The end of the Cul-de-sac?

by: commonweal

Sun Feb 07, 2010 at 10:56:17 AM EST

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).

 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.

The other study examined the effect of connectivity on fire station service area and capital facilities planning, based on research by Matt Magnasco of the Charlotte (N.C.) Department of Transportation.  

The key is this: The cost to operate a fire station generally is fixed. The size of the service area and the number of properties served per station don't really affect operating cost. Therefore, the bigger the service area and the more properties that can be served, the more efficiently the fire department is using taxpayer money. If the fire department can serve more properties with fewer stations while meeting response time standards, it can save taxpayer money.

The study examined eight fire stations in the Charlotte area and found as street connectivity increased, the number of households served by each fire station increased as well. The least-connected service areas served 5,700 to 7,300 households; the most-connected service areas served 20,800 to 25,900 households. That means there are dramatic differences in the fiscal efficiency of individual fire stations. The stations in least-connected areas cost $586 to $740 per capita annually; the stations in most-connected areas cost $159 to $206 per capita annually.

The study also looked at the trend of fire response times over the past 38 years. As the Charlotte region developed with increasingly disconnected street patterns, average fire response times increased from 4.5 to 5.5 minutes. However, in 2001 an ordinance was passed to require connectivity in new subdivisions. Since 2001, average response times have dropped below the 5-minute mark. This has occurred even though the rate of new fire station openings has remained nearly flat.

Furthermore, a study completed by the American Society of Civil Engineers compared travel demand in a status-quo suburban pattern to a traditional neighbourhood pattern. The study evaluated the performance of each pattern with respect to vehicular capacity of the street system, travel speeds, and impacts of travel times and delays, and land required for rights of ways.

the study found that traffic volume/capacity is reduced on arterials and
collectors by about 10% in the traditional pattern over status-quo developments, while local street volume/capacity is nearly equal. Thus, both patterns achieve close to the same capacity, however, where the traditional pattern allows traffic to be dispersed among a dense network of local streets, the status-quo
pattern relies on a sparse network of major arterials. In terms of travel demand (measured in daily VKT), the conventional pattern generated 75% more travel demand on arterials than the traditional development, and up to 80% more demand on collector streets. VKT on local streets was considerably higher in the traditional pattern compared to the status-quo pattern, again, due to the integration of local streets within the traditional pattern. The total VKT in the traditional development pattern was found to be 43% lower than the conventional development pattern.