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3 Introduction Purpose of the Guidance Intersection sight distance (ISD) is a key design element at intersections. ISD is the actual, measurable unobstructed view a driver has of an intersection, including traffic control devices and views along the intersecting roadways. Adequate ISD at intersections with stop control on the minor road contributes to the ability of drivers stopped on the minor road to identify an appro- priate gap for departing from the intersection and entering or crossing the major road. Designs that provide adequate ISD also allow drivers on the major-road approaches to see stopped vehi- cles on the minor road and prepare to slow or stop if needed. A Policy on Geometric Design of Highways and Streets, 6th Edition, also known as the Green Book, provides design criteria for dif- ferent types of sight distances, including ISD (AASHTO 2011). The ISD criteria in the Green Book vary according to traffic control on the minor road, design speed of the major road, and turning movement from the minor road. Minimum ISD values are based on driver gap-acceptance behav- iors. Assumptions based on field observations are made about physical conditions (e.g., object height and driver eye height), vehicle performance capabilities, and driver behavior. Informa- tion does not currently exist in the Green Book or the Highway Safety Manual (HSM; AASHTO 2010) on how ISD affects the expected frequency and severity of intersection crashes. NCHRP Research Report 875: Guidance for Evaluating the Safety Impacts of Intersection Sight Distance has been developed as a resource for practitioners who are involved in evaluating inter- section safety. It describes data collection methods and analysis steps for making safety-informed decisions about ISD. The safety performance evaluation methods draw on concepts from the HSM; therefore, a general understanding of the HSM methods is important for the appropriate application of the crash modification factors (CMFs) for ISD. Intended Audience This guidance is intended for practitioners involved in the planning, design, operation, and traffic safety management of stop-controlled intersections. It provides information for practi- tioners on how to estimate the effect of ISD on crash frequency at intersections. The guidance also provides basic information on the importance of ISD that can be shared with decision makers and other stakeholders. Guidance Organization Following the introduction and overview, the guidance presents ISD material in four main sections: â¢ Measuring available ISD and other critical information, â¢ Safety performance and ISD, C h a p t e r 1
4 Guidance for evaluating the Safety Impacts of Intersection Sight Distance â¢ Example applications of the content of this guidance, and â¢ Other countermeasures and resources. The final chapter provides the base equations from the research. Guidance Development and Research Objective This guidance is a product of NCHRP Project 17-59, âSafety Impacts of Intersection Sight Distance.â The overall objectives of this research project were to determine the relationship between safety (measured as expected crash frequency) and available ISD and to develop guide- lines for transportation agencies to consider when making decisions about ISD. The objectives were accomplished through several research activities centered on extensive field testing and analysis of data collected in North Carolina, Ohio, and Washington. The data were modeled to explore the relationship between the field-measured ISD and expected crash frequency at minor-road stop-controlled intersections. While the information presented in subsequent chapters focuses on a limited number of variables and intersection characteristics, many dif- ferent types of sites and risk factors were included in the model development. The studied sites were diverse in characteristics such as area type, number of major-road lanes, number of intersection legs, presence of a median on the major road, and speed limit. This allows the information provided in this guidance to be applied to a myriad of intersections with a range of characteristics. A summary of all the research activities conducted as part of this project can be found in the technical report for NCHRP Project 17-59, published as NCHRP Web-Only Document 228. Scope of This Guidance The information presented in this guidance is intended for three- and four-leg intersections with stop control on the minor-road approaches. It is applicable to both planned and existing conditions. How to Use This Guidance The entire guidance document should be reviewed before application. âPractitioner Tipsâ text boxes throughout the document provide information and simple step-by-step instructions on how to put the information into practice. Practitioner Tips: Determine Appropriateness of the Guide This guidance will be useful if one or more of the following situations exist: â¢ Sight distance has previously been noted as an issue or factor related to crashes, and countermeasures to improve ISD are being considered. Sight distance issues can be identified informally through public feedback or more formally through safety reviews such as road safety audits. â¢ There is a proposed change that may affect intersection sight distance at a specific intersection. â¢ Design alternatives are being considered that vary in ISD, and a comparison of the expected crash performance of the alternatives is needed. â¢ An agency is considering systemic approaches to improve intersection safety.
Introduction 5 This guidance can be used to inform decision making that is part of the following activities: â¢ Conducting performance-based planning and design, â¢ Making Highway Safety Improvement Program project selections, â¢ Performing traditional and systemic road safety management, and â¢ Implementing recommendations stemming from road safety audits. Additionally, it can be used as part of alternatives analysis when considering multiple options for treatment (e.g., informing an economic analysis of tree removal versus implementing geo- metric changes). Engineering judgment should be used in interpreting and implementing the results of this guidance. There are many variables that must be considered when examining ISD, and the prac- titioner should balance the information presented here with his or her knowledge and under- standing of a particular intersection. Definitions and Acronyms Understanding the terms discussed in this section is essential for any practitioner interested in putting this guidance to use. Drivers on the minor-road approach of an intersection with minor-road stop control must judge the speed and distance of approaching vehicles to select an appropriate gap in traffic before entering or crossing the major road. Gap-acceptance crashes are crashes that result at these intersections when drivers on the stop-controlled approach attempt to enter an insuf- ficient gap in traffic on the major road and enter the intersection into the path of an oncom- ing vehicle. A driverâs ability to select an appropriate gap is affected in part by the available intersection sight distance, which is the actual, measurable unobstructed view a driver has of an intersection, including traffic control devices and views along the intersecting roadways. Adequate ISD at intersections with minor-road stop control contributes to the ability of driv- ers stopped on the minor road to identify an appropriate gap for departing from the inter- section and entering or crossing the major road. Designs that provide adequate ISD also allow drivers on the major-road approaches to see stopped vehicles on the minor road and prepare to slow or stop if needed. This guidance refers to several considerations related to ISD. Design ISD is the minimum ISD as identified in the Green Book. Existing (or available) ISD is the ISD as measured in the field. Proposed ISD is the ISD that is being considered and is expected to result from a change in the intersection environment such as the removal of trees or other objects in the sight triangle. For the purpose of this guidance, the minor road is the roadway that is stop controlled at the intersection. Drivers on the minor-road approaches must yield to vehicles approaching on the major road, which is uncontrolled at the intersection. The minor road defined in this way usually has lower traffic volumes than the major road. This guidance is still applicable if the minor road has a higher traffic volume than the major road if the ratio between the volumes of the major road and minor road is close to 1.00. Judgment should be used if the volume of the minor road significantly exceeds that of the major road. In the field, available ISD is measured on the approach from the minor road. The decision point (DP) is defined as the location where drivers on the minor road stop their vehicles, view approaching traffic, and decide when to enter the intersection. Clear sight triangles extending from the decision point must be provided along both the major and minor approaches. This
6 Guidance for evaluating the Safety Impacts of Intersection Sight Distance is illustrated in Figure 1. The Green Book includes a detailed discussion of how the minimum design ISD for an approach is determined (AASHTO 2011). This guidance provides practitioners useful crash modification factors for estimating the change in the expected number of crashes after implementing a given countermeasure at a spe- cific siteâin this case, increasing or decreasing ISD. A CMF is an index of the expected change in safety performance following a modification in a traffic control strategy or design element. CMFs can be used to compare the safety effectiveness of changes to available ISD or to compare different decisions as part of an alternatives analysis. More information on implementing CMFs can be found at the FHWAâs webpage for CMFs in Practice (http://safety.fhwa.dot.gov/tools/crf/ resources/cmfs/). The CMFs included in this guidance take the form of CMFunctions that consider the following characteristics of ISD: â¢ The sensitivity of the expected number of target crashes to changes in ISD is expected to be highest when ISD is shorter and decrease as ISD increases (e.g., the safety benefit of increasing ISD from 300 to 600 ft is expected to be substantially larger than the safety benefit of increas- ing ISD from 1,000 to 1,300 ft). â¢ The sensitivity of the expected number of target crashes to changes in ISD depends on major- road speed limit and two-way average annual daily traffic (AADT). CMFunctions are based on cross-sectional regression models developed from numerous observations to estimate the effect on expected crash frequency based on changes in key variables. In this case, the CMFunctions were developed by collecting data (including ISD) at more than 800 intersection approaches and comparing the number of crashes that occurred across these intersections. The CMFs in this guidance are applied at the intersection approach level. There are two differ- ent ISDs for each minor-road approach leg of an intersection: one looking left and one looking right. At a four-leg intersection, there are four different minor approach ISDs. These ISDs can vary greatly. Improvement to the ISD at an intersection would involve consideration of each direction on each approach separately in the selection of a countermeasure. Therefore, the CMFs in this guidance are intended to be applied to crashes associated with an individual approach direction. Source: AASHTO 2011 M in or R oa d M in or R oa d Major Road Major Road Clear Sight Triangle Clear Sight Triangle Decision Point a a b b Decision Point Departure Sight Triangles Departure Sight Triangle for viewing traffic approaching from the right Departure Sight Triangle for viewing traffic approaching from the left Figure 1. Sight triangles.
Introduction 7 The CMFs in this guidance are applied to target crashes. Target crashes are defined as crashes involving a vehicle from the major road colliding with a vehicle entering from a minor road. This guidance also provides tools to analyze subsets of target crashes. For example, a target fatal and injury crash in this document is defined as a target crash that results in at least one injury or fatality. In addition to the terms discussed in this section, practitioners should familiarize themselves with the following acronyms and abbreviations used throughout the guidance: AADT Average annual daily traffic CMF Crash modification factor CP Critical point GIS Geographic information system