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6 Description Techniques in this group eliminate or reduce left-turn movements through a range of actions, including physically precluding left turns by constructing a non-traversable median and prohibiting left turns by installing regulatory signage. Vehicles that previously made a direct left turn must make a right turn followed by a U-turn (if exiting the access) or a U-turn followed by a right turn (if entering the access). The U-turns can be made at downstream intersections or at U-turn crossovers developed in the roadway median. Tables 2 and 3 follow. Quantitative Analysis Methods Motor Vehicle Operations A study in Florida (6) found that making a right turn followed by a U-turn at a downstream median opening was faster on average than making a direct left turn, accounting for the extra travel time involved, under the following conditions on 6- and 8-lane arterials: â¢ > 5,500 vehicles/hour (sum of both directions) on the arterial and 50 vehicles/hour exiting the driveway; â¢ > 5,200 vehicles/hour on the arterial and 100 vehicles/hour exiting the driveway; and â¢ > 5,000 vehicles/hour on the arterial and 150 vehicles/hour exiting the driveway. C H A P T E R 1 Restrict Left-Turn Movements at an Access Point Source: Photograph provided by the authors.
Restrict Left-Turn Movements at an Access Point 7 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Close existing median openings. Replace full median opening with one serving only left turns from the major roadway. Install U-turn crossovers in conjunction with left-turn restrictions. Prohibit left turns through signage, channelizing islands, or both. Prohibit left turns into driveways on undivided highways. â â â â â â â â â â â â â â â â â â â â Table 2. Multimodal operations and safety performance summary. Mode Operations Safety At higher traffic volumes and with diversion distances under 0.5 mile, the total time taken to make a right turn followed by a U-turn can be less than the time required for a direct left turn (1, 2), when the left turn is made in one continuous movement. A U-turn crossover can be signalized without impairing traffic progression (1). Reduces the number and location of conflict points (2). Median U-turn intersections, which require similar movements, typically experience 20â50% fewer crashes relative to full- movement intersections (3). Vehicles may violate left-turn restrictions enforced only by signs, channelizing islands, or both (1, 2). Back-to-back signalized U-turn crossovers provide an opportunity to create a signalized midblock pedestrian crossing. Reduces the number of conflicts turning motorists must observe at a given time. Motorists making left turns from TWLTL lanes and undivided roadways may speed up when a pedestrian is approaching a driveway (4). May require out-of-direction travel for bicycles exiting the access. Design unsignalized U-turn crossovers such that vehicles can make the U-turn without encroaching on a bicycle lane. Reduces the number of conflicts turning motorists must observe at a given time. U-turn crossovers preclude placing a midblock bus stop in the travel lane at or just downstream of the crossover (5). No documented effect beyond that generally observed for motor vehicle traffic. A âloonâ (widened pavement area on the edge of the roadway) or âbulbâ may need to be constructed to accommodate U-turning trucks with large turning radii at locations with narrow medians (3). No documented effect beyond that generally observed for motor vehicle traffic. Table 3. General trends associated with restricting left turns at an access point.
8 Guide for the Analysis of Multimodal Corridor Access Management Under lower-volume conditions on 6- and 8-lane arterials, the average extra time to make a right-turn and U-turn movement was up to 30â40 seconds longer for driveway volumes of 50 and 150 vehicles/hour, respectively (6). On 4-lane arterials, a study in Florida (7) found that making a right turn followed by a U-turn at a downstream median opening was always slower than making a direct left turn (25 to 40 seconds slower on average, depending on the combination of arterial and driveway volumes). At the same time, the total control delay involved in making a right turn and a U-turn at a downstream median opening (i.e., considering only the waiting time to make the turns and ignoring the extra travel distance and time) was always less at any combination of volumes than making a direct left turn (7). This finding suggests that the right-turn and U-turn movements themselves were faster and less stressful than making a direct left turn. Making a U-turn at a traffic signal took an extra 40 to 65 seconds than making a U-turn at a median opening, depending on the combination of arterial and driveway volumes (7). The delay to make a direct left turn will be higher when the median is not wide enough to store a vehicle (i.e., the entire left-turn maneuver must be made in one stage) (1). Under these conditions, the right-turn and U-turn movement will be faster at lower volumes than found in the Florida studies (6, 7). HCM6 methods (8) can be used to precisely compare delays and total travel times with and without a direct left turn at an access point, using traffic volumes, median storage width, lane configurations, and distance to the U-turn location as inputs. When traffic volumes (and corresponding driveway delays) are high, motorists will accept shorter (i.e., less safe) gaps than are suggested by the HCM6âs default gap-acceptance values (1). Motor Vehicle Safety A motorist making a right turn followed by a U-turn experiences 30% fewer conflict points in making the maneuver, compared with a direct left turn (2). A study in Florida found 34% to 38% fewer actual conflicts (defined as a motorist having to brake, swerve, or noticeably decelerate) in situations with a right turn followed by a U-turn, compared with a direct left turn (9). Median U-turn intersections, which operate in a similar manner as situations with a right turn followed by a U-turn, typically experience 20% to 50% fewer crashes relative to full-movement intersections (3). In Table 14-25 of the Highway Safety Manual, 1st ed. (HSM) a crash modifica- tion factor of 0.80 when replacing direct left turns with a right-turn and U-turn combination is reported (10). Pedestrian and Bicycle Safety No study was found that directly addressed pedestrian and bicycle safety when converting direct left turns to right-turn and U-turn movements. However, a study in New York City found that restricting left turns either part-time or full-time at intersections resulted in 41% fewer left-turn crashes involving pedestrian and bicycle injuries and 21% fewer overall crashes involving pedestrian and bicycle injuries (11). In New York City, left-turn crashes account for three times as many serious injuries and fatalities to pedestrians and bicyclists as do right-turn crashes (11). Additional Information â¢ Chapters 2, 11, and 14 in this guide. â¢ Access Management Manual, Second ed.: Sections 17.3.3, 17.3.4, 20.2.8, and 20.5.6.
Restrict Left-Turn Movements at an Access Point 9 â¢ Access Management Application Guidelines: Chapter 17, U-Turn Lane Requirements. â¢ NCHRP Report 420: Chapter 8, U-Turns as Alternatives to Direct Left Turns. References 1. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999. 2. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 3. Jagannathan, R. Synthesis of the Median U-Turn Intersection Treatment, Safety, and Operational Benefits. Publication FHWA-HRT-07-033, Federal Highway Administration, Washington, D.C., 2007. 4. Layton, R., G. Hodgson, and K. Hunter-Zaworski. Pedestrian and Bicyclist Impacts of Access Management. Proceedings of the Third National Access Management Conference, Fort Lauderdale, Fla., 1998. 5. Reid, J., L. Sutherland, B. Ray, A. Daleiden, P. Jenior, and J. Knudsen. Median U-Turn Intersection Informa- tional Guide. Report FHWA-SA-14-069, Federal Highway Administration, Washington, D.C., Aug. 2014. 6. Lu, J., P. Liu, J. Fan, and J. Pernia. Operational Evaluation of Right Turns Followed by U-Turns at Signalized Intersections (6 or More Lanes) as an Alternative to Direct Left Turns. Florida Department of Transportation, Traffic Operations Office, Tallahassee, July 2004. 7. Lu, J., and P. Liu. Operational Evaluation of Right Turns Followed by U-Turns (4-lane Arterials) as an Alternative to Direct Left Turns. Florida Department of Transportation, Traffic Operations Office, Tallahassee, March 2005. 8. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 9. Lu, J., S. Dissanayake, N. Castillo, and K. Williams. Safety Evaluation of Right Turns Followed by U-Turns as an Alternative to Direct Left TurnsâConflict Analysis. Volume II. Florida Department of Transportation, Traffic Operations Office, Tallahassee, Aug. 2001. 10. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 11. Brunson, C., A. Getman, S. Hostetter, and R. Viola. Donât Cut Corners: Left Turn Pedestrian & Bicyclist Crash Study. Presented at 96th Annual Meeting of the Transportation Research Board, Washington, D.C., 2017.