Some urban roads are prime for bike lane retrofits

By Chris Spahr

An April 11 post on FiveThirtyEight provides a thoughtful analysis of what happens when a bike lane is constructed on an urban road where commuters drive on a daily basis. This analysis counters opponents of a Brooklyn bike lane who made claims that it reduces room for cars and restricts views of pedestrians crossing the street. In their analysis, Gretchen Johnson, a transportation consultant in Boston, and Aaron Johnson, a Ph.D. candidate at MIT, analyzed how the addition of bike lanes impact congestion in cities. Their data analysis shows that while peak hour traffic volumes on Prospect Park West in Brooklyn remained the same after the installation of a protected bike lane along the one-mile road, the volume-to-capacity ratio (a measure of how “full” the road is) did not reach a point that could be considered heavy congestion. This means the capacity of the road was sufficient to allow removal of a lane of traffic when the bike lane was installed without causing major disruptions to commuters traveling in cars. Moreover, the New York City DOT reported other interesting statistics, including an increase in the number of cyclists and decreases in speeding cars, cyclists riding on the sidewalk, and injury-causing crashes.

50%-75% = mild to moderate congestion 75%-90% =  heavy congestion Above 90%  = severe congestion

50%-75% = mild to moderate congestion
75%-90% = heavy congestion
Above 90% = severe congestion

The study also analyzed the 45 miles of bike lanes constructed in Minneapolis in 2010. Looking at the 10 road segments that gained a bike lane at the cost of a driving lane, the researchers studied Average Annual Daily Traffic in 2008-2009—prior to construction of the bike lanes—and again in 2012 after the bike lanes were installed. After adjusting the data to estimate congestion during rush hour, the researchers found that the volume-to-capacity ratio increased significantly on some roads, but not enough to be considered heavy congestion.

The results of this study raise a question about how we traditionally build our roads. If roads are built that have a much larger capacity than what is needed for the volume of traffic, are we making good use of public resources? Fortunately, the past mistake of overbuilding roads now creates opportunities for retrofits with bike lanes, which has been done in numerous cities. The valuable analysis provided by Gretchen and Aaron Johnson provides strong support for urban bike lanes and inspires other questions about how we build our roads.

Chris Spahr is a Graduate Assistant with SSTI.