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Stone Way lane reduction study: speeders & collisions down

By Athima Chansanchai · May 25th, 2010 · 8 Comments

The Seattle Department of Transportation (SDOT) today released its before and after study of the Stone Way N. rechannelization, which the department said demonstrates that Seattle’s streets can be made safer through lane reconfigurations.

The report documents that the lane changes on Stone Way N. from 34th St. to 50th St. have had the effect of reduced speeds for motorists and fewer motor and bicycle collisions – while still maintaining the road’s capacity.

Highlights from the study include:
• Motor vehicles now travel at speeds nearer the legal limit;
• Total collisions dropped 14 percent with injury collisions down 33 percent;
• Pedestrian collisions declined significantly;
• Bike trips increased 35 percent but collisions per bicycle trip have declined; and
• Volumes show the roadway still easily accommodates motor vehicle traffic.

Data collected by SDOT on the rechannelized street shows motorists drive at speeds closer to the posted 30 m.p.h. limit and a decline of more than 80 percent in those traveling over 40 m.p.h. The study also reveals a decrease in total collisions by 14 percent and pedestrian collisions by 80 percent.

The study compared the before period (April 2005-August 2007) to a period after the rechannelization (August 2007-December 2009.) SDOT converted the street to two general travel lanes with a center turn lane from N. 40th to N. 50th streets in August 2007 and from N. 34th to N. 40th streets in April 2008. Additional bike and pedestrian facilities were also installed at those times. Before the lane reduction, the street consisted of four general purpose travel lanes.

National studies show that lane reductions can result in lower motor vehicle speeds, improved pedestrian safety, increased bicycling trips and fewer collisions, while maintaining a corridor’s capacity. Starting in 1972 with California Ave. SW and N. 45th St, the city of Seattle has implemented 24 rechannelizations as of April 2010.

There was one interesting increase, buried at the end of the study:

The number of rear‐end collisions increased, especially between N. 39th St. and N. 41st St. This type of collision increased 65% during the study period (from 17 to 28) in spite of the overall reduction in the number of collisions. Since most of the increase was attributable to the
section of roadway where there was a transition from 4 lanes to 3 lanes during the interim
period between August 2007 and April 2008, one possible explanation is that drivers had
difficulty making left turns in this transition area.  While these types of collisions tend to be
minor in nature, SDOT will again examine the rate of rear‐end collisions within the next two
years to determine if the rate in fact declined after the roadway was rechannelized south of
40th Street.

For more information: check out SDOT’s website.




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