Cars parked on the motorway

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Cars parked on the motorway

Post by Bobinwales » Thu Mar 16, 2006 1:58 pm

Can anyone remember the word for the effect whereby traffic comes to a stop even though there is no apparent reason for it?

Someone brakes, the person behind brakes a little harder, the person behind brakes a little harder… Eventually I am sitting in a stationary car. It’s always me!
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Cars parked on the motorway

Post by minjeff » Thu Mar 16, 2006 5:27 pm

Isn't it a "jam" regardless of the reason for stoppage? I prefer the German word for it though: Stau (its sound describes the mess better).
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Cars parked on the motorway

Post by Shelley » Thu Mar 16, 2006 8:02 pm

While we're on the road, here: what do you call it when you're IN one of those things (a jam, a bottle-neck, rubber-necking, whatever) and you keep switching from lane to lane in order to get into the one that's moving, only every time you decide the other lane is moving faster and you switch over, suddenly the lane you switched INTO stops and the one you switched OUT OF starts moving? Hm?
Bobinwales, my husband is also mystified by the occurrence of jams -- total traffic stoppages -- FOR NO APPARENT REASON!
"Stau" is good, minjeff. Sounds like it might be related to "stall" . . .
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Cars parked on the motorway

Post by Erik_Kowal » Thu Mar 16, 2006 9:15 pm

Bob and Shelley, if you think about what happens when traffic moves off from traffic lights, the car at the front starts accelerating sooner than the one behind it, and so on down the line. The distances between the vehicles correspondingly increase. This is due partly to the delay from the successive reaction times of the drivers behind, and partly to most drivers' desire to maintain a safe distance from the vehicle in front.

Now consider what happens when the car at the front of a line of dense traffic decelerates. There is a slight delay in the braking of the car behind due to the time it takes the driver of that vehicle to react (unless he too started braking at the same time in response to whatever caused the first driver to begin braking). During this delay period the gap between the second car and the car in front of it continues to close because the front car is slowing while the second car is still continuing to travel at the previous speed for a few moments due to the reaction time of the driver of that car. And so on down the line.

If the distances between the cars in the line are small and the line of traffic is more than a few cars long, unless some of the drivers in the middle of the line were paying attention to the traffic several cars in front of them and therefore began braking before the car immediately in front of them started to do so, rear-end collisions will be inevitable.

All this is by way of demonstrating that although I don't know the name for this phenomenon (or even if it has one), there is a reason for the traffic eventually coming to a stop one way or another.
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Cars parked on the motorway

Post by Ken Greenwald » Thu Mar 16, 2006 9:44 pm

Erik, The general phenomenon you have described, which doesn't just apply to traffic flow, is often referred to as the SLINKY EFFECT.
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Cars parked on the motorway

Post by haro » Thu Mar 16, 2006 9:57 pm

The German word 'Stau' means about the same as the English 'traffic jam.' There's no etymological connection to the English 'stall,' but there is one to 'stow,' in that 'stauen' also means 'to pile up' - not in the sense of a 'pileup' but in the sense of 'keeping things together in a heap.'

The German term 'Harmonika-Effekt' (literally 'concertina effect' or 'accordion effect') is exactly what Bob described. Maybe it's time for another coinage.
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Cars parked on the motorway

Post by Phil White » Thu Mar 16, 2006 10:49 pm

One of my favourites is the reason given on the German Radio for the notorious traffic jams on my local motorway: "hohes Verkehrsaufkommen". It roughly translates as "there's a lot of traffic about". Duh!
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Cars parked on the motorway

Post by Edwin Ashworth » Thu Mar 16, 2006 11:28 pm

It doesn't travel well, Phil.
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Post by Bobinwales » Fri Mar 17, 2006 9:16 am

I seem to remember that some university had a lot of money to research this effect, which is caused exactly as Erik explains as far as I can see, but it had a term, and although I can see the logic in slinky effect, I don't think it was that. Any more offers?
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Cars parked on the motorway

Post by Edwin Ashworth » Sat Mar 18, 2006 8:43 am

The M25? "Bunching" is a term I seem to remember.
At http://www.sciencenews.org/pages/sn_arc ... 9/bob1.htm there is a recent article - an extract:

More recently, Helbing and other colleagues came up with a new analysis of traffic that defines six phases. Besides free flow and traffic jam, the researchers identify four phases of congestion, all characterized by waves of rising and falling vehicle density. The phases may overlap with Kerner's synchonized traffic state.

The new findings distinguish the phases by how often waves pass through the stream of vehicles and how much the density drops off between waves. In a phase called a "pinned localized cluster," for instance, an enduring but very localized bunching haunts the immediate vicinity of an on ramp.
.....
Instead of the excitement of the physicists who have created it, the new picture of traffic has aroused the ire of traditional traffic researchers. Traffic engineers are particularly disturbed by the physicists' notion that traffic flow can spontaneously break down into a slow-moving or stopped state.

"When congestion arises for no apparent reason, this just means that its cause has not been identified," asserts Carlos F. Daganzo of the University of California, Berkeley. There are many possible causes to choose from, such as accidents, merges, stalled cars, and slowpoke drivers.

Studies of actual traffic by Michael J. Cassidy, also of Berkeley, show that congestion recurs reliably at the same bottlenecks along highways. Uncovering the causes usually suggests cures, such as ramp metering or redesigning the geometry of a stretch of highway. However, "if congestion really does form for reasons we cannot pinpoint, then dealing with congestion in those instances would be much harder," Cassidy says.
.....
Daganzo also notes that engineers have long been aware of phase transitions in traffic, such as the onset of a jam, but have simply used different words to describe the phenomena. The recent models, for the most part, rediscover and relabel well-known concepts and observations in their field, he and other engineers claim....
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Post by aelnamer » Tue Mar 21, 2006 11:02 pm

I am always amazed that we do not have that much traffic congestion in Dawson Creek; especially at our traffic circle. It is a small community of 14,000 but starting in the spring and during summer we get many tourists that drive through the city. For those of you that travelled to Alaska starting at the Mile 0 Alaska Highway know what I am talking about. The traffic circle receives traffic from one main street and 2 highways (8th Street- North South), the Alaska Highway (west) and highway 49(East) to the Alberta border. In either direction you must yield to the traffic in the circle. Most of the tourists especially the ones that drive their big rigs(RV’s) pay no attention to the yield sign, or do not know it is there, and keep on driving narrowly missing other vehicles. The stoppage never occurs in the circle, but on the roads leading to the circle.

Ahmed
21st of March,2006
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