Measuring the Safety Effects of Signal Installation and Left-Turn Phasing Schemes

There are several ways that a traffic signal system copes with left turns at an intersection: The “permitted phase” allows drivers to choose a safe gap in oncoming traffic; the “protected phase” provides turning vehicles with an exclusive turn phase and the “permitted/protected phase” combines the two to accommodate different left-turn and through patterns. The Mn/DOT Signal Design Manual provides guidance for implementing safe and efficient left-turn phasing.