The Price Of Anarchy In Traffic Design

Anarchy on the highways isn’t something that normally crosses my mind when I pull onto the freeway for my morning commute, but in fact our roads are governed by so few laws that it amounts to anarchy each time we get behind the wheel. I’m not suggesting that we don’t follow the rules, but that in most cases we have many different routes to get to the same place and we are free to pick whichever one suits us. Drivers usually pick the route that seems to be the fastest or the one with the least number of stops or turns.

This “selfish anarchy” has created an interesting paradox for traffic engineers. They found that by limiting the number of choices for drivers, traffic actually moves more smoothly and efficiently; the more choices presented to drivers, the greater the potential cumulative delays. Given their selfish nature, drivers seek out a route that appears to be most advantageous to them personally, even if in reality that route actually takes them longer.

Several years ago, city planners in Seoul, Korea, tore down a six-lane freeway and replaced it with a five-mile long park. You would think traffic on other streets and highways would be severely impacted, but the reverse proved to be true. Using road closures to channel traffic into designed routes can actually increase efficiency rather than slow traffic down as would be assumed. In part, this is because of another counterintuitive paradox.

By limiting the number of cars a system can carry, driving itself is no longer the most efficient method of getting around a city. In the past, engineers hoped that by increasing capacity, traffic flows would be improved. What they found was that by making it easier to drive a car to work, more people drove cars to work, so the net effect was either no improvement or deteriorating conditions. The City of San Francisco, back in the 50s, required a minimum number of free parking spaces for each new building; they now limit the number of free parking spaces allowed. By making it more expensive and less convenient to drive, more people are using public transportation, bicycling, or even walking! In a time frame that saw an overall increase in employment in the City, traffic flows have actually decreased. An interesting corollary can be seen in the price of gasoline versus the amount people drive and the car choices they make. Cheap gas fosters greater consumption. The more expensive the gas, the less people drive and the smaller the cars they drive.

Perhaps the most poignant aspect of anarchy on the roads is that people will act selfishly, even when presented with evidence that their decisions may not guarantee the best outcome.

Michael Morin is the owner of Morin Bros., a full service automotive shop in San Luis Obispo. For answers to car questions email: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Much of this article is based on Scientific American, Feb ‘09, Detours by Design p 20

For an interesting definition of the mathematical principles involved, see:

http://en.wikipedia.org/wiki/Braess_paradox