The cul-de-sac is perhaps the quintessential symbol of suburban America. Perhaps millions of them have paved over greenways throughout the country. Hailed for their safety (no traffic that can run over kids) and prized by developers because they allow more houses to be built into oddly shaped tracts and right up to the edges of rivers and property lines, planners and town officials are beginning to realize their downside.
Early last year the state of Virginia became the first state to severely limit cul-de-sacs from future development. Similar actions have been taken in Portland Oregon, Austin, Texas, and Charlotte, North Carolina. What they are beginning to realize is that the cul-de-sac street grid uses land inefficiently, discourages walking and biking, and causes an almost complete dependence on driving, with attendant pollution and energy use. Furthermore, town officials are beginning to realize that unconnected streets cost more money to provide services to and force traffic onto increasingly crowded arterial roads, which then, in many cases, need to be widened (more tax money).
Two interesting studies were presented at the CNU Transportation Summit in 2008 that examined the public safety and financial implications of street
connectivity. They both seem to support what critics have begun to voice about the downside of cul-de-sacs grid networks. The first study was conducted by Norman Garrick and Wesley Marshall, of the University of Connecticut’s Center for Transportation and Urban Planning. Their study investigated the relationships between connectivity, network configuration, density, severe vehicle crashes, and mode choice.
Twenty-four California cities were analyzed at the
block level; half were classified as “safe cites” (severe/fatal crash
rates one-third of the state average), and half as “less safe cities”
(severe/fatal crash rates close to the state average). Interestingly,
the results showed that the safe cities were well established prior to
1950; the less safe cities were largely developed after that time. Even
within the safe cities, the changes in street network patterns over
time were related to big differences in performance. In the example of
Davis, CA, the pre-1940s sections of town (intersection density 211/sq
mi) had a fatal/severe crash rate that was half the rate of the
post-1970 sections of town (intersection density 111-132/sq mi). The
walking/biking/transit mode share was 59 percent in the pre-1940
sections of town; in the post-1980 sections of town the
walking/biking/transit mode share was 14 percent.In addition to intersection density, the researchers also
investigated street network configuration – grid patterns, cul-de-sac
patterns, and everything in between. The results were consistent across
the board, with highly connected networks of small blocks exhibiting
the best performance in all categories.
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I wanted to point out a neighborhood in Davis that has several cul-de-sacs, which are preferential bike streets. While the cul-de-sacs are not through streets for cars, they are for bikes. There are several examples that you can find using Google aerials. In these locations, there is a network of off-street paths for bicycles and pedestrians that are free of heavy traffic volumes. I think in this situation (whether the paths were retrofitted or built at the time of the street) is an example where cul-de-sacs can benefit cyclists.