Estimation of de Facto Left-Turn Lanes at Signalized Intersections

Depending on vehicle delays or turning restrictions at a multilane approach to a signalized intersection, vehicles permitted to make left turns either share the leftmost lane with through vehicles or use the leftmost lane exclusively. An exclusive left-turn lane is explicitly identified by a turning restriction, whereas a de facto left-turn lane occurs when excessive delays and queues discourage through vehicle drivers from using the left-turn lane. However, Highway Capacity Manual (HCM) procedures for the identification of a de facto left-turn lane require left-turn volumes and opposing volumes to be excessively high for a de facto left-turn lane to occur because HCM overestimates the effective green time of a shared left-turn lane. As a result, HCM may not indicate a de facto left-turn lane, even though its delay is much higher than the delay of adjacent lanes. The HCM estimation of percent left turns in the left-turn lane is modified such that a de facto left-turn lane is indicated when its average delay is reasonably greater than the average delay for the adjacent lanes. Examples show the effects of these modifications in comparison with the effects of the HCM procedure. These suggested improvements need to be refined with further research and validation, as recommended.