Seattle redesigned Stone Way with two traffic lanes, a center turn lane and bike facilities; total crashes fell 14%, injury crashes 33% and pedestrian crashes 80%

In July 2007, Seattle repaved part of Stone Way North, a four-lane street linking Fremont and Wallingford neighbourhoods. The city used the repaving to restripe it with two travel lanes, a centre turn lane, bike lanes and sharrows. Local business owners pushed back because they saw Stone Way as a vital arterial, a detail recorded in the road diet case study from the Federal Highway Administration (FHWA). The Seattle Department of Transportation (SDOT) evaluated the results in a May 2010 report, Stone Way N Rechannelization: Before and After Study. Total collisions fell 14%, injury collisions 33% and pedestrian collisions 80%. The numbers behind those percentages are smaller than they sound.Seattle restriped Stone Way in two phasesSDOT’s study area ran approximately 1.2 miles, from N 34th Street to N 50th Street, on a street that carried about 13,000 vehicles a day. As mentioned in the above study, N 40th Street to N 50th Street was restriped in August 2007, while N 34th Street to N 40th Street followed in April 2008. The report provides a number of reasons that led to this change. There were new crosswalk standards in 2004, so existing crosswalks would need to be striped out.Collisions fell from 159 to 137 across matching 28-month periodsIn the report’s comparison of April 5, 2005 to August 6, 2007 with August 7, 2007 to December 4, 2009, total collisions fell from 159 to 137 and injury collisions from 52 to 35. Pedestrian crashes decreased from 5 to 1, making the 80% decrease four crashes. Angle collisions fell 56%, and the share of collisions involving an injury dropped from 33% to 25%. Bicycle crashes were still seven in both time periods, but with an increase in bicycle trips, the number of crashes per bicycle trip had decreased.One type of collision went the wrong way. Rear-end collisions rose 65%, from 17 to 28, mostly between N 39th and N 41st Streets, where the road shifted from four lanes to three during the interim period between August 2007 and April 2008, when only part of the corridor had been restriped. The same report states that the driver had trouble making left turns at the location and acknowledged the two-year period to be difficult to quantify statically.Top-end speeding dropped by about 75%The posted speed limit was 30 mph. Before the change, the report put the 85th percentile speed at approximately 37 mph and it dropped by about 1 mph northbound and 3 mph southbound. Those driving at 10 mph or more over the posted speed limit decreased by about 75%. Traffic in excess of 40 mph reduced from about 4% or 150 vehicles per day to 1%, or less than 25 vehicles per day. The speed drop matters for pedestrians, since the United Kingdom study Killing Speed and Saving Lives, cited in the report, put a pedestrian’s chance of surviving a hit at 85% at 20 mph and 15% at 40 mph.Throughput was maintained and cycling increasedThere was a reduction in average daily traffic of vehicles by about 6%, whereas there was an increase in the number of bicycles by 35% between 2007 and 2010. In SDOT’s counts, daily traffic on parallel streets fell 12% to 34%, which suggests drivers did not divert onto side streets. At N 42nd Street, peak hour volume has been reduced by less than 5% and at N 48th Street by 13%.Other road diets show a similar rangeA November 2014 FHWA newsletter, Using Road Diets to Advance Healthy, Safe Communities, put the crash rate reduction from road diets at 19% to 47%. The newsletter’s other example, East Boulevard in Charlotte, North Carolina, is a 35 mph street where the 85th percentile speed dropped from 43 mph to 40 mph and crashes on one segment fell from 2.64 to 1.67 per month. On the other segment of East Boulevard, the drop was smaller, from 1.97 to 1.86 crashes per month. It should be noted from the newsletter that road diets are not suitable for all roads and that streets with less than 20,000 vehicles per day may be considered for further study.You use AI every day. Now get your AI Quotient. Take the AIQ test.