  Transit signal priority (TSP) has been implemented to transit systems in many
cities of the United States. In evaluating TSP systems, more attention has been
given to its operational effects than to its safety effects. Existing studies
assessing safety effects of TSP reported mixed results, indicating that the
safety effects of TSP vary in different contexts. In this study, TSP
implementations in Portland, Oregon, were assessed using interrupted time
series analysis (ITSA) on month-to-month changes in number of crashes from
January 1995 to December 2010. Single-group and controlled ITSA were conducted
for all crashes, property-damage-only crashes, fatal and injury crashes,
pedestrian-involved crashes, and bike-involved crashes. Evaluation of the
post-intervention period (2003 to 2010) showed a reduction in all crashes on
street sections with TSP (-4.5 percent), comparing with the counterfactual
estimations based on the control group data. The reduction in
property-damage-only crashes (-10.0 percent) contributed the most to the
overall reduction. Fatal and injury crashes leveled out after TSP
implementation but did not change significantly comparing with the control
group. Pedestrian and bike-involved crashes were found to increase in the
post-intervention period with TSP, comparing with the control group. Potential
reasons to these TSP effects on traffic safety were discussed.
