Operational Performance of Two-Way Left-Turn Lanes

This study evaluated the operational performance of two-way left-turn lanes (TWLTLs) of varying widths in urban and suburban settings, in support of more flexible lane width standards for the South Carolina Highway Design Manual (SCHDM) update. Currently, the SCHDM only allows a TWLTL width of 15 ft to be used in design, and the distribution of TWLTL widths observed in a field study were justly limited; this limitation disallowed a proper evaluation of operational effects of TWLTL width. A fully integrated, high-fidelity driving simulator was used to test the operational effects of a range of TWLTL widths. Sixty participants made a series of left turns from a development driveway into TWLTLs of various widths. The TWLTL simulator scenarios included three-lane and five-lane road cross-section configurations and TWLTL widths of 12, 14, and 16 ft. Analyses were performed to determine the effects of (a) TWLTL width on driver gap acceptance and (b) turning-vehicle encroachments into through lanes. Analysis of variance tests indicated that TWLTL widths of 12, 14, and 16 ft had no effect on gap acceptance for three-lane and five-lane roadway sections. Trajectories were compared for a random sample of participants to evaluate the effect of TWLTL widths on vehicle encroachments into through lanes. On the basis of a subset of 30 participants, only one encroachment was observed for the scenario with a width of 12 ft, and two encroachments for the scenarios with widths of 14 and 16 ft. Greater variation in lane position was observed for wider TWLTLs, as participants took advantage of the larger space for maneuvering.