Yes, you read it right. More roads leads to more congestion and slower travel times, even if the amount of cars/drivers remains the same. It seems that the Nash equilibrium, which drivers achieve simply by acting in their own self interest is not the most efficient way to move people across our cities. An article in this month’s Scientific American gives us the details:
Conventional traffic engineering assumes that given no increase in vehicles, more roads mean less congestion. So when planners in Seoul tore down a six-lane highway a few years ago and replaced it with a five-mile-long park, many transportation professionals were surprised to learn that the city’s traffic flow had actually improved, instead of worsening. “People were freaking out,” recalls Anna Nagurney, a researcher at the University of Massachusetts Amherst, who studies computer and transportation networks. “It was like an inverse of Braess’s paradox.”
The brainchild of mathematician Dietrich Braess of Ruhr University Bochum in Germany, the eponymous paradox unfolds as an abstraction: it states that in a network in which all the moving entities rationally seek the most efficient route, adding extra capacity can actually reduce the network’s overall efficiency. The Seoul project inverts this dynamic: closing a highway-that is, reducing network capacity-improves the system’s effectiveness.
Although Braess’s paradox was first identified in the 1960s and is rooted in 1920s economic theory, the concept never gained traction in the automobile-oriented U.S. But in the 21st century, economic and environmental problems are bringing new scrutiny to the idea that limiting spaces for cars may move more people more efficiently. A key to this counterintuitive approach to traffic design lies in manipulating the inherent self-interest of all drivers.
A case in point is “The Price of Anarchy in Transportation Networks,” published last September in Physical Review Letters by Michael Gastner, a computer scientist at the Santa Fe Institute, and his colleagues. Using hypothetical and real-world road networks, they explain that drivers seeking the shortest route to a given destination eventually reach what is known as the Nash equilibrium, in which no single driver can do any better by changing his or her strategy unilaterally. The problem is that the Nash equilibrium is less efficient than the equilibrium reached when drivers act unselfishly-that is, when they coordinate their movements to benefit the entire group.
The solution hinges on Braess’s paradox, Gastner says. “Because selfish drivers optimize a wrong function, they can be led to a better solution if you remove some of the network links,” he explains. Why? In part because closing roads makes it more difficult for individual drivers to choose the best (and most selfish) route.
The article goes on to discuss the phenomenon known as shared streets, where all traffic lights, signage and lanes are removed from roads and space is shared equally between cars, trucks, bikes and pedestrians. One would think that anarchy is the result, but in fact traffic flows and safety both improve. Who would have thought that the French method of driving was optimal, or even safe? 😉
Finally, there is a discussion about zoning that requires developers to provide x number of parking spaces per square metre of building, and how this seemingly good idea subsidises car use and makes congestion in our cities much, much worse.
In a misguided effort to reduce congestion, planners in the 1950s required developers to provide a minimum number of free parking spaces-a strategy that “completely ignored” basic economics, Siegman says, referring to how lower prices increase demand.
Now limited urban space and concerns about global warming are inspiring city planners to eliminate these requirements. In San Francisco, for example, developers must restrict parking to a maximum of 7 percent of a building’s square footage, a negligible amount. Although downtown employment has increased, traffic congestion is actually declining, Seigard says. With fewer free spaces to park, drivers seem to be switching modes, relying more on mass transit, cycling and just plain walking.
While I am not advocating the removal of any roads in New Zealand, (I was thrilled when they finally completed spaghetti junction in Auckland), the building of new roads is clearly uneconomic. Let’s fix up and otimise the ones we have, while providing real alternatives that reduce our dependence on foreign oil.