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NCHRP 3-74 Chapter 6 Selection of Speed Reduction Treatments at High-Speed Intersections Guidelines Page 6-1 6 Guidelines NCHRP Report 3-74: Guidelines for Selecting Speed Reduction Treatments at High Speed Intersections (ÂGuidelinesÂ) assists roadway planners, designers, and operators as they consider and select appropriate speed reduction treatments at intersections located in high-speed environments. While the application of these treatments most often applies to existing intersections that experience undesirably high speeds, the information is also relevant to new intersection designs. The Guidelines are not a new standard for implementing treatments. Rather, they are informational, describing good practices for selecting treatments. Drivers generally choose a reasonable travel speed based on their perception of safety and comfort; however, there are a variety of conditions and circumstances that can lead a driver to misinterpret what is safe and comfortable and result in speeds that are undesirably high for the conditions present at a specific intersection approach. Some of these circumstances are human factors that may be unique to the individual driver and their behavior, and others are physical. Physical changes to the roadway and surrounding environment may influence driver behavior, which can indirectly reduce speed, improve operations, and/or enhance environmental quality. The Guidelines provide relevant information about the effects of speed, the conditions that may contribute to undesirably high speeds at intersection approaches, and the state of the practice related to speed reduction treatments used in the U.S. and abroad, including their effectiveness and implementation considerations. The Guidelines also provide insights on the relationship between speed and facility operations. There are many popular beliefs about the relationship between speed and safety, and it is common for people to assume a direct relationship between the two. In fact, there is little published data to actually link speed with safety performance. This report does not discuss whether speed reduction is appropriate for a condition or what amount of reduction is necessary. This document assumes the user already desires reduced speeds. The Guidelines provide users with information about speed, speed considerations at intersections, and the potential application of treatments to affect speed. Additional research is needed to fully understand the effects that speed reduction treatments and reduced speed may have on safety. 6.1 SCOPE OF GUIDELINES The Guidelines apply to intersections with approach speeds of 45 miles per hour (mph) or greater. Stop-controlled, yield-controlled, and uncontrolled approaches to signalized and unsignalized intersections are addressed. Because speeds tend to be lower in urban areas, the Guidelines primarily apply to suburban and rural roadways. Speeds on roadway segments outside the influence area of an intersection are not addressed; however, the relationship between segment speed and speed within the intersection influence area is addressed. This document focuses on public roadway intersections; however, many principles also apply to private driveways that include public-roadwayÂlike features. Intersections are discrete roadway features that occur on roadway or corridor segments. They frequently occur in urban conditions, but more sporadically in rural and suburban areas. Some of
Chapter 6 NCHRP 3-74 Guidelines Selection of Speed Reduction Treatments at High-Speed Intersections Page 6-2 the visual cues, physical features, and perceptible qualities that influence operating speeds on roadway segments are different from those that influence operating speed at intersections. The Guidelines focus on speed reduction treatments within an intersectionÂs influence areas and do not specifically address speed reduction in roadway segments. The influence area of an intersection includes the area within which the typical section of the roadway segment is modified and which is influence by traffic operations (ex. Queuing and deceleration) related to the intersection. Section 1 introduces the Guidelines and presents their purpose, scope and applicability. This section also introduces the importance of differentiating the intersection influence area from adjacent roadway segments. Section 2 focuses on the role of speed in an intersection environment and discusses the ways in which speed affects intersection performance and the adjacent environment. It details ways in which speed is affected by roadway design and elements of the adjacent environment and highlights some physical conditions and user characteristics that may make an intersection particularly sensitive to speed. The purpose of this section is to describe many considerations that may be useful in understanding the nature of a speed concern. Speed is a product of many roadway and intersection features and, in turn, speed affects the performance of roadway facilities and the quality of adjacent environments. Excessive speeds generally result when environmental and operational elements are incompatible, sending a mixed message about appropriate behavior to motorists. Speed reduction does not necessarily guarantee safety or environmental benefits. Speed, when considered as a design criterion or consideration, can affect the roadway design while also influencing the environmental context. Understanding how speed affects intersection conditions and how those conditions affect speed is a first consideration in evaluating and selecting an appropriate speed reduction treatment. Section 3 guides users through the process of selecting speed reduction treatments for intersection approaches. The process includes: intersection pre-screening; treatment screening; and treatment implementation. This section provides information about the speed-reduction treatments, including their applicability, cost, secondary impacts, implementation considerations, and potential effectiveness in reducing speeds and increasing safety. This information was compiled from a variety of national and international sources. Some of the treatments included in the Guidelines have had limited or no documented applications at high-speed intersections; however, based on their function, they are considered potentially applicable at certain high-speed locations. The information provided in this section highlights considerations that will help determine which treatments may be appropriate at a specific location. In many cases, one or more treatments may be appropriate for a given intersection. Section 3 also provides guidance to help users layout and design selected treatments based on a target speed and the desired driver behavior. Finally, this section gives some direction about
NCHRP 3-74 Chapter 6 Selection of Speed Reduction Treatments at High-Speed Intersections Guidelines Page 6-3 evaluating treatments. Section 4 describes speed reduction treatments in detail. This section contains an overview of each treatment, as well as more detailed descriptions, summaries of applicability and pertinent considerations such as maintenance, discussions of potential layouts and designs, and summaries of the documented effectiveness of each treatment in reducing speeds and improving safety. Photos and diagrams are also provided. The information provided in this section should be considered in combination with appropriate local engineering practices and engineering judgment related to the site-specific situation. The appendices to the Guidelines provide a diagnostic flow chart, scenario-based case studies, testing data and results from the testing plan, and references to other relevant studies. Appendix A, the diagnostic flow chart schematically depicts the treatment selection process and key considerations. Appendix B, the scenario-based case studies, describes several specific applications of the treatment selection process. Appendix C provides a list of references that provide additional information about the treatments.
