Operational Effects of U-Turns as Alternatives to Direct Left Turns from Driveways

Abstract

Recently, many state and local transportation agencies have begun considering using U-turns as alternatives to direct left turns (DLT) from driveways through installing nontraversable medians on multi-lane highways. Some of the concerns are that this proposed change may transfer operational problems to the downstream median opening and increase delay or travel time of vehicles making a left-turn egress from driveways. However, the effect of this treatment on roadway operations is not clear. A comprehensive study of operational effects of this treatment would be beneficial, both in setting design policy and in project-level design. Field studies were conducted to obtain the data necessary to quantify operational effects of this treatment at eight sites in the Tampa and Clearwater areas of Florida. About 300 h of field data were collected, including delay, travel time, traffic volume, speed, traffic control, and geometric data. An operational effects database was developed to perform statistical analyses. Delay and travel-time models of DLT and right turn plus U-turn (RTUT) were developed as a function of major and minor road-traffic-flow rates. The curves developed based on delay and travel-time models present a clear picture of the operational effects of the two types of movement. Moreover, a ratio model was developed to estimate how many drivers would prefer to make a RTUT rather than a DLT under certain traffic-flow conditions. Finally, the operations models were used to measure system performance of a full median opening versus a directional median opening from the standpoint of weighted-average total delay.