A “sharrow”, or shared lane marking, is a symbol painted on roads to indicate that vehicles on the road should share the road with cyclists. Sharrows have been in DC since around 2008, but they have been used in Denver, Colorado since the 1990s. They were added in 2009 to the Manual on Uniform Traffic Control Devices (MUTCD), which established them as an accepted form of bike infrastructure. The MUTCD includes provisions for shared lane markings, specifically the sharrow design, with guidance that the markings should be placed at least 11 feet from the curb face or the edge of pavement on a street with parallel parking. Also, on streets with no parking and an outside lane less than 14-ft wide, the centers of the sharrows should be placed at least 4 feet from the curb or the edge of pavement.
A recent study by the University of Colorado at Denver concluded that streets with sharrows are no safer than those without. The researchers divided Chicago into three groups: blocks with no bike infrastructure, blocks with only sharrows, or blocks with bike lanes. The researchers discovered that blocks with bike lanes experienced a significantly larger increase in bike commuters than blocks with sharrows or no bike infrastructure at all. Injury crashes per year, per 100 bike commuters decreased in all three groups, but blocks with only sharrows had the smallest drop in injuries per year per 100 bike commuters. Blocks with bike lanes had the greatest decrease in bike injuries.
The University of Colorado’s study runs contrary to a prior study by the Federal Highway Administration published in December, 2010, which examined the effect of sharrows in three cities. With respect to one street in Cambridge, Massachusetts, the study found that dooring events (when a driver opens his door into oncoming bike traffic) decreased from 4 percent to less than 1 percent on streets with sharrows. Injuries from cars pulling into or out of parking spaces decreased from 11 percent to 4.5 percent. The study also concluded that sharrows increased spacing between cyclists and motor vehicles. Similar results were found in the study’s examination of a street in Chapel Hill, NC.
The authors of the University of Colorado study told Streetsblog that sharrows have a small effect on encouraging people to bike, but no other benefit. As our friends at the Washington Area Bicyclist Association have been advocating, the best measure to improve cyclists’ safety is probably protected bike lanes.