Conflict Analysis for Double Left-Turn Lanes with Protected-Plus-Permitted Signal Phases

A conflict study evaluated double left-turn lanes with protected-plus-permitted signal phasing. The data collection team observed traffic behavior at four intersections in the metro Atlanta region. Included in the data set are three intersections operating under protected-plus-permitted signal phasing and a fourth intersection with a before and after examination of a protected-plus-permitted signal phase changed to protected-only phasing. The authors calculated the traffic rates for five conflict types and one traffic event unique to double left-turn movements. A statistically significant decrease in traffic conflicts was identified for the before and after comparison site. The study also identifies unique intersection geometry and traffic volumes at each site and compares traffic conflicts associated with the features. The results generally show that the enhanced capacity of double left-turn lane intersections operating under protected-plus-permitted signal phasing may occur at the expense of the safety of the traveling public.