just my attempt to clarify!

Traffic laws allow running a yellow if stopping would require slamming the brakes and cause a traffic hazard. In a fuel economy oriented world I could imagine traffic laws permitting running a yellow or even a fresh red if the color change caught you towards the end of your pulse. On the occasions when I ran a yellow because it was almost impossible to stop on short notice at the end of my pulse, at least 2-3 other cars behind me would also keep going, I'm sure they caught the red. I keep an eye on them in the mirror, expecting red/blue flashing lights...

The "freshness" of the red is in the mind of the beholder! This is a line that cannot be crossed, I think...

Add me to the list of those who continue gliding. I've wasted enough late pulses to have figured out it's not worth the gamble, especially when there's already cross traffic waiting.

Hello -

I will admit to the occasional "im-pulse" on occasion. However, I think that "just saying no" to the pulse evens out over time. It's cool when I let fate decide and it goes yellow as I cross the intersection (I win!).

CarloSW2

I couldn't agree more. I was by no means advocating that reds should be run, only imagining a reality more hypermiler-friendly: Longer red overlap with cross-traffic, or maybe longer yellows, and of course an LED display showing seconds until color change.

Displays showing seconds to next green are a commonplace in India. Also sometimes the red lights have "RELAX" or "CHILLAX" written on them. People are encouraged to shut off engine when waiting for a green.

There is something called PCRA - Petroleum Conservation Research Association, that funds such advertising, and promises a blanket 20% increase in FE to everyone on taking a few simple actions. Good for us!

Waste no inertia is my basic rule. The Insight fuel economy display clearly shows a significant loss in average mileage from a single forced stop from any decent speed, with the resulting acceleration from the forced stop.

Gliding is your best friend when it comes to extracting the best possible mileage. The key to your scenario is knowing you can coast to a virtual stop regardless of the light condition.

I would only pulse when I knew I could make it through the intersection safely and without any legal risk.

I always give myself plenty of time to reach my destination, so there is no impulse to push for time saving.

regards
Mech

This depends on a lot of things. Here's a mini check list I go through:

How long is this yellow? (if it's a light I go through regularly)
Sensor triggered or timed?
How large of a road? (smaller triggered roads tend to trigger faster for stopped traffic than larger roads)
How fast am I going?
How close is the car behind, and what has his behavior been like thus far?
Cops?

95% of the lights in my city are "trigger" lights, so I coast down if there is cross traffic that has been there. Most of the time I have a commit distance made up in my head well before light switches yellow, if I am passed that number I accelerate or continue coasting. If I am behind I usually coast to a stop/brake.

yah, those are all good inputs, I might also weigh in if it looks like a high pedestrian area or not, don't want to be terrorizing the peds w/my speeding metal box.

But usually I keep coasting, nothing more frustrating than slamming on the brakes after a long burn.

Coast. Nearly every time I break that rule I end up slamming into the red. Each time I swear I'll never do that again.

I have one of these on my route home, in the middle of the worst uphill. I hate it. I usually hold steady speed ~30 mph in 5th gear until the "commit" point - either eoc and stop or give more gas and go. At 30 mph uphill, the commit point is very close to the light.

Ditto!

I almost never pulse up to a green anymore. I've had too many "D-oh!s". Just one of those gambles lost murders your FE.

1) Our indecision is telling us that our fuzzy logic processors (our brains) say it's a close call or there's too much uncertainty to tell.
So if we trust our gut instinct and experience and decide instinctively on a case-by-case basis, then on average we've probably optimized it.

But it just hurts so much to slam on the brakes right after a burn that I usually just coast and enjoy the scenery; I've placed my bet, now fate decides.

2) To those fellow red/green color blind: LED's are very monochromatic compared to incandescent stoplights. So if that single wavelength happens to be in the part of color space that you can't distinguish well, then you're outta luck. With multichromatic light maybe there are other wavelengths that still allow you to differentiate red and green lights. But I usually have more problems at night distinguishing between red and yellow, and not between red and green (which is much more whitish to me).

3) Getting information on when stoplights will switch: I spoke to the traffic control center in my city, but they said that because so many lights are traffic- and pedestrian-controlled there's no way to provide, say, a car's navigation system with reliable data to inform the driver. In Ingolstadt, Germany there was a pilot project called Travolution
Audi Travolution Project Minimizes Red Lights and Emissions
in which, among other things, each stoplight informed approaching drivers over their infotainment system about what speed they need to maintain in order to roll through the green light. Presumably you could do this on a large scale if every stoplight had a small FM transmitter, which the car radio receives and passes the data on to the navigation system. Or better yet, the cars ahead of you tell you if they're standing at a light or in traffic, or moving (or for that matter, if the road is slippery i.e. they're using ABS or ESP alot).

Sometimes you coast up to a green and it's the wrong decision. I did that this morning. I coasted towards a stale green, down to 40mph, and got close enough that I'd have to use my friction brakes if the light turned. Then I decided it wasn't going to turn, and I departed from optimal BSFC to get back up to 60mph quickly.

Coast.

This one of the reasons I drive the Insight in local trips and the VX on highway trips.

The first 7 miles of my daily drive only involve 1 traffic light. I know when I see the light as I come around the approaching curve, if its green, there is no chance of making it through the light, so I slow down and generate some battery juice.

If the left portion of the intersecting road is moving I know the light will be green when I get to the intersection so I maintain my speed. Sometimes I will pulse to a higher speed when the same traffic has been moving for a few seconds.

If the right intersecting traffic is moving I will coast without regeneration, which puts me in a perfect position to reach the light at the highest possible speed, even when there is stationary traffic in my lane headed in the same direction.

The next light is 3 miles further down the road on a slight uphill climb. I can see the light about .4 to .5 tenths of a mile away. The last sequence of traffic flow is the oncoming left turn signal. Ideally I want to be about .2 mile from the light when the left turn oncoming traffic flow stops and my light turns green.

After this first 7 miles, there are 12 lights in the next 3 miles, some of which are newly installed and very poorly timed. It used to be that you could average 47 MPH and hit every light green. The speed limit is 45, but if you go that speed you would catch a yellow light at exactly the wrong time and have to stop quickly.

Sadly the newly installed lights are not timed properly and you are forced to make a decision as to how you react. It's almost impossible to maintain any decent average speed without getting nailed by one of the newly installed lights. Its sad because the traffic at these lights is almost non existent and it is especially stupid to have them individually sequenced which causes 25 cars to slam on their brakes for a single car to enter from the intersection.

In this case I revert to the coasting scenario, and all the cars around me get aggravated by my extended coast to the light, only to have me glide past them as they stomp on the gas when the light turns green again, on this 6 lane stretch of road.

regards
Mech

Yeah, I am still learning to glide/coast to a stale light. Almost every time I decide to test it and hit the gas it changes and I have to hit the brakes.