摘要：Objectives. We evaluated the effects of on-street bicycle lanes installed prior to 2007 on different categories of crashes (total crashes, bicyclist crashes, pedestrian crashes, multiple-vehicle crashes, and injurious or fatal crashes) occurring on roadway segments and at intersections in New York City. Methods. We used generalized estimating equation methodology to compare changes in police-reported crashes in a treatment group and a comparison group before and after installation of bicycle lanes. Our study approach allowed us to control confounding factors, such as built environment characteristics, that cannot typically be controlled when a comparison group is used. Results. Installation of bicycle lanes did not lead to an increase in crashes, despite the probable increase in the number of bicyclists. The most likely explanations for the lack of increase in crashes are reduced vehicular speeds and fewer conflicts between vehicles and bicyclists after installation of these lanes. Conclusions. Our results indicate that characteristics of the built environment have a direct impact on crashes and that they should thus be controlled in studies evaluating traffic countermeasures such as bicycle lanes. To prevent crashes at intersections, we recommend installation of “bike boxes” and markings that indicate the path of bicycle lanes across intersections. Bicycling is a healthy, environmentally friendly alternative to automobile use. 1 – 3 Yet, in the United States bicycling is primarily considered a recreational pursuit rather than a means of utilitarian travel. Among the nearly 140 million commuting trips made every day, slightly less than 0.5% are made by bicycle. 4 Of trips for all purposes in the United States, only 1% are made by bicycle. 5 Approximately 25% of all trips made are less than 1 mile, and 75% of these short trips are made by automobile. 6 If some of these short trips were made by active modes such as walking or cycling, more people would reach the recommended 150 minutes of moderate-intensity physical activity per week (at present, fewer than 5% of adults engage in this amount of physical activity 7 , 8 ). Integrating physical activity into daily routines such as bicycling to work 9 would also lead to sustained increases in habitual physical activity. 7 The health and environmental benefits of cycling are clear and significant. However, bicyclists are vulnerable in that they share the same roadway with motorized vehicles. At intersections, they must maneuver their way through conflicting vehicular movements if they need to make a turn. Indeed, safety is a major concern that discourages people from bicycling. 10 , 11 When a crash occurs, bicyclists are much more likely than motor vehicle users to sustain an injury, and the injury is likely to be more severe. Therefore, there is a need to gain a full understanding of the factors associated with cycling safety, particularly because many American cities are installing extensive bicycle lane networks to encourage the use of cycling for commutes. 12 Studies of the safety effects of bicycle lanes in the United States date back to the 1970s. Some of the early studies, based on self-reported data from surveys of bicyclists or police reports, compared bicycle crash rates on different types of roadways such as roads with or without marked bicycle lanes and off-road trails. These studies reported lower bicycle crash rates on roads with bicycle lanes than on roads without such lanes. 13 – 17 However, causality cannot be inferred from these neighborhood-level studies because of confounding factors. Results from studies conducted at the roadway segment or intersection level have been mixed. 18 – 20 A major limitation of the existing studies is their lack of a rigorous quasi-experimental design that included a treatment group and a comparison group and that compared crashes in these groups before and after the installation of bicycle lanes. 21 A report published by the Transportation Research Board and the Institute of Medicine described the existing literature on the built environment and physical activity as follows: [M]ost of the studies conducted to date have been cross sectional. Longitudinal study designs using time-series data are also needed to investigate causal relationships between the built environment and physical activity. 22 (p7) The report went on to state that [w]hen changes are made to the built environment—whether retrofitting existing environments or constructing new developments or communities—researchers should view such natural experiments as ‘demonstration’ projects and analyze their impacts on physical activity. 22 (p229) The same limitations apply to later studies evaluating the impact of the installation of bicycle lanes on safety. In a before–after study of bicycle lanes on arterial roads in Madison, Wisconsin, Smith found an increase in bicyclist crashes on the 2 roads with bicycle lanes; however, the increase was insignificant relative to the increase in city-wide bicyclist crashes observed. 18 Increases in crashes were also found in a before–after study of bicycle lanes in Oxford, England. 19 To our knowledge, only 1 before–after study involved the use of both a treatment group and a comparison group to evaluate the safety impact of bicycle lanes. 20 This study, which focused on bicycle lanes installed in Copenhagen, Denmark, between 1988 and 2002, revealed increases in most types of crashes and injuries on roadway segments and at intersections with bicycle lanes; however, none of these increases were significant at the 5% level. Using a quasi-experimental design that included a treatment group and a comparison group, we conducted a before–after analysis of 43 miles of bicycle lanes installed in the 5 boroughs of New York City from 1996 through 2006. The city's 5 boroughs vary greatly in built environment characteristics, 23 and this large variation helps strengthen the validity of our model results. We used generalized estimating equation (GEE) methodology 24 to account for correlations within repeated observations and to control factors (e.g., built environment factors) that could not be controlled through the use of a comparison group.
