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4 Access management is the process that provides (or manages) access to land development while preserving the flow of traffic on the surrounding road network in terms of safety, capacity, and speed. Previous studies and empirical evidence have shown positive safety benefits asso- ciated with good access management practices. There have been several research studies to evaluate the safety impacts of individual access management strategies; however, there have been relatively few scientifically rigorous evaluations to quantify the safety effectiveness of multiple access management strategies in combination, particularly at the corridor level (Gross et al. 2018). Further, there are relatively few highly rated crash modification factors or functions (CMFs) for access management strategies in the two national resources for safety effectiveness estimates: the Highway Safety Manual (1st Edition) (AASHTO 2010) and the FHWA Crash Modification Factors Clearinghouse (widely known as the CMF Clearinghouse) (FHWA n.d.). Part C of the Highway Safety Manual (1st Edition) presents a method to predict the safety performance of an individual segment or intersection based on the geometric design and traffic operations characteristics. While this method does account for multiple variables in the crash pre- diction process, there are relatively few access management variables included in the methodology. Further, it is not well-suited to account for corridor-level access management strategies such as intersection density and median opening spacing. For these reasons, some have questioned the performance and reliability of the Part C Predictive Method across sites with different combina- tions of access management characteristics. For example, the same Part C Predictive Method is used to estimate the safety performance of the two roadways shown in Figure 1 because both represent the same facility type (i.e., urban/suburban, four-lane, median-divided arterials); how- ever, the question remains as to whether or not the Part C Predictive Method adequately captures the differences in access management features. Part D of the Highway Safety Manual (1st Edition) presents CMFs and supporting infor- mation regarding the safety effectiveness of various individual engineering strategies. To be included in Part D, the CMFs must pass a screening process or meet expert panel approval. The CMFs in Part D that relate to access management strategies are limited, and many relate to traffic control and turn lanes at intersections or median type along roadway segments. Within the section titled Crash Effects of Roadway Access Management, there are CMFs for only one segment-related strategy (i.e., modify access density). The CMF Clearinghouse is an online repository of CMFs compiled from research studies. The CMF Clearinghouse employs a star rating system to indicate the relative quality of CMFs on a scale of one to five stars, where a rating of five stars is the highest level of quality. The CMF Clearinghouse includes all CMFs from Part D of the Highway Safety Manual (1st Edition) and a separate notation identifies CMFs that meet the Highway Safety Manual inclusion criteria. Based on the literature review for this research (NCHRP Project 17-74), there were no four- or C H A P T E R  1 Introduction
Introduction 5  five-star rated CMFs for many access management strategies, including changing driveway design elements (e.g., change type of driveway, change movement restrictions, change driveway width), changing median type [e.g., replace two-way left-turn lane (TWLTL) with non-traversable median or convert undivided to divided by traversable median], providing frontage roads (e.g., change proportion of primary roadway with frontage road), modifying turn lanes (e.g., change design elements of existing turn lanes), managing the location and spacing of signalized and unsignalized access (e.g., establish intersection and driveway density and spacing criteria), and managing the location and spacing of median openings (e.g., create directional median opening and establish median opening density and spacing criteria). This does not indicate that these strategies are unimportant or ineffective; it simply indicates a lack of related research. The focus throughout this guide is on relatively high-quality CMFs, established as three stars or higher, because these CMFs are based on relatively more reliable methods that account for more poten- tial sources of bias in CMF development. Objective and Scope The objective of this guidance is to help transportation practitioners quantify the safety impacts of access management strategies to inform access-related decisions on urban and sub- urban arterials. To achieve this objective, the guidance presents methods to quantify safety performance of individual locations (i.e., intersections or segments) as well as corridors that represent multiple adjacent intersections and segments. The guidance provides several potential benefits by supporting the following activities. ⢠Safety-conscious, performance-based practical design and construction. Quantitative safety analysis can inform agencies as to which of several project designs is expected to provide the greatest safety benefit to the public. ⢠Understanding of complex projects. Quantitative safety analysis can help practitioners to understand the comprehensive costs and benefits of projects and to compare the safety impacts with other factors such as mobility, environmental impacts, and regional economies. ⢠Return on investment. Quantitative safety analysis supports economic analysis, which can help practitioners to make more-informed decisions in planning, programming, and imple- menting transportation programs, which can improve the rate of return for a given budget. Figure 1. Example of two roads that represent the same facility type in the first edition of the Highway Safety Manual (AASHTO 2010).
6 Application of Crash Modification Factors for Access Management ⢠Documentation of decision process and public involvement. Quantifying the benefits and costs of highway projects also provides documentation to justify and explain the decision process to legislatures and the public. The guidance addresses the cumulative and interactive effects of access management strate- gies, including discussions of the appropriate use of the methods to explore the safety tradeoffs associated with each strategy individually and in combination. The guidance also describes how to estimate safety performance in terms of specific crash types and severities. While the guidance includes components to predict safety performance in terms of pedestrian and bicycle crashes, it focuses primarily on motor vehicle crashes and severity due to the limited data and CMFs to predict pedestrian and bicycle safety. Target Audience The target audience for this guide is transportation professionals (e.g., planners, designers, and traffic engineers) with or without prior experience in access management and highway safety. The guide is also applicable to highway agency executives that oversee, administer, and implement programs. Guide Organization Chapter 2 provides an overview of quantitative safety performance, including definitions of various terms and basic information on the application of quantitative safety methods. Chapter 3 provides an overview of access management and background information on the purpose of specific access management strategies. Chapter 3 also presents high-quality CMFs for access management strategies on urban and suburban arterials along with details to understand the proper application in practice. Chapter 4 presents the Highway Safety Manual (1st Edition) Part C Predictive Method to estimate the safety performance of individual intersections and segments, including the use of CMFs from Chapter 3 to adjust the predictions accordingly. Chapter 5 presents two methods for estimating corridor-level safety performance; one that combines predictions for individual locations based on Chapter 4, and another that is based on corridor-level prediction models. The corridor-level models allow the analyst to enter variables describing the characteristics of the corridor, which provide a more reliable method to account for interactions among adjacent sites and among multiple access management strategies. Chapter 6 explains how to communicate analysis results and document access management planning and design decisions. Appendix A provides a summary of resources related to access manage- ment and safety, indicating how each resource relates to this guidance and would support specific analyses or decisions. Appendix B provides an overview of quantitative safety performance, how it can inform decisions, the related methods for quantifying safety performance, and a decision process for selecting an appropriate method. Appendix C provides details on how to calibrate predictive methods. Appendix D presents summary statistics by land use and region. Appendix E presents a summary of models by land use and crash type, and Appendix F presents correlation coefficients by land use.
