Guidelines for the Use of Protected/ Permissive Lefi-Turn Phasing

T urning left across opposing traffic at a signalized intersection could result in a traffic accident, as well as create motorist delay. A solution to the left-turn problem may be the addition of a left-turn phase when certain guidelines are met.’ After a decision has been made to add a left-turn phase, one of two basic alternative phasing methods is commonly used. In Kentucky, the predominant type of left-turn phasing has been a protectedonly type, in which the left-turning driver is allowed to turn left only during the green arrow portion of the phasing, during which time opposing traffic is stopped. An alternative type of phasing that has been used sparingly in Kentucky in recent years involves a combination of protected and permissive leftturn movements. During a portion of the left-turn phase, the left-turn movement is made on a green arrow and is protected from opposing traffic. In addition, left turns may be made during the remaining green through phase for the subject street when there are available gaps in opposing traffic. This is the permissive portion of the phase. Split phasing also is used in some instances. In a previous research study, the results of replacing protected-only (exclusive) with protected/permissive (permissive) phasing at four trial intersections were studied.’ The permissive phasing provided a substantial reduction in delay and was popular with local drivers. However, several left-turn related accidents occurred at those locations. Since those initial trial installations, permissive phasing has been used at several intersections across the state. This created a larger data base that could be used to determine when permissive phasing could be used without causing an accident problem. The objective of this study was to develop guidelines to aid traffic engineers in deciding whether permissive left-turn phasing is appropriate for use at a given location. A review of the literature was conducted. Several studies were found that investigated the effects permissive phasing has on traffic accidents. Several studies were also found that listed guidelines for the use of permissive phasing. The results of those studies were summarized in the literature review included in the full report.’ The recommended guidelines found in the literature were considered in the development of guidelines in this study.