Complete Streets and Adult Bicyclist Fatalities: Applying G-Computation to Evaluate an Intervention That Affects the Size of a Population at Risk

Abstract

"Complete Streets" policies require transportation engineers to make provisions for pedestrians, cyclists and transit users. These policies may make bicycling safer for individual cyclists while increasing overall bicycle fatalities if more individuals cycle due to improved infrastructure. We merged county-level records of Complete Streets policies with Fatality Analysis Reporting System counts of cyclist fatalities occurring between January 2000 and December 2015. Because comprehensive county cycling estimates were not available, we used bicycle commute estimates from the American Community Survey and US Census as a proxy for the cycling population, and limited analysis to 183 counties (accounting for over half the US population) for which cycle commute estimates were consistently non-zero. We used G-computation to estimate the effect of policies on overall cyclist fatalities while also accounting for potential policy effects on the size of the cycling population. Over 16 years, 5,254 cyclists died in these counties, representing 34 fatalities per 100,000 cyclist-years. We estimated that Complete Streets policies made cycling safer, averting 0.6 fatalities per 100,000 cyclist-years (95% CI: 0.3, 1.0) by encouraging a 2.4% increase in cycling and a 0.7% increase in cyclist fatalities. G-computation is a useful tool for understanding policy impact on risk and exposure.