Slower is Faster in D.C.

One of the themes readers will discover in Traffic is the idea that slower can be faster. Ramp meters on highway entrances, for example, keep the mainline flow going more smoothly by temporarily holding up drivers on the ramps. The individual driver suffers a moment of time loss, but the whole system moves better.

I’m in D.C. for the day, and I’ve been interested to note that an idea that’s had success in places like England’s M25 motorway is being introduced here. It’s called “variable speed limits” (wait, aren’t they all variable, you’re asking?), and the basic idea is that when a section of highway has become congested, rather than having upstream vehicles simply drive at full speed into the gelling pack, those drivers are given instructions to drive at specific speeds, lower than the typical speed limit. Instead of driving into a stop-and-go mess (in which a lot of time and fuel is wasted stopping and restarting), following cars approach at a slower, smoother pace. When the new speeds are obeyed (in the U.K. they’ve mounted cameras to enforce this), engineers have found they can achieve greater “throughput” through bottlenecks.

It’s counterintuitive, but slower is faster. As individual drivers, we pursue our immediate interest, which is to get ahead as quickly as possible. But in traffic, this works against the system as a whole. As Phil Goodwin once described it in another context, “It is one of those cases where Adam Smith’s individuals pursuing their own best interests do not add up to Jeremy Bentham’s greatest good for the greatest number.” These sorts of initiatives, which are lumped under the heading of “ITS,” are just one of the ways we can “think,” and not build, our way out of congestion.

This entry was posted on Thursday, July 31st, 2008 at 6:40 am and is filed under Cars, Congestion, Drivers, 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.

4 Responses to “Slower is Faster in D.C.”

  1. Lea Croteau Says:

    The germans have been doing this for years also. Around Munich where there is congestion, the electronic speed limit signs post variable speeds for the time of day and flow. We’ve all seen the person who insists on dodging and weaving through traffic to get ahead, only to see the same car some minutes later. I moved to LA recently, also having been from the WashDC area, and have noticed that at rush hour, the flow of traffic here actually works pretty well. And no cameras are necessary because of the amount of traffic. Thru traffic stays in the leftmost lanes, while merging and exiting traffic occupy the right lanes. Slow downs occur at on/off ramps, where people are changing lanes. Many drivers out here have the right idea: drive a slower, consistent speed to avoid the accordion effect. There are meters here too, as well as HOV lanes which are also home to hybrid vehicle traffic.

  2. Ted Says:

    I just found out about this book in a newspaper (Wall Street Journal, I think?). It is going straight to the top of my reading list. I have been living in the Research Triangle area of North Carolina for the past three years, being a transplant from Upstate New York. One of the biggest culture shocks of living down here is the absolutely astounding aggressive, or just plain poor, driving that goes on around here. I used to never see accidents or come close to death as a result of other drivers; now it seems it is a daily occurrence.

  3. Christopher Monnier Says:

    The bit about variable speed limits made me think of a slightly different way to implement them. Instead of using traffic cameras to enforce variable speed limits (which seems like a heavy-handed tactic that many drivers will likely ignore), why not just charge drivers different rates for different speeds traveled? Assuming that a “pay-per-mile” tolling scheme is in place, drivers could be charged not only for where and how far they drove but also how fast they drove. As drivers approach a traffic jam, there could be one rate for traveling at 65 mph, another for 55 mph, and another for 45 mph, with the cheapest rate assigned to the speed that optimizes traffic flow.

    This solution provides a transparent set of incentives for drivers to reduce their speed without resorting to some draconian enforcement mechanism (i.e. cameras that rigidly enforce lower speed limits).

  4. Christopher Monnier Says:

    > “It is one of those cases where Adam Smith’s individuals pursuing their own best interests do not add up to Jeremy Bentham’s greatest good for the greatest number.”

    I think this is because the individuals don’t know what their own best interests are. If a mechanism were put in place that directly linked individual financial incentives with the overall flow of traffic, I think Adam Smith would be vindicated. In other words, given the proper information and incentives, optimal flow will be reached more effectively if individual drivers are charged with acting in their own best [financial] interest than if a heavy-handed bureaucracy (i.e. cameras) exhorts top-down control.

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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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