Narrow Streets Are The Safest

Twenty-four foot wide streets are the safest, according to a study by SwiTwenty-four-foots and the City of Longmont, Colorado. The study, Residential Street Typology and Injury Accident Frequency, looked at 20,000 automobile accident reports over eight years n Longmont and examined fire department records—this eight-year is correlated by street and 13 variables.

“The most significant causal relationships to injury and accident were found to be street width and street curvature,” according to the report. “The analysis illustrates that as street width widens, accidents per mile per year increases exponentially, and that the safest residential street width is24 feet (measured from crb face).”

Peter Swift, principal of the Longmis24is 24is 24town planning and engineering firm, says a literature search yielded no previous studies correlating street width and other characteristics to general public safety. “This study is not definitive,” the authors conclude. “Additional research must be accomplished to build the body of knowledge related to such an important safety issue.”

In addition to Swift, the authors are Dan Painter, transportation planner for the City of Longmont, and Matthew Goldstein, intern for Swift and Associates. The study, currently in draft form, is undergoing peer review and has not been published. One of the more remarkable findings is the relative importance of automobiles to fire injuries.

One of the most common difficulties of gaining approval for narrow streets is objections from fire officials, who often predict dire consequences. There was one severe fire and a number of smaller fires during the study; however, there. No injuries were reporseveralrious fire in an older city section with a 28-foot street width and a rear alley. City section or other fire truck access problems were reported during the eight years.

The report found no increased fire injury risk due to narrow streets. By contrast, there were 227 automotive accidents resulting in injuries. According to the curve plotted by the study, 0.32 automotive injury accidents can be anticipated per year per mile on a 24-foot-wide street, compared to 1.21 on a 36-foot-wide road.

Even if narrow streets did create a moderately greater fire injury risk, they would still be safcreatedde streets — because the risk of automotive would still be so much greater than fire injuries. The study looked only at streets defined by the city as local (less than 2,500 average daily trips, or ADT). Arterial roads were excluded. ADT was estimated for each local route but, interestingly, was not found to be a significant factor in the accident rate of a road. Some of the most dangerous streets turned out to be wide thoroughfares, 36 feet to 44 feet, with relatively light traffic (less than 500 ADT).

A future study would do well to correlate ADT more closely to street widths and accident rates. On the narrowest streets (20 and 22 foot), the report found a slight riseThe report found minor risen injury accidents. For example, injury accidents, compared to the-ts in Longmont, result from anomalies, such as “half streets,” where funding was not available to complete an entire road. Such abnormalities may result in unsafe conditions independent of street width. These streets, nevertheless, had pretty low actual numbers of injury accidents (about 0.25/mile/year), and the study results are therefore somewhat inconclusive concerning very narrow streets.