Evaluating the Safety Performance of Left-Turn Signal Phasing: A Roadway-Level Case Study in Utah

Transportation agencies widely use left-turn signal phasing treatments to enhance safety at signalized intersections, particularly to mitigate left-turn crashes that result in severe injuries and fatalities. Existing studies primarily focus on intersection-level analysis, but conversions can occur at specific roadways within an intersection or the entire intersection. To address this gap, this study conducted a roadway-level analysis using data from 256 intersections in the State of Utah, United States. The results showed that converting from both-roadway permissive to one-roadway permissive-protected intersections had no significant impact on total crash frequency but increased left-turn crashes. The conversion from one-roadway to both-roadway permissive-protected left-turn phasing led to increases in both total crashes and left-turn crashes. Conversely, conversions from both-roadway permissive-protected to one-roadway protected, and from one-roadway to both-roadway protected left-turn phasing, reduced left-turn crashes but had no significant impact on total crash frequency, especially when converting to both-roadway protected phasing. Notably, the safety effects varied between converting to one-roadway and both-roadway left-turn phasing treatments, underscoring the importance of roadway-level analyses for accurate and context-specific evaluations. The increase in crashes after conversions to permissive-protected left-turn phasing in Utah may be attributed to the state’s operational approach, which may result in a shorter protected portion, according to a nationwide survey.