Left-Turn Accommodations at Unsignalized Intersections (2013)

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3 Background of Study Left-turn movements at unsignalized intersections, includ- ing driveways—especially movements that are made from lanes that are shared with through traffic—cause delays and adversely impact safety. The warrants used by many jurisdic- tions 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. Part of the concerns regarding the existing warrants were other factors that should be considered when researching left-turn lane warrants, such as the cost element, particularly when right-of-way issues are involved, and the rational nexus, when the left-turn accommodation may be related to a proposed new development. These and related issues led to the creation of NCHRP Project 3-91, “Left-Turn Accommodations at Unsignalized Intersections.” The project 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 these goals led to the decision to create this design guide to facilitate the distribution 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. Documentation of how the left- turn lane warrants were developed is contained in the final research report from the project (1). The left-turn lane warrants developed from NCHRP Proj- ect 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. Technical warrants are an important element of the decision-making process; however, other factors also should be considered when deciding whether to install a left-turn lane, including: • Sight distance relative to the position of the driver and • Design consistency within the corridor. These factors should be considered in conjunction with the numerical warrants. For example, if volumes indicate that a left-turn lane is not warranted but there is insufficient sight distance at the location for the left-turning vehicles, then the left-turn lane should be considered along with other poten- tial changes (e.g., remove sight obstructions, realign the high- way, etc.). The practitioner must consider a number of issues when making decisions about planning and designing the lane. Some of these issues include: • Design vehicle, • Width of the turn lane, • Need for an island of appropriate size for pedestrian refuge, • Pedestrian facilities, • Length for deceleration, • Taper/transition length, • Length storage for turning vehicles, • Signage, • Pavement markings, • Illumination, • Position of driveways/intersections in the vicinity of the left-turn lane, and C h a p t e r 1 Introduction

4• Potential of signalizing the intersection in the foreseeable future. Proper consideration of these issues, and the ability to make informed decisions about them, will improve the likelihood that a left-turn lane is installed at an appropriate location and that its design will lead to improved operations during its ser- vice life. Purpose and Scope of Guidelines The design guide was developed with a focus on practi- tioner use. It does not contain a comprehensive description of the research project or how the conclusions were devel- oped. Rather, it focuses on designs and traffic control treat- ments for left-turn lanes. Details about the research project are documented elsewhere (1). Organization of Design Guide This design guide has five chapters, including this intro- ductory chapter. Chapter 2 discusses key steps in the plan- ning and design process and how to compile and use the necessary information to complete those steps. It also con- tains the warrants the practitioner should consider when deciding whether to install a left-turn lane at a particular location. Chapter 3 provides a summary of key geometric design criteria and considerations specific to left-turn lanes at unsignalized intersections. Chapter 4 discusses appropriate traffic control devices and illumination, including provisions for future signalization. Chapter 5 contains a variety of case studies. The practitioner can refer to these case studies for examples of prior left-turn lane installations, which illustrate decisions and considerations made by other practitioners in similar situations.