TWO-WAY LEFT-TURN LANE GUIDELINES FOR URBAN FOUR-LANE ROADWAYS

Two-way left-turn lane (TWLTL) medians are commonly used to solve the safety and operational problems on four-lane undivided roadways caused by conflicts between through and left-turning traffic. Although the potential safety and operational effects of TWLTL medians are well-recognized, there are no generally accepted guidelines that define the circumstances under which the costs of TWLTL medians are justified by the benefits they provide. The objectives of the research on which this paper was based were (1) to evaluate the safety and operational effects of TWLTL medians on urban four-lane roadways, (2) to develop a methodology for evaluating their cost-effectiveness, and (3) to use this methodology to develop guidelines for their cost-effective use. The formulation of the cost-effectiveness methodology was based on a benefit-cost analysis approach. The benefits were the accident and operational cost savings provided by TWLTL medians. The costs were the costs of installing and maintaining them. The cost-effectiveness methodology was used to develop guidelines that indicate the average daily traffic levels (ADTs), left-turn percentages, and driveway densities at which TWLTL medians on urban four-lane roadways are cost-effective. Their development was based on conditions and costs representative of those on urban four-lane roadways in Nebraska during 1986. Over the range of conditions considered, TWLTL medians were cost-effective at lower ADTs on roadways with higher left-turn percentages and fewer driveways per mile. The minimum ADT required for TWLTL medians to be cost-effective ranges from 6,200 to 6,600 vehicles per day (vpd), depending on the left-turn percentage and driveway density.