Left-Turn Accommodations at Unsignalized Intersections (2013)

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1 Left-Turn Accommodations at Unsignalized Intersections NCHRP Project 3-91, “Left-Turn Accommodations at Unsignalized Intersections,” had two primary research goals: • Develop an objective and clear process for the selection of left-turn accommodations at unsignalized intersections and • Provide guidance on the design of these accommodations. The second of those goals led to the decision to create this design guide to facilitate the distri- bution of findings from that project and to assist practitioners in their efforts when installing, or deciding when to install, left-turn lanes at unsignalized intersections. Left-turn movements at unsignalized intersections and driveways, especially those that are made from lanes that are shared with through traffic, cause delays and adversely impact safety. The warrants used by many jurisdictions for when to provide left-turn lanes are based on volume thresholds and a queuing model introduced in research from the mid-1960s. Recent research has indicted that many of the assumptions used in those warrants are dated and need to be reexamined. The left-turn lane warrants developed from NCHRP Project 3-91 include consideration of the following: • Rural or urban/suburban environment, • Number of lanes on the major roadway, • Number of approaches to the intersection, • Peak hour left-turn lane volume, and • Major roadway volume. This design guide was developed with a focus on practitioner use. It does not contain a com- prehensive description of NCHRP Project 3-91 or how the conclusions were developed. Rather, it focuses on designs and traffic control treatments for left-turn lanes. Details of the research project are documented in the final research report, which is separate from this design guide. The design guide discusses the basic information (e.g., traffic volumes, speeds, crash history, and delay or gap acceptance data) a practitioner needs to make a decision on whether to install a left-turn lane and how the practitioner can compile that information. It also sets forth a process by which the practitioner can establish and apply design criteria, both in the decision to install a left-turn lane and in the process of determining the specific design elements for that lane. Warrants for installing left-turn lanes are provided in tabular and graphical form for rural two-lane roadways, rural four-lane roadways, and urban/suburban arterials. Warrants for bypass lanes on rural two-lane roadways also are included. The volume thresholds for these warrants are S u m m a r y

2typically lower than those described in various states’ manuals and other guidance documents, with installations warranted for as few as five left-turning vehicles in the peak hour, depending on the opposing through volume. These warrants are based on a benefit-cost analysis conducted in NCHRP Project 3-91. The design guide also contains recommendations for appropriate dimensions for specific design elements, such as: • Lane width (as wide as the adjacent through lane, but at least 10 ft), • Taper length (8:1 [L:T] for design speeds up to 30 mph and 15:1 [L:T] for design speeds of 50 mph) and type (bay taper or approach taper; “shadowed” or direct entry), • Deceleration length (preference for no deceleration in the through lane, though 10 mph may be allowed), and • Storage length (enough to store expected number of design vehicles during a critical period, but at least 40–50 ft, or two passenger car lengths). A discussion of intersection sight distance, median design, and channelization and offset are included, as well as general principles for bypass lane design and pedestrian accommodation. Traffic control devices are important complements to the geometric design of unsignalized intersections. The guide discusses commonly used signs, markings, and illumination, based on guidance from the MUTCD and lighting guides from AASHTO, ITE, and IESNA. The appropri- ate types (e.g., regulatory, warning, guide), sizes, and placement of signs are described, as are application guidelines for both signs and markings. Commonly used principles related to type, intensity, and location of lighting are explained, and provisions for accommodating future sig- nalization are also included. The guide also contains a series of 11 design example case studies, based on actual inter sections where left-turn treatments were evaluated and installed. These case studies include installations of exclusive left-turn lanes in developing areas, a left-turn lane as part of a road diet program, a J-turn intersection, a bypass lane (or “passing blister”), and a signalized left-turn lane. Design examples provide the practitioner with “real-world” scenarios that are similar to those the prac- titioner would actually encounter, along with possible solutions and the methods by which those solutions can be evaluated and installed.