Evaluation of Safety and Development of Guidelines for Protected/Permitted Left-Turn Signal Controls
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This research addresses two particular issues associated with protected/permitted left-turn signal controls: the safety of a special protected/permitted left-turn control display implemented at several intersections in the Las Vegas area, and the selection of appropriate left-turn control types. The safety of the special protected/permitted left-turn control was evaluated based on the crash statistics from before-after implementation of the special left-turn control. Guidelines were developed using a multiple attribute decision-making approach to determining left-turn control types. The guidelines were tested in a case study based on intersections selected from three major transportation agencies in the State of Nevada. Results from the before-after safety analyses did not reveal any obvious safety concerns about the special protected/permitted left-turn display. However, the crash data were drawn from a very small sample size, thus additional analyses may be necessary to enhance the research findings and conclusions. The additional analyses may require more observation years for the "after" period to increase the sample size, or the analyses may be conducted from other perspectives such as surveying drivers' understanding of the special display. The left-turn control guidelines developed in this research are sensitive to the weighting-and-scoring scheme employed in the guidelines, which can be modified to tailor agencies' specific policies. Based on the results from the case study, the current weight-and-scoring scheme included in this report tends to favor a more restrictive control policy when both protected/permitted control and protected control are plausible; there a protected control is more likely selected. Such a difference in policy was clearly shown among the three agencies in Nevada, where the City of Reno adopts a more restrictive control policy while the City of Las Vegas adopts a less restrictive policy. The City of Sparks, however, adopts a somewhat moderate policy.