Monday, September 19, 2011

Cul-de-sacs may be unsafe

This month's Atlantic has an interesting article about the work of two University of Connecticut engineering professors, Norman Garrick and Wesley Marshall, on the relationship between street design and traffic safety. (Garrick is an engineering professor at the University of Connecticut). The article notes that cul-de-sacs are in part a result of government pressure rather than the free market, and goes on to discuss Garrick's empirical work on traffic safety.

Garrick examined crash data from 24 medium-sized California cities, and discovered that:

the safest cities had an element in common: They were all incorporated before 1930. Something about the way they were designed made them safer. The key wasn’t necessarily that large numbers of bikers produced safer cities, but that the design elements of cities that encouraged people to bike in places like Davis were the same ones that were yielding fewer traffic fatalities.

These cities were built the old way: along those monotonous grids. In general, they didn’t have fewer accidents overall, but they had far fewer deadly ones. Marshall and Garrick figured that cars (and cars with bikes) must be colliding at lower speeds on these types of street networks. At first glance such tightly interconnected communities might appear more dangerous, with cars traveling from all directions and constantly intersecting with each other. But what if such patterns actually force people to drive slower and pay more attention?

“A lot of people feel that they want to live in a cul-de-sac, they feel like it’s a safer place to be,” Marshall says. “The reality is yes, you’re safer – if you never leave your cul-de-sac. But if you actually move around town like a normal person, your town as a whole is much more dangerous.”

In other words, a cul-de-sac alone is safer than a gridded street alone. But cities dominated by cul-de-sacs force more traffic onto a few big commercial streets- and because those commercial streets tend to be wider and accommodate faster traffic, accidents on those streets are more likely to lead to death or serious.

The article discussed in the Atlantic article can be found at Marshall's home page, and Garrick has a home page full of presentations and slide shows.

Michael Lewyn

Transportation, Urbanism | Permalink

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