Median U-turn design as an alternative treatment for left turns at signalized intersections

Institute of Transportation Engineers. ITE Journal, Feb 2002 by Bared, Joe G, Kaisar, Evangelos I

THIS FEATURE STUDIES THE TRAFFIC-OPERATIONAL BENEFITS OF SIGNALIZED MEDIAN U-TURNS FOR LEFT-TURNING VEHICLES MAKING INDIRECT MANEUVERS. A DISCUSSION OF POSSIBLE SAFETY BENEFITS IS ALSO PROVIDED.

CONVENTIONAL INTERSECTION designs often are not capable of alleviating congestion without incurring significant improvement costs and increased conflicts. A few states are considering and applying nonconventional treatments for high traffic-volume intersections, especially those with high left-turning traffic volumes. Nonconventional intersections provide unique channelization of particular turning movements and indirect traffic movements (Figure 1). Consequently, capacity is enhanced (with an increase in effective green) and delay may be reduced when the number of signal phases is reduced. Moreover, safety benefits are possible with separation and reductions of potential conflicts. Michigan has many boulevards that allow wide median U-turns to divert direct left turns from the main signalized intersection. A paper by Levinson, et al. has reviewed the safety and operational performance of the Michigan experience over long corridors that were implemented in the 1960s.1 Reported accident reduction is about one third less than direct dual left-turn lanes. Capacity gains are about 18 percent higher for indirect median crossover vs. dual-leftturn lanes. Florida and North Carolina use continuous green T-intersections where the right lane of the major road has a constant green signal. New York has built a continuous-flow intersection with three legs where the left-turning traffic is channelized to the left side of the approach. The left-turning vehicles take advantage of the through green phase to turn left on the cross road without the need for an exclusive phase. Another similar intersection has recently been opened in Maryland. According to the National Cooperative Highway Research Programs Synthesis on Left-Turn Treatments at Intersections, 26 agencies use indirect left-turn treatments.2 Public education and proper advance signing are critical to achieving a successful redesign. This feature studies the trafficoperational benefits of signalized median U-turns for left-turning vehicles making indirect maneuvers. A discussion of possible safety benefits is also provided.

CONFIGURATION

Figure 2 illustrates the intersection treatment analyzed in this study, which is similar to the illustration in Figure 1 (second row, on the right). Drivers who intend to turn left at the intersection have the advantage to naturally stay in the left lane. They cross the main intersection and take the left-turn lane downstream to make a U-turn. A lane is added on the right side of the other direction with a jug handle to cover the swept paths of large vehicles. The U-turning traffic is also provided with an acceleration lane on the right side of the crossroad. In this study, U-turn crossovers are provided on both sides of the major road only at 137 meters (in) [450 feet (ft.)] from the main intersection (stop-bar to stop-bar). The left-turning pockets downstream of the intersection are 122 in (400 ft.) long and are protected by a U-turn signal. Minor-- road left-turning traffic is allowed to turn at the main intersection. This latter design is the selected case study and is based on the assumption that minor left-turning volumes are not high and that right-turning maneuvers for indirect left turns could initially be counterintuitive. Lengths of the left-turn lanes and offsets could vary according to the peak left-turning traffic demand. A longer offset was used only for the highest flow scenario in this analysis with 500 vehicles per hour (vph) of left-- turning volume. Spillback is less likely because the cycle lengths can be shorter with fewer required phases, and the U-- turning traffic is mostly able to clear the left-turn lane in only one cycle, with longer phase splits. Moreover, the through movement always has a continuous green (in the direction of the U-turning movement) that can store excess left-turning vehicles and prevent a spillback. A wide median is necessary to accommodate large vehicles as in the Michigan U-turn design, unless a jughandle is provided (Figure 2).

According to the American Association of State Highway and Transportation Officials' (AASHTO's) Policy on Geometric Design,3 a WB- 15 design vehicle making a 180 deg turn will require a minimum inside turning radius of 5.9 in (19 ft.) and a maximum outside turning radius of 14.1 m (46 ft.). When converting a conventional intersection, additional land will only be required for the jughandle (or flaring/widening the median). The described geometric characteristics per approach are symmetrical along the four-lane major highway and the four-lane minor road.

A median U-turn design separates potential conflict points and reduces their locations in comparison to the conventional four-legged intersection (Figure 3). From a safety perspective, dual left-turn lanes are likely to be more hazardous than median Uturn design. Dual left-turn lanes have wider exposed pavements that add extensive potential conflict area and require more travel time for the cross-street traffic. In particular, it will increase the yellow clearance time needed for the cross-street phases. Hakkert and Mahalel derived a regression equation relating accidents to conflict points.4 The conflict points are converted into an index of traffic flows (x) that was calculated as the sum of the product of the flows at each conflict point for the entire intersection. The equation is of the simple linear regression form and explained 60 percent of the variation for a sample of intersections that are mostly without signals. This fitness measure is higher than most accident prediction models that relate accidents to highway factors. A reduction of conflicts combined with a reduction in the number of signal phases is likely to have positive safety implications. If the number of phases is reduced, drivers are faced with lesser critcal decisions during yellow clearance time and all red (or dilemma zone).