Reducing red light running through longer yellow signal timing and red light camera enforcement: results of a field investigation

January 2008

Red light running is estimated to account for 900 intersection crash fatalities annually. Two principal methods used to reduce red light running involve lengthening the duration of yellow change intervals and automated red light enforcement. The present study evaluated the incremental effects on red light running of first lengthening yellow signal timing, followed by the introduction of red light cameras. At six approaches to two intersections in Philadelphia, Pennsylvania, yellow change intervals were increased by about 1s, followed several months later by red light camera enforcement. The number of red light violations was monitored before changes were implemented, several weeks after yellow timing changes were made, and about 1 year after commencement of red light camera enforcement. Similar observations were conducted at three comparison intersections in a neighboring state where red light cameras were not used and yellow timing remained constant. Results showed that yellow timing changes reduced red light violations by 36%. The addition of red light camera enforcement further reduced red light violations by 96% beyond levels achieved by the longer yellow timing. This study shows that the provision of adequate yellow signal timing reduces red light running, but longer yellow timing alone does not eliminate the need for better enforcement, which can be provided effectively by red light cameras.