Evaluating U-left turn and direct left turn movements at signalized intersections using traffic conflict indices

This research aims to analyze and compare the impact of U-left turn (ULT) and direct left turn (DLT) movements on safety and traffic flow using traffic conflict indices as a non-crash data methodology. The primary focus of this study is to assess the risks associated with left-turn movements at intersections controlled by traffic lights, providing a basis for evaluating the safety of DLT and ULT maneuvers. To collect data on left-turn conflicts with through-traffic vehicles, video analysis was employed to capture conflict indicators. By classifying the base time and base speed indices, the study compared collision probability, collision severity, and collision risk between DLT and ULT movements. The results revealed that conflict events, as measured by the speed distribution, predominantly had a time to collision (TTC) value greater than 4 s, indicating a safe passage. Additionally, conflict events assessed using the time index distribution showed a high percentage of conflicts with post encroachment time (PET) values exceeding 2.5 s, suggesting a low collision probability. While ULT conflicts exhibit a slightly higher average severity, an ANOVA test indicated no significant difference in severity between ULT and DLT conflicts at a 95% confidence level. The probability of conflicts was analyzed using objective scores based on PET. ULT maneuvers exhibited a higher likelihood of collisions due to limited visibility and a somewhat experimental approach during the merging process. However, an ANOVA test indicated no significant difference in collision probability between ULT and DLT maneuvers at a 95% confidence level.