Influencing Busy Four-Lane Highway/Freeway Speeds to Increase FE
 

Is it just me, or does driving the speed limit help to reduce the risk of running into traffic jams? I feel like when it's busy on the Merritt / Wilbur Cross Parkway (a four-lane limited access highway, two lanes each way), if I just drive the speed limit of 55, cars have to slow down and get over to the left lane to pass me, so they kind of "trickle through". If I were to drive 65-70 like everyone else, everyone would kinda just be "pouring through" at a higher speed.

Eventually, this would lead to a bottleneck, because people would be merging on and off the highway and everyone would have to slow down to the speed limit or lower anyway. The way I do it, no one really has to slow down more than about 5 mph at any given time if someone merges on up ahead. So, does this idea of "influencing the speed of the road" ultimately help or hurt our fuel economy? Obviously this wouldn't work as well for roads with two passing lanes, but it seems like the basic idea deserves credence.

Its not just you.... it happens to me every day. Even if there is a decent amount of cars on the highway, you can actually do less than the speedlimit and bottlenecking will still occur. I like to refer to it as a slinky effect. People speed up as soon as they can best they think the speed limit is the minimum they should be doing, only to come to a complete stop again a mile down the road.

I find it really amusing how angry people get when they cant do atleast the speed limit, especially in rural areas where there is stop lights every half a mile. While i'm hypermiling in between lights, people behind me sometimes get instantly pissed off, some even hang their head out the window looking at me like i'm and idiot.... then i catch them at the red light, me coasting up to it. I just look over and give them a nod, with an ecomodder grin on my face.

Even if there was no traffic on the road, driving 55mph vs 65mph only costs me 10 minutes of driving just about. I'd rather get 10-15% better FE and drive 10 more minutes.

newhavenpatriot -

Here's someone who has a whole theory for it :

SCIENCE HOBBYIST: Traffic Waves, physics for bored commuters

CarloSW2

I know, I read that already! :D A lot of it kinda made sense, but then a lot of it also seemed really weird, too. I do agree with his basic premise that the closer together cars are, the slower the drivers go. The way I see "traffic theory", it's kind of like when you pour oil into a funnel or something. If you pour all of it in at once, it's gonna back up. If you pour it slower, everything will go through smoothly. If nothing else, I think allowing fewer cars to go by you quickly on the highway at least decreases the length of time you're stuck in traffic jams, even if encountering one is basically unavoidable due to the sheer volume of cars on the road at a given time. (see NYC-CT rush hour traffic)

Also, I think we (meaning state/federal government) could improve traffic flow greatly (but not cheaply) by making all exit ramps exit on both the left and the right. This would allow for fewer lane changes, which slow down traffic dramatically in high-volume situations. I don't envision putting offramps between lanes or anything like that, just because of the risk of running into the side of the offramp, but exits on both sides would help a lot, I think, especially on highways with two lanes in each direction.

newhavenpatriot -

Ok, you're wayyyyyy ahead of the game, :thumbup: . An interesting test of your observation would be to have a set of equally spaced right-lane-only speed-limit-only cars along a route. By what you write, this might keep the passing cars from ever getting "too fast" to cause a traffic jam.

CarloSW2

That's an awesome idea that I've actually thought about in the past, and I firmly believe it would work. I'd love to see the government/police/whoever hire "pace cars" to purposely get highway speeds down to the speed limit consistently along a very long stretch of road. This would create a ton of jobs for people, as well as improve safety and congestion on state highways. It would be bad for the environment because gas would be wasted from all those cars, but hey, I never said I cared about the environment. It might help the environment, though, because people might stay at a constant efficient speed, thereby reducing emissions from constantly braking and accelerating. I dunno...

Maybe we should have an ecomodder get-together to test this speed-limit-only theory... :)

That's actually how they manage highways with variable speed limits. Whenever there is something causing congestion, they lower the limit upstream so people do not come in rushing. It gives more time for the congestion to dissipate and keep a steadier traffic flow.

good theory but....

This only works when the "Others" smoothly merge around us. In my experience most drivers race up to my bumper, get mad, make an obscene gesture, pass wildly while making more gestures and cursing, and ultimately burning far more gas than I'm saving. This happens even when there are no cars in the passing lane to keep my gesturing friend from passing smoothly. Go figure?

Oh, I'm not worried about the jerks' mileage. They can go light themselves on fire, for all I care. I'm just theorizing that the more cars that work in tandem to reduce the flow of traffic down to the speed limit, the more those cars will enjoy less traffic jams down the road (assuming a road with one travel lane and one passing lane).

If there's a group of hypermilers, how about have at least one in each lane driving at the limit such that it is impossible to pass? I remember reading about a group actually doing that and video recording the results.

I remember reading an article and seeing a video about that being done in some city... three people drove their cars at exactly 55 mph and it actually caused a gigantic gathering of 55 mph-driving cars behind them. The cops actually pulled them over and tried to say they were disrupting the normal flow of traffic, and I don't know what happened in the end. My guess is, depending on the state, they might have been cited for not passing when in the left (passing) lane.

Most?

When I slow down and treat the speed limit as an upper limit I am constantly passed by people driving faster and they all smoothly flow around me. I have not had a single person make gestures and curse, and only a tiny handful stomp on the gas and "pass wildly".

Really, the only times I can remember people getting pissed off and honking, swearing, etc is when someone drives real slow in the left lane rather than the right. And somewhat rightly so; it's not your job to police other people's speed. Driving slow in the left lane is just as unlawful as speeding is.

I agree with that - drive on the right, pass in the left.

That said, I have had people get upset with me for going the speed limit on a two-lane road with a no-passing zone. And I have been tailgated going the speed limit in the right lane on a four-lane highway (where I could have easily been passed.)

I made a thread once upon a time re: hypermiling wastes gas?!? Because if you look at the macro of the thing, one hypermiler can "cause" hundreds of lead-footed SUV-drivin' eh holes to slam on the brakes then goose it getting around you. Of course the gas consumed is much greater than if they wouldn't have done that i.e. if the hypermiler would have been going with the flow too.

Not saying don't hypermile, not at all. Just sayin', that's what I think the macro view of this thing is.

That is a very fair point. I don't think that scenario holds all of the time, or even necessarily the majority of times, but it is still a possibility to be considered if your goal is to reduce fuel consumption generally (instead of just your particular use of it.)

Can't save everything, I suppose, but still it's one reason why it's a particular pet peeve of mine to see people get self-righteous about their preferred method of driving (regardless of whether they have a lead foot or if they hypermile.) Driving and traffic are such complex things, there's always some new factor to be considered, something new to be learned. There are often unintended consequences to consider.

As an aside, has anyone read Tom Vanderbilt's book, Traffic: Why We Drive the Way We Do? It's a pretty good read on the subject - just came out in paperback recently.

I have to disagree if we're talking about a four-lane highway. Driving the speed limit reduces the average speed of all the traffic around you. How? Well, I'm glad you asked. A driver merging into the passing lane to get around you causes everyone else in the passing lane to slow down, since there's no third lane, and eventually this brings it down to a safer, less frenetic pace. Sure, they can just accelerate once they get past your car, but if you're driving the speed limit of 55, the people behind you and to the left aren't going 70 anymore, maybe they're only going 60-65 now. And the people behind you are going 55 and then changing lanes. If you continue to "go with the flow" and drive 70, everyone in both lanes will go that speed or higher for that same distance. Does that make sense at all? I can see how your thinking holds true on roads with two passing lanes, but with only one, I don't see how tons of people going 70 vs. 55, 60, or 65 can generate a net savings of fuel.

I'm sorry. That was an exaggeration. It is a lot though.

And of course I drive slow only in the right lane, I have no interest in policing others.
I'll tell you this, where I drive, residents are known as "Mass-holes" for a reason.

My point was, if my goal is to reduce total fuel burned as opposed to fuel *I* burn. I failed if others are accelerating rapidly to get back up to speed past me. (like too many do here)

I have learned that if I'm at less than 15 over the posted limit, I have to watch the mirrors NEARLY as much as my front view. (not exaggerating) even if it is only for that 1 in 100 driver.

Newhaven: if passers executed their maneuvers smoothly it wouldn't be so bad. Many don't. As noted, even on a 4-lane people will come barrelling up behind you and at the last second either slam on the brakes or swerve around, even if the left lane is clear. Why? I think cell phones have a lot to do with it. :mad:

Even, as you describe, if on a 4-lane people "slow", they only do it next to and behind you; then they accel back up to where they were.

You're spot-on re: cell phones. I don't know how all those people talk on the phone without crashing. I get so annoyed when I see someone tailgating me and yakking away, because they're putting my safety on the line. :mad:

I suppose the question becomes this: If you can get most people to slow down around you, and only some of them fly wildly around you, then does hypermiling save gas in a macro sense? I'm not sure there's a way to weigh one vs. the other, because it has so many variables. You have to take into account whether drivers brake hard behind you and accelerate hard to get around or whether they just merge left and go around a bit faster, how many people are on the road, whether they drive faster once they get ahead because they feel they "lost time" by driving slower, how many lanes there are, etc.

Sometimes people just sit behind me and match my speed, which I think is wicked cool, because then no one can rear-end me if they're distracted with their cell phone, and it just makes me feel less nervous than people speeding right up to my back bumper and then moving.

I have no faith in the average motorist therefore I think you'd need to block all lanes to keep people back there and "slow". Not a good plan.

...can you say "O.J.Simpson-slowspeed rolling blockade?" Sure you can.

I've done a "staggered formation" setup with some friends before, just to see what happened. Guess what? Every driver on the road turned into Tony Freakin' Stewart, weaving around us all like morons, flooring it to get into the other lane, then braking heavily as they approached the rear of the next car in that lane, then flooring it to change lanes again, etc.

Me, on the other hand, when people used to block the highway and go slower than the speed limit, I'd just pass them on the shoulder, usually in third gear, high RPM, as smoothly as though I'd just executed a normal pass under normal circumstances. They normally figure out that I'm passing on the shoulder, and end up swerving toward their "friend" to avoid the perceived accident situation. Often, in the rear view mirror, I see that afterward, they decide that they should fall in line like everyone else, and stop blocking traffic.

I don't feel like it's unsafe to travel in the left lane slower than everyone else, especially after traffic builds up. I do feel that it's completely discourteous, though.

Found that video again on Youtube... it was made in 2007 by some students from the Atlanta metro area. This really demonstrates how instead of a traffic jam, you get clear sailing up ahead, and all the cars are going 55, so there is no problem with traffic flow. Unless of course you consider not being able to go faster than the speed limit a problem. I don't. :)

http://www.youtube.com/watch?v=kuM2A...layer_embedded

I remember the news stories about the traffic thing
Local news had several "irate" people on talking about how this impacted thier day - "Late for work" / "MIssed an oppoitment"
Umm..
1st of all - in ATLANTA traffic - you ASSUME there will be an accident and it will take you twice as long to get where you need to be - so the SPEEDLIMIT should NOT cause to be late - ever.
2nd of all - if you are leaving at a time where you have to speed to get there - you are an idiot.

Was a great event.
I do not recall if the charges of "rolling roadblock" were dropped or not...

I think they could have protected themselves better with a rotating car formation.
have 4 cars passing then pulling over and re-passing . this way no 1 car is "blocking the left lane" and causing a roadblock. you are actually passing the car on your right... he just re-passes you ( with a few blocker cars behind you....

Steve

I worked a project several years ago doing core analysis on LA traffic patterns.

The goal was to determine what the primary cause of traffic stalls were.

After 2 years of analysis and simulation testing despite all the preconceived theories, the major cause of traffic jams and accidents was determined to be cars going 10 miles an hour or more UNDER the speed limit. Cars that were speeding and causing accidents were in excess of 30 miles an hour over the limit and accounted for < 1 in 40 accidents and delays. The split was something like 15% for speeders 55% were slow drivers the rest was mechanical failure or stupidity..

And the 55 magic number is pulled out of thin air. There is nothing magic about 55.
My BMW gets peak mileage at 65 not 55. (34MPG to 36 MPG depending on the terrain.)
Drops to a consistent 30 to 32 mpg at 55. I still get 30 MPG at 75, My state the limit is 65 and I don't save significant time by going to 75 for a < 300 Mile trip so 65 it is...

On the other hand WA lets me go 75 and when its a drive from Portland to Spokane thats 400 Miles and Its all flat, I let the car find its sweet spot and that's usually approaching 75. I tend to exploit the terrain, turnoff the cruise down hill and let it coast up.

Portland to Spokane About 400 miles.
5.3 hours at 75,
6.2 at 65
7.5 at 55

I balance the time against the fuel cost, 45 minutes, not a big deal 2 hours gets my attention, I'm usually working and that travel time is 200 bucks an hour.... vs about 1 gallon of gas. 65 Being the min because I'm not going to lose the time AND the mileage.
I fly when I can but there is a lot of distance between airports in the west and the small towns with no air service.

At the end of the day economics control a lot....

Dave

Side tidbit, the CD of the BMW 318ti is about .34 stock.
The engine is a 1.8 liter with a 5 speed transmission car weighs in at about 2800.

I'd like to know what it is about that car that gives it better fe the faster it goes :confused:

Lots of guys on some of the Minivan forums I'm on are claiming better FE at 62-65 than 55 as well, and it's understandable, because they're geared to hold in the most efficient RPM range at those speeds, since many of the highways have 65 and 70MPH speed limits. Many of those same people are using scan gauges, as well.

This isn't to say that they wouldn't be getting better mileage at 45 than 65, with the transmission locked, but FE curves don't specifically follow speed, either.

Of course, another part of it is that most of the people "trying" to get better mileage by going slower, may just be very squirrely at that speed, since they're used to driving faster, and perceive that they're doing something that could be potentially dangerous.

Of course, I have to add that re: slow people causing traffic jams, that's all well and good, but why did the slow people cause the traffic jam? Because they created a bottle neck when others tried to pass them? Yeah, that's what I thought... therefore, it's not the fault of the slow drivers entirely. Your study, thus, was flawed. From the data gathered, I have to wonder if there wasn't some agenda behind it, as well. Something irks me about it, not sure what.

There are a couple oddballs like that, it may have more to do with how he drives at different speeds or possibly because the slushbox is too slushy at 55 or even because his big motor is choked off too far at 55 who knows.

Or it might even be because of another reason.

Small engine, no funky slushbox weirdness.

I have my doubts that even if the engine has a better BSFC at 65 vs 55, that that is enough to overcome the added loads.

The FE kinda makes sense to me, first, the car has the hands down best cruise control I've ever driven, it actually feathers the throttle instead of shutting it off on long down hills. The speed stays dead on up or down hill as well.. Second, I think the hatch tail hits a aero rollover at the right speed.

Google a image of the 318Ti and you'll see what I mean.

It also has a dual path intake manifold, it has a set of low RPM long runners and a set of short High rpm runners, so in cruise it really pulls the torque.

I'm also only noticing a 2 to 4 MPG rating which I find easy to accept because it peaks fairly sharply which would imply a tuned spot in the engine management.(On purpose or accidentally who knows.

As far as the slow drivers causing accidents, if the speed limit is 65, and your not going at least 55, what happens when a car driving legally, comes around a corner and a car with no brake lights is moving slow? He may not hit it but he has to check up which causes a chain reaction of reflex checking down the lane.

The goal of the study was to identify the moments that cause the traffic jam, and they identified lots of reasons. But by far rigs more than 10 under the limit consistently generated a traffic bottleneck. My logic is, if you don't want to drive the limit, drive to the right. Whats the deal with people trying to "enforce" the speed limit on others to save fuel.? Its dangerous, as dangerous as road rage. And you might meet the guy out there driving 5 tons of steel clad utility truck who is quite willing to Nerf you into the guard rail, not that I know anyone like that or anything....... :rolleyes:

The study was used to develop the traffic software that updates all the GPS systems and emergency vehicles with safer faster routes around ongoing or even future traffic events. (It can actually predict a traffic event by up to a minute.

Wow sorry I spewed on so long....
Dave

I don't advocate long trips on the highway at less than 55 MPH. I will not go under 45 on the highway, unless I'm speeding up or slowing down.

Part of your mileage effect at higher speeds can also be derived from a "corridor" effect, if you're keeping with traffic at that time.

I've seen trucks that would run you off the road if they were so inclined. I've also followed them for less than legal reasons, after a couple of them tried. ;)

Also, one of the only two accidents I've been involved in on the road was a matter of some moron driving like a moron. I eventually ran him off the road because of the potential for a head-on collision. The cop arrested him. Details would include that I was driving the speed limit, he was on the side of the road "coasting" (I guess) with his hazard lights on, so I leaned left and passed him about halfway over the double yellow line in a passing zone at 35-40 MPH. I was just pulling off a side road, so I wasn't going 55 yet, and he decided while I was passing him, he was going to basically floor it, force me all the way into the other lane, then attempt to keep speed with me so that I couldn't get back over. I saw what appeared to be headlights in front of me (night time), so I slammed my Mazda B2200 into his rear door as "gently" as I could, to "push" him over some so we'd fit three wide on the rural road. Turns out I saw a four wheeler's spot light, and there was no real reason to push him over, but the officer understood the situation and made the arrest anyway.

That's the only situation I've been in like that, and hopefully, I never have to make the choice again like that. I try to drive safely first, economically second. There is a point where safety is relative, though. The driver can't always control the situation, I agree to a point, but many of us could do so much more to at least prepare for a situation.

BTW - I know what the 318Ti looks like. I'd like to get an older 3 series or a newer 1 series.

I just wanted to make sure I said what I meant :)

And I am a big fan of the 318ti, its a really amazing little car, simple, fuel efficient, fun to drive comfortable, just wish the Diesel version was available in the US!

And YES a 135 is a cool ride. No throttle all valves....

Dave

dwtaylorpdx -

This is the picture I always cite when it comes to higher speeds and MPG :

Fuel Consumption at Higher Speeds - May 2006
Green Car Congress: Fuel Consumption at Higher Speeds
http://ecomodder.com/forum/member-cf...her-speeds.jpg

I don't doubt your experience, but I think it falls outside the norm. Your BMW is probably like the Mercedes C 180 K.

I think that getting "better" MPG at higher speeds can sometimes mean "getting almost the same" MPG at higher speeds. It's like a rising fastball, it doesn't actually rise, it just looks like it rises in comparison to other fastballs.

CarloSW2