The objective of this research project was to develop a methodology for evaluating alternative midblock left-turn treatments on urban and suburban arterials. This methodology is applicable to three commonly found midblock left-turn treatments: the raised-curb median, the flush median with two-way left-turn lane (TWLTL) delineation, and the undivided cross section. The approach taken in conducting this research was to develop a comprehensive midblock left-turn treatment evaluation methodology, collect field data to calibrate this methodology, and then use the calibrated methodology to develop treatment selection guidelines. This approach was applied to the parallel development of three models that comprise the evaluation methodology. These three models can be used to evaluate the operational effects, safety effects, and the access impacts associated with a specific midblock left-turn treatment. The operations and safety models were used to develop midblock left-turn treatment selection guidelines based on a benefit-cost analysis approach. The following conclusions have been reached as a result of this research. First, the raised-curb median and the TWLTL yield similar delays to arterial drivers while the undivided cross section yields significantly higher delays than either the raised-curb median or TWLTL. Second, any of the left-turn treatment types can function without congestion to the arterial traffic movements at average daily traffic demands of 40,000 vpd or less. Third, the difference in safety between the undivided and TWLTL treatments is highly dependent on whether parallel parking is permitted; however, the raised-curb median treatment appears to be associated with fewer accidents than either the undivided cross section or the TWLTL, when all other factors are equal. Fourth, business owners believe that the conversion from an undivided cross section to either a raised-curb median (with frequent median openings) or a TWLTL will improve arterial traffic conditions and business conditions; however, they also believe that a raised-curb median with infrequent median openings will not improve business opportunities. (A)
Abstract