Chaos and Your Everyday Traffic Jam 477
An anonymous reader writes "What causes these mysterious traffic jams that continually appear throughout the day for no reason whatsoever? Is it simply the fact that most people just don't have a clue how to drive? That's very possible, and in reality there are so many variables involved in something like a traffic jam. But is it possible that the entire traffic jam could be both the continuing and end result of a chain reaction set in motion by a single driver who was in too much of a hurry?"
Field of Study (Score:4, Informative)
This mainly deals with optimizing freeways and the like, based on people's behavior in traffic, and the ripple affects.
Re:On a very busy road... (Score:4, Informative)
Re:Roads and CSMA/CD (Score:4, Informative)
Re:It's both! (Score:4, Informative)
the assumption is that the average person requires about 2/5th of a second to react,
That's very fast. Reaction to an unexpected event is 0.5-1 second. Most safety studies put 1 second as the reaction time.
What are the actual chances of this happening to any responsible, alert driver doing 70 MPH?
Responsible and alert drivers are the minority. Rules are made for everyone, so they take into account the fact that people is chatting with passengers, looking at the mountain on the left, thinking about their children, and so on. When you factor in the boringness of a long drive and all the possible distractions, even 1 second may be too low.
You need to plan for the worst case, not for the best.
Re:slow ass drivers (Score:3, Informative)
I lived outside of LA for a couple of years, and found that the OP is correct, and it applies somewhat everywhere. There is an "efficient" speed at which 90-95% of interstate highway drivers find comfortable. This varies slightly with terrain and time on the road (California has coined a term - "velocitation" - for the effect), as you get comfortable at increasingly higher speeds while driving, but is generally somewhere between 75 and 90 mph in good weather. The upper limit has a lot to do with road and wind noise, along with the responsiveness of steering of your car at higher speeds.
The problem (yes, I believe it's a problem in this context) is that the speed limit is generally 10-20mph below that limit. Conscientious drivers and the small percentage who are uncomfortable at speed (generally older drivers, but includes tentative inexperienced drivers of all ages) view the limits as a "reason" to drive slower than the flow of traffic. Yes, yes, speed limits are the law...blah, blah, blah. Stay with me for a minute - we're talking physics and human interaction, not legislation right now. Because a majority of drivers are comfortable driving significatnly faster than the limit, they tend to drive at/near the limit of enforcement, or about 7-9mph above the posted limit. A minority will carefully abide by the law, maintaining the speed limit plus or minus a couple of mph. This creates a differential in the traffic flow, and the result is platoons on single (and some double) lane roads, with the platoon leader, aka slow driver, at the head. Yes, they're really called that, according to the USDOT. In multi-lane conditions, this forms a moving blockage, with a net 10-15mph differential. Looking at it as a particle flow problem, and knowing that the cars going around the obstruction will not speed up to equalize the pressure (think Bernoulli's principle), you get a build up of traffic and an eventual blockage / traffic jam.
Trucks can cause horrible traffic jams in hilly terrain. Here in Virginia, I-81 has some hellacious slow-downs due to a two-lane traffic area and some significant grades. Full trucks will drop to 25-40mph on the up hill climbs, and then do 80+ down the next hill. Talk about a recipe for disaster. Add that to the relative inexperience in dense traffic for most of the local residents, and we have a bunch of accidents. It's not the speed, it's the speed differential. Hitting a tractor-trailer at 30mph is just about as deadly as hitting one at 70mph when there's just a guardrail between you and a 50-200' dropoff in the mountains.
OTOH, on the 210 east of Pasadena outside of LA, it's not uncommon to find 85-90mph "traffic flow" with very few incidents of jams or wrecks. Of the two I can remember, both were caused by a driver either falling asleep or drifting into the guardrail (distracted?), both were "one driver" accidents that led to messes. They probably would have caused their damage if they were going 40. One part is that there is a limited flow volume on 4 lanes - lots of space for the slow folks (i.e. anyone going less than 10 over the limit) and everyone else to have their own lanes. Often there wouldn't be but a 4-6mph difference between lanes. Very orderly, few variables.
Re:It's both! (Score:2, Informative)
Re:Roads and CSMA/CD (Score:2, Informative)