An evaluation of leading versus lagging left turn signal phasing
Abstract
The use of optimum phase sequences at signalized intersections could save Indiana motorists many hours of delay and could result in fewer accidents. However, very little factual information has been available to guide engineers in choosing between the various signal phasing alternatives. To close that gap, leading and lagging left turn signal sequences were evaluated in this research using a survey of licensed drivers, and examination of traffic conflicts, an analysis of accident records, and a simulation model of traffic flow. The research results, in general, showed that lagging sequences at selected types of intersections can provide safety and delay advantages over the (more common in Indiana) leading sequence. The motorist survey revealed that leading sequences were preferred to lagging sequences, but not strongly. The lagging sequence intersections studied had fewer traffic conflicts than similar leading sequence intersections. The lagging sequence sites examined had lower accident rates and far fewer injury accidents over a four-year period than comparable leading sequence sites. The lagging sequence was found to cause more vehicles to turn "unprotected" on the green ball phase than the leading sequence, but was also found to cause many fewer vehicles to turn at or after the yellow signal at the end of the signal cycle. Finally, lagging sequences generally led to lower vehicle delay than leading sequences for a wide variety of traffic and other conditions simulated. Guidelines were presented based on the results of the research for the use of the lagging and leading phase sequences in Indiana. Other research results were presented which did not directly apply to the leading and lagging issue but may have an impact on some aspect of left turn signals.
Degree
Ph.D.
Advisors
Sinha, Purdue University.
Subject Area
Civil engineering
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