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Old 02-22-2008, 10:02 PM   #21 (permalink)
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That's not a traffic jam, that is congestion. Speed differentials and sudden braking especially cause standing waves which lead to traffic jams. This is modeled by a compressional wave. Regardless of the initial velocity there will be points of no movement in a compressional wave. As your link clearly illustrates maintaining a herd at a constant pace alleviates any chance of traffic jams.

Anyways, if everyone were driving the speed limit, things would have been fine. In fact, the view from the bridge shows the side abiding by the posted limit is about as congested as the side speeding!


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Old 02-26-2008, 08:00 PM   #22 (permalink)
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I though I81

through Tennessee had a speed limit of 80mph.

Personally, I would prefer seperate speed limits per lane, if we can't enforce "pass on left only". Where I live (off I-270 in MD), running 15-20 mph over the "speed limit" is typical. The one thing that stands out in my mind is a recent flyby by someone doing 120+ in the right lane while I was getting ready to merge. I can't see any means of "enforce 55" that would have stopped him from easily killing me, but other ways might have a chance.

The other thing to remember is just how wildly popular 55 was, and how badly it gets blown by to this day. I think we need to think in terms of getting a few lanes for our own turtle crawl, rather than try to steal the time of others lives.
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Old 02-26-2008, 08:03 PM   #23 (permalink)
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Quote:
Originally Posted by wumpus View Post
Personally, I would prefer separate speed limits per lane, if we can't enforce "pass on left only".
That idea makes a lot of sense but it would take a lot of political will power and education in order to get something like that enacted into law.
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Old 02-26-2008, 08:28 PM   #24 (permalink)
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I've seen that more then once in my area. Speed doesn't cause traffic jams, people getting irritated, like that white van in the video, at people like us that go the speed limit (saving our gas) and doing stupid things like passing in the median and tailgating/heavy braking which causes the wave effect. I will admit that driving the speed limit on the highway in with any decent amount of congestion is dangerous, however, I do stay in the far right lane at all times and if someone has a problem with it they can pass me. There is a reason why there is more then one lane in each direction on the highway, it's so that those going faster can go around those going slower to avoid exactly what happened in that video. Looking at it in that way, I'll be the traffic would have been worse had they left the left lane open...
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Old 02-26-2008, 09:31 PM   #25 (permalink)
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Quote:
Originally Posted by tjts1 View Post
Higher speeds are also a cheap way of increasing the capacity of existing freeways without having to add more lanes of traffic, something nobody wants to talk about.
That doesn't seem to make sense...

The faster you go, the more space you need to maintain a safe following distance. If you increase speeds, and drivers increase the gaps between cars, you end up with less capacity.

If you increase speed without increasing following distances, you end up with a much less safe road and more catastrophic crashes when they happen.
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Old 02-27-2008, 02:00 AM   #26 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
That doesn't seem to make sense...

The faster you go, the more space you need to maintain a safe following distance. If you increase speeds, and drivers increase the gaps between cars, you end up with less capacity.

If you increase speed without increasing following distances, you end up with a much less safe road and more catastrophic crashes when they happen.
A good read on the subject.
http://www.usroads.com/journals/aruj/9709/ru970901.htm

Quote:
By and large, those results lend credence to the argument that motorists drive at speeds that they feel are appropriate, apparently independent of the posted speed. On urban-55 segments, where drivers are more confined by the geometric characteristics and more likely to encounter congestion, speeds are considerably slower than in fringe-55 areas, which are more open and less congested. This also indicates that freeway speed limits set closer to the 85th-percentile speed may be more appropriate and would lead to better compliance.
And another one.
http://www.azdot.gov/highways/traffic/Speed.asp

How the Germans deal with speed limits.
http://www.gettingaroundgermany.info...tm#speedlimits

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Old 02-27-2008, 02:32 AM   #27 (permalink)
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Quote:
Originally Posted by tjts1 View Post
That seems to confirm that lowering the limit increases capacity...

From second link
Quote:
Traffic Volume

At some point, the volume of traffic affects its flow and forces motorists to slow. This factor is important because:

* Traffic volume is often used as a reason to lower speed limits.
* Peak volume occurs at certain times and for certain durations.
* Merging volumes, densities of traffic, and increased speeds all affect drivers' abilities to merge and change lanes.
* Each highway section has a certain carrying capacity.
While it doesn't specifically say capacity increases with lower speeds (it just says lower speeds are used when volume is high) - its last point doesn't link carrying capacity with speed.

And in the third link - limits are set in areas of heavy congestion....

This is actually rather interesting - I wonder if there's a Bernoulli like model for this.

..... researching......

So it looks like Metro was right...
http://www.vtpi.org/tdm/tdm96.htm

Vehicle density and vehicle flow (vehicles per hour per lane) increase in a nonlinear fashion as speed reduces. That is, maximum traffic volume (capacity) is higher for slower moving traffic.
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Old 02-27-2008, 03:45 AM   #28 (permalink)
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Quote:
Originally Posted by trebuchet03 View Post
That seems to confirm that lowering the limit increases capacity...
Not really. If you read the article carefully, they never link limit to capacity, wrather traffic self regulates the speed at any given point based on congestion. So to infer that a speed limit would increase capacity is wrong. You can cherry pick from these articles to support any point of view you want. I just thought the first article was an interesting study, the second article pointed out the process by which the 85% rule is applied and the completely different approach in Germany. If anything the study shows that the arbitrary speed limit is completely irrelevant to most drivers.
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Old 02-27-2008, 11:49 AM   #29 (permalink)
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fwiw when i lived in Florida I drove 90+ everyday, never got pulled over (i even drove 65 in a 45 with a police officer literally right next to me didn't get a second, much less a first glance from the officer) anyhow, point being that law enforcement on average is pretty week when it comes to speeding in SOME areas. as for fuel economy, difference between 65 mph and 90 mph was minimal, but then again the car is engineered a little different then most.. now as for my jeeps, they never go over 65 and they love the right lane, different vehicle different driving habits, I know that my jeeps fuel economy drops quickly over 55ish.
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Old 02-28-2008, 11:41 PM   #30 (permalink)
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Contrast & compare: Switzerland cutting speed limits to reduce emissions

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