Selection criteria for left-turn phasing, indication sequence, and auxiliary sign

This report documents the development of guidelines and recommendations for the selection of the left-turn phasing, indication sequences and auxiliary signs. The guidelines developed are based on field studies of over 100 intersections in nine Texas counties and utilize easy to obtain data for the selection process. A three-level decision process regarding the suitability of left-turn phasing treatment to be used is established. The process favors the least restrictive permissive left-turn phase unless traffic and geometric conditions warrant the more restrictive protected/permissive or protected only phasings. The guidelines developed are based on threshold values which statistically determine what constitutes an excessive value for any particular decision variable, beyond which more restrictive left-turn phasing treatments may be justified. Recommendations concerning signal indications and auxiliary signs are made based on delay, safety and degree of motorists' understanding. The study results show that selection of any particular phasing is a multi-objective process involving a number of factors, and in many cases more than one condition must be met to justify the selection of a particular phase to ensure an optimum solution.