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From: dafa@ihuxs.UUCP
Newsgroups: net.auto,net.physics
Subject: Re: physical laws of freeway traffic?
Message-ID: <333@ihuxs.UUCP>
Date: Sat, 18-Jun-83 21:38:25 EDT
Article-I.D.: ihuxs.333
Posted: Sat Jun 18 21:38:25 1983
Date-Received: Thu, 23-Jun-83 23:27:49 EDT
Organization: BTL Naperville, Il.
Lines: 45

Apropos the discussion of traffic jams, here is
"The Straight Dope" from Cecil Adams in the
Chicago Reader:

	My friends and I were trapped in the middle
of the Santa Monica freeway, unable to move in any direction,
when the conversation turned to the cause of our condition.
"Why," one friend asked, "does traffic come to a stop on a highway
that presumably offers nothing to stop it? We should be
able to drive across the country and back without stoppping,
except for gas." It sounds like a silly question, but what
stops the first car in the daily freeway tie-up? - D. S.,
nearing th Vermont off-ramp, Los Angeles

	I don't know much about the Santa Monica freeway, D.,
but I would venture to say that if you are near the Vermont
off-ramp, part of your problem is that you are in the wrong
damned state. See if you can work on this. In the meantime
you may be interested to know that engineers have devoted
considerable study to expressway traffic, and they have
concluded that there is a compelling psychological principle
that causes the cars to stop, namely the fear of flaming death.
Here's what happens. In theory, given the old rule about
maintaining one car length ahead of you for each ten miles
per hour driving speed, the capacity of a single lane of
expressway is 40 cars per minute (2,400 per hour) at 60
MPH. In practice, however, drivers instinctively begin to slow
down at loads higher than 25 cars per minute (1,500 per hour).
At 33 cars per minute (2,000 per hour), average speed drops
to 35 MPH. At this critical juncture, drivers are extremely
jumpy, and they will slam on the brakes at the slightest
provocation - anything from an accident or a stall to a couple
of extra cars trying to merge into traffic at an on-ramp. The
first guy slows down a little, the second guy slows down a
lot, and the third, fourth, or fifth guy may stop altogether,
bringing traffic to a halt. That's why you almost never find
smoothly flowing expressway traffic at speeds below 35 MPH -
it's usually stop-and-go. It also explains why relatively
minor increases in traffic volume, such as those caused by
mass transit fare increases, frequently result in chaos on
the highways. For this reason, if suburbanites had brains
instead of mush, they would always support transit subsidies.
As usual, however, you can never get people to understand
what is good for them.