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Freeing Up ‘Freeways’

Over at Freakonomics, UCLA’s Eric Morris lays out the rational arguments for variable road tolling (a subject that people tend to get pretty irrational about). I wonder if we should drop all reference to pricing/tolls/charges and simply call such things part of the “smart grid” (and there are a lot of interesting comparisons to be drawn between transportation and this emerging concept from the world of energy).

This bit about California’s S.R. 91 caught my eye:

By pricing to keep traffic speeds at 45 m.p.h. or a bit higher, the toll lanes will work with maximum efficiency. They’ll move a lot more cars through than they did when they were congested. During the peak periods on SR 91, the toll lanes handle 40 percent of the traffic despite the fact that they constitute only one-third of the road surface.So the toll lanes will actually ease the burden on the free lanes, hence the benefit even to those who never choose to pay.

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This entry was posted on Thursday, January 8th, 2009 at 6:05 pm and is filed under Congestion, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

One Response to “Freeing Up ‘Freeways’”

  1. aaron Says:
    January 11th, 2009 at 10:14 am

    A simple way to explain this is that people all use some form of the equation for stopping distance we learned in drivers ed to allow safe following distance to react to traffic changes.

    distance = velocity * reaction time + braking coefficient * velocity^2 + some constant

    Reaction distance increases linearly with speed, slowing distance increases quadratically. Once the speed is reached where reaction distance is equal to stopping distance, more speed requires much more stopping distance. So at high speeds, more space is needed to follow safely. At 45mph, much less space is needed than at 70, so more cars fit onto the roads.

    Also, once capacity for a given speed has been reached, when new cars enter the system, traffic must slow down to make space. This is true whether traffic is flowing above or below the ~45mph threshold. So preventing new cars from entering the system allows traffic to flow at a more efficient speed.

    Interestingly, the 45mph speed is very near the optimal speed for fuel efficiency. 55mph gets optimal fuel efficiency, but there is little difference between 55 and 45mph efficiency.

    Below 55mph, faster is more fuel efficient. This is probably a big part of why high gas prices cause fuel efficiency to go down, as has been happening for several years. Despite less driving and more efficient vehicles being bought.

    People mistakenly think slow is more fuel efficient, both in acceleration and speed. Accelerating quickly, but smoothly, is more efficient than accelerating slowly. And faster is more efficient except for freeway speeds above 55mph.

    When people bring this slow is efficient mentality to the surface streets, congestion at bottlenecks compounds this problem.

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Traffic Tom Vanderbilt

How We Drive is the companion blog to Tom Vanderbilt’s New York Times bestselling book, Traffic: Why We Drive the Way We Do (and What It Says About Us), published by Alfred A. Knopf in the U.S. and Canada, Penguin in the U.K, and in languages other than English by a number of other fine publishers worldwide.

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