Effects of Traffic Signal Installation on Accidents

A before-after accident study of 31 recently signalized intersections within the City of Milwaukee was performed, using three years of accident data before, and at least two and usually three years of data after signalization. City-wide volume and accident trends were used as a control to adjust the data. Upon signalization, overall, little or no change was noted either in the number of accidents, or in severity as measured by property damage only equivalent (PDOE) accidents. Individually, only four intersections showed significant changes in PDOE accidents, three increasing and one decreasing. A significant decrease of 23% in rightangle PDOE accidents was accompanied by a significant increase of 41% in rear-end PDOE accidents (includes sideswipes), and a significant increase of 56% in other (head-on, vehicle/bicycle, fixed-object, etc.) PDOE accidents. Vehicle/pedestrian and opposing direction one turning left type accidents did not change significantly. Various parameters involving geometries, traffic control, and vehicular volumes were analyzed. These parameters had little effect on the change in PDOE accidents upon signalization of the intersections in this study. A significant shift in accidents was noted, however, between periods of flash and normal operation. The increase in PDOE accidents of 42% during flash, and a corresponding decrease of 8% during normal operation is due almost entirely to a change in severity. Whether or not a signal was warranted seemed to have little bearing on whether accidents would be reduced. There does appear to be some correlation between the number of correctable accidents before, and the change in accidents upon signalization, but even this is weak. Therefore, the traffic engineer should use the established (MUTCD) warrants as merely guidelines and still analyze each location for both accidents and operational efficiency. Only if overall benefits can be expected for the entire system effected by the signal in study, should signalization be considered