Because drivers respond differently to different signal\nphasing sequences, engineers need to evaluate how drivers interpret the message\nthat each phasing sequence is trying to convey. This research evaluated the safety and operational impacts of\ndifferent left turn movement treatments at signalized intersections by using\ncrash, video, and survey data. The results should help decision makers select\nmore suitable signal phasing plans under various operational conditions.<\/p>\n\n\n\n
Less than ten percent of intersections are signalized;\nhowever, more than 30 percent of intersection fatalities occur at signalized\nintersections. Signal phasing strategy is considered to be one of the most important\nfactors influencing the safety and operations of signalized intersections. This\nproject compared the safety of protected\nleft turns with that of protected-permissive\nleft turns with a flashing yellow arrow. Protected-permissive\nleft turn phases have the potential to improve traffic operations by allowing\nmore vehicles to complete their left turns during the permissive phase,\nespecially in off-peak hours. <\/p>\n\n\n\n
The researchers also compared the safety of doghouse displays with that of four-section vertical displays for protected-permissive\nleft turns with a flashing yellow arrow. Doghouse\ndisplays accommodate permissive left turn movements during a circular green\nsignal without a flashing yellow arrow. As a result, doghouse displays may\ncreate some confusions among drivers because a circular green signal indicates\nthe allocation of the right-of-way, while left turning vehicles must yield to opposing\ntraffic during the permissive phase with a circular green signal. <\/p>\n\n\n\n
Finally, signal plans may change from a\nprotected-permissive with a flashing yellow arrow to a protected-only phase at\ndifferent times of the day, given traffic\nconditions. Previous evidence has suggested that protected-permissive phases\nwith a flashing yellow arrow may contribute to more crashes during off-peak\nperiods because of driver confusion.<\/p>\n\n\n\n
The results of the driver\ncomprehension survey showed that half of the drivers who had encountered\nintersections with a left-turn control mode that varied by time of day felt\nconfused by that phasing strategy. On the other hand, a simulation-based\nanalysis showed that changing the left-turn control mode by time of day yielded\nmore efficient traffic operations and lower average delays.<\/p>\n\n\n\n