a mystery

[cellio]

Consider a lane-closing on a highway. You have, say, two lanes of traffic going 60mph that have to merge into one lane. Why is the resulting speed near the merge point not close to 30mph? It's usually around 5mph. "Friction" (that is, wiggle-room from imperfect communication) should slow it down from 30mph, but not that much.

For extended construction projects (like the one I drove through on the way to Pennsic), wouldn't be worth the hassle of installing a traffic signal in order to improve flow? I wonder why they never do this. (They obviously have electricity available to power all the message-board signs.)

Comments

[tangerinpenguin.livejournal.com]

The problem is that the individual effects of "friction" tend to multiply as they propagate back through a series of moving cars, like the beats when matching pitch or diffraction rings. In some cases, with very dense traffic at highway speeds, a 10 MPH slowdown at one point can build into a complete stop a mile earlier.

There's a cool simulation of this at http://www.amasci.com/amateur/traffic/traffic1.html that also discusses some interesting science experiments that the overly clever driver can play with other people's rush hours.

[cellio]

Neat site! Thanks for the link. Someone at Pennsic tried to explain this as fluid dynamics, but it took seeing the animation and wave explanation to really grok it.

[goljerp]

Also, don't neglect the game theory factor. Fluid flow is random, while drivers (theoretically) are thinking. The problem is, a merge has aspects of a non-iterative zero-sum game, so there's great incentive to do selfish things which may slow down traffic flow in general, but benefit an individual driver. And there's a lot of reinforcement in this - once one person has acted selfishly (slowing down traffic for others), there's more of a payoff for selfish behavior. Now, I've thought on occasion that if there were a way to make traffic merges iterative (i.e. each driver has a transponder which tells other drivers how obnoxious they were at the last merge, or something) then people would be more polite. Although the traffic light idea is probably more likely. I did see some traffic lights on highway on-ramps while I was recently in California, although I avoided driving during the (rush-hour) times when they were in use.