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NCHRP 17-59 4 CHAPTER 1: BACKGROUND BACKGROUND ON STUDY Most multivehicle crashes at stop-controlled intersections can be classified as either gap acceptance crashes or traffic control device violations (e.g., stop sign violations), with gap acceptance crashes generally occurring more frequently (Eccles et al. 2012). Gap acceptance crashes include several crossing path crash types: right turn into path, left turn into path, left turn across path, and straight crossing path. One of the critical elements to the driverâs ability to identify an appropriate gap is the intersection sight distance (ISD). The design ISD is set forth by the American Association of State Highway and Transportation Officials (AASHTO) A Policy on the Geometric Design of Highways and Streets, also known as the Green Book (AASHTO 2011). The methodology in the Green Book uses sight triangles (both approach and departure) to determine the appropriate sight distance needed for a driver to safely clear or turn onto the intersecting major roadway. The ISD represents the leg of the triangle along the major road to the approaching vehicle. The area in between this point, the decision point (e.g., where the driver is stopped on the minor road), and the center of the approaching vehiclesâ crossing path in the intersection, constitutes the sight triangle and should be free of obstructions, providing sufficient view for the driver to avoid the potential collisions. The ISD specified in the Green Book varies according to the minor road traffic control and design speed of the major road. ISD values are derived from driver gap acceptance behavior. Assumptions are made on physical conditions (e.g., object and driverâs eye height) and, perhaps most importantly, driver behavior. For example, at a stop-controlled intersection, the Green Book recommends that the decision point be 14.5 feet from the edge of the major road. This value is based on driver behavior research that has shown drivers will typically stop with the front of their vehicle 6.5 feet or less from the roadway. However, it recommends increasing this distance to 18 feet where practical, although there remains ambiguity as to what is considered practical in this context. Depending on the site conditions of the intersection, drivers may either (1) remain stopped at or behind the stop bar or (2) inch closer to the edge of the major road. While the Green Book provides the calculations to determine the ISD, it provides little guidance on how to determine the location of the decision point, which in turn determines the area that should be clear of obstruction to provide adequate view of approaching vehicles. Having the minimum required ISD does not necessarily mean that gap acceptance crashes will not occur. The available sight distance may change over time (e.g., through development of adjacent land parcels, overgrown vegetation, seasonal crop growth, etc.), or additional sight distance may be needed in certain situations (e.g., at intersections where the vehicle mix includes farm equipment). Additionally, adequate sight distance alone is not all the driver needs to identify a suitable gap.
NCHRP 17-59 5 The relationship between sight distance and safety is not fully understood. Several studies (Mitchell 1972; Agent et al. 1996; Harwood et al. 2000; Golembiewksi and Chandlre 2011; Fitzpatrick et al. 2000) have attempted to quantify the relationship. Early studies of ISD found removing sight obstructions reduced crashes as much as 30 to 67 percent (Mitchell 1972; Agent et al. 1996). More recent studies have found that increased ISD results in more moderate crash reductions (Harwood et al. 2000; Golembiewksi and Chandlre 2011; Fitzpatrick et al. 2000) and perhaps even adverse effects (Pick v. Gratiot County Road Commission 1993). Harwood et al. (2000) examined multiple studies and through the work of an expert panel determined that improving limited sight distance in each quadrant of the intersection can reduce total crashes by 5 to 17 percent. An understanding of the relationship between ISD and crashes is needed to improve safety at uncontrolled intersections, both at the design level and at the safety improvement level. Volume 5 of the National Cooperative Highway Research Program (NCHRP) 500 Series presents strategies for improving safety at unsignalized intersections (Neuman et al. 2003). All four strategies for improving sight distance are categorized as tried, which means that while they may be used frequently or even incorporated into standards, they should still be applied with caution, as there have been limited evaluations. This reiterates the importance of this research in determining the relationship between available ISD and safety. STUDY OBJECTIVES AND SCOPE The overall objective of this research was to determine the relationship between safety and available ISD, and to develop guidelines for transportation agencies in making decisions about ISD. The overall objectives of this research were accomplished through the following sub- objectives: 1. Identify appropriate definitions and methods to measure ISD. 2. Quantify the relationship between safety and available ISD for stop-controlled intersections, considering the variance in conditions at stop-controlled intersections such as temporal variations (e.g., those caused by foliage), environmental and traffic conditions, and the quality of sight distance. 3. Develop guidance for transportation agencies on how to apply the quantified relationships to evaluate the safety impacts of available ISD. 4. Identify the limitations and constraints of the methodologies presented in the guidelines. 5. Develop a process for incorporation of the results into appropriate manuals such as the AASHTO Green Book, the Highway Safety Manual (HSM), and the Interactive Highway Safety Design Model (IHSDM). 6. Disseminate results to key groups to facilitate implementation.
NCHRP 17-59 6 The results of this research are practice-ready and provide meaningful and clear guidance for transportation agencies. APPROACH TO ACHIEVE OBJECTIVES The project team employed a three-phase study to achieve the objectives of this project: ï· Phase 1 â Develop and Refine Approach. This phase included a literature review methodology reviews, and the development of a data collection plan and analytical methodology. ï· Phase 2 â Implement Work Plan. In this phase, the data were collected and analyzed. ï· Phase 3 â Refine Models, Develop Guidance document, and Disseminate Results. In this phase, the models were translated into practical guidance in the form of a practitionerâs guidance document. SCOPE OF THIS DOCUMENT This document is the final report for the NCHRP Project 17-59: Safety Impact of Intersection Sight Distance. The first interim report presented the detailed the findings of Phase 1 and presented a work plan for Phase 2. The second interim report presented the results of Phase 2, the implementation of the work plan. The third interim report presented the guidance document and a dissemination plan for the guidance document. The report is presented in four chapters and two appendices, with this chapter (Chapter 1) presenting the introduction and background. Chapter 2 presents a summary of the research approach that was used to achieve the objectives of the contract. It describes the literature review, the field data collection method, and the analytical method. Chapter 3 presents the findings and applications of the research. This includes the final model selection, how the research results were translated into practice, and information on the development of the guidance document that is the primary product of this research. Chapter 4 presents the conclusions of the research and recommendations based on the results. The chapter also includes a brief section on suggested future research on intersection sight distance. Appendix A presents the detailed data collection methodology that was used in Phase 2 of the project to collect the field measurements, roadway, crash, and volume data that was used in this research.
