Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
184 C H A P T E R 8 Practitioner Guide Introduction The Practitioner Guide is available as a standalone document. This chapter describes the objective of the guide, identifies benefits of using the guide, and provides an overview of each chapter. Objective of the Guide The objective of the Practitioner Guide is to assist transportation planners, designers, and traffic engineers quantify the safety impacts of access management strategies and make more informed access-related decisions on urban and suburban arterials. To achieve this objective, the guide presents methods to quantify the safety performance of individual locations (i.e., intersections or segments) as well as corridors that represent multiple adjacent intersections and segments. Benefits of Using the Guide The primary benefit of using the guide is to quantify the safety performance of access management strategies. The safety performance can then be used in the decision process to compare against other quantitative measures (e.g., costs, operational efficiency, and environmental impacts) or perceptions (e.g., fairness, convenience, competitiveness related to property access and businesses). By quantifying safety performance and considering safety alongside other factors, agencies will better understand the comprehensive costs and benefits of projects, which will lead to more informed decisions and more impactful investments. Other benefits of using the guide include the following: ï· Safety-conscious, performance-based practical design and construction: quantitative safety analysis informs agencies as to which of several project designs is expected to provide the greatest safety benefit to the public, ï· Understanding complex projects: quantitative safety analysis can help to understand the comprehensive costs and benefits of projects and to compare the safety performance with other factors such as mobility, environmental impacts, and regional economies, ï· Return on investment: quantitative safety analysis supports economic analysis, which can help to make more informed decisions in planning, programming, and implementing transportation programs, which can improve the rate of return for a given budget, and ï· 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. Chapter 1: Introduction The introduction identifies the purpose of the guide, which is to assist transportation agencies in estimating and assessing the potential safety effects of access management strategies. It also identifies the target audience, which includes transportation professionals (e.g., planners, designers, and traffic engineers)
185 with or without prior experience in access management and highway safety. Finally, it orients readers to the structure of the guide. Chapter 2: Definitions for Quantitative Safety Analysis Chapter 2 provides a brief introduction to quantitative safety performance, including definitions of key terms such as CMFs, SPFs, observed crashes, predicted crashes, and expected crashes. An appendix provides a more thorough overview of quantitative safety performance, including basics on the application of quantitative safety methods. Chapter 3: Safety Effects of Access Management Chapter 3 provides an overview of access management and background information on the purpose of specific access management strategies. The strategies are divided into four areas: access spacing, roadway cross-section, intersections, and property access. The access spacing strategies include unsignalized access density and spacing criteria, signal density and spacing criteria, functional area and corner clearance criteria, and spacing criteria for interchange crossroads. The roadway cross-section strategies include non- traversable medians, spacing criteria for median openings/crossovers, two-way left-turn lanes, and two-way versus one-way street operation. The intersection strategies include left-turn lanes, right-turn lanes, and alternative intersection designs. The property access strategies include driveway design elements, sight distance at unsignalized access, frontage/backage roads, and shared driveways and internal cross connectivity. Chapter 3 also presents high-quality CMFs for these access management strategies on urban and suburban arterials. Analysts can use the CMFs to compare the relative safety impacts of access management strategies or apply the CMFs the observed, predicted, or expected crashes to estimate the magnitude of the expected change in safety. As discussed in the appendix of the guide, the use of observed, predicted, and expected crashes result in varying degrees of reliability. Specifically, applying CMFs to expected or predicted crashes is typically more reliable than applying CMFs to observed crashes. In some instances, the CMFs presented in this chapter can be applied in the segment- and intersection-level predictive methods presented in chapter 4; however, prior to applying any CMFs that were not developed specifically for use with the predictive method in chapter 4, there is a need to consider the applicability of the CMFs with respect to crash type, crash severity, and base condition as well as the potential for double-counting crash reductions with each additional CMF. Chapter 3 includes CMFs inferred from the Highway Safety Manual (1st Edition). Inferred CMFs are based on the ratio of predicted crashes from one or more statistical models for two conditions of interest. For example, the Highway Safety Manual (1st Edition) provides models for multivehicle driveway and non- driveway crashes per mile, considering the traffic volume, number and type of driveways, and median type (divided or undivided). Using the models, one could predict crashes for various combinations of driveway density, driveway type, median type, and traffic volume, and use the ratio of two predictions to infer the CMF (i.e., the expected change in crashes when converting from one condition to another). In some cases, the inferred CMFs provide a logical relationship. In other cases, it is not reasonable to use the results from cross-sectional models to infer CMFs. Specifically, when cross-sectional models do not account for all differences in safety between the two site types, the comparison of results from two different cross-sectional models may not produce reasonable and reliable CMFs. Further, it is necessary to calibrate cross-sectional models to the same spatial and temporal conditions before using the models to infer CMFs. Counterintuitive results are presented in a separate section of chapter 3 with discussion and cautions to not use the CMFs to estimate the safety effect of the specific variables.
186 Chapter 4: Segment- and Intersection-Level Analysis Chapter 4 presents a method to estimate the safety performance of individual intersections and segments, including instructions on how to apply CMFs from chapter 3 to adjust the predictions. The method is consistent with the Highway Safety Manual (1st Edition) Part C Predictive Method but expands the method to incorporate the safety impacts of additional access management strategies. The Part C Predictive Method already facilitates the consideration of a limited number of access management variables. For segment- level predictions, the existing method accounts for the number and type of driveways along the segment. For intersection-level predictions, the existing method accounts for the presence of left- and right-turn lanes, left-turn signal phasing (at signalized intersections), and right-turn-on-red restrictions (at signalized intersections). The research from NCHRP Project 17-74 confirmed the Highway Safety Manual (1st Edition) Part C Predictive Method performs relatively well across a range of several other access management features not accounted for in the existing predictive method. Specifically, the existing Part C Predictive Method (i.e., combination of SPFs and CMFs) performs well for sites with similar geometry, but different access management features such as median opening spacing, number of median openings by type, and corner clearance along a segment. There are, however, a few scenarios where the existing models do not perform well across sites with different access management features. Chapter 4 identifies these scenarios and presents adjustment factors to account for differences in the predictions, including channelized right-turn lanes and distance to ramp terminal. Chapter 4 presents another significant limitation of the Part C Predictive Method with respect to access management. Specifically, the Part C Predictive Method should not be used to estimate the safety effect of variables related to access spacing and density. Chapter 4 is only applicable to estimating the safety performance of individual segments and intersections, assuming independence among each unit of analysis. While the results can be aggregated from multiple segments and intersections to estimate the safety performance of a corridor, as suggested in the Highway Safety Manual (1st Edition), this method does not consider the potential interactions among adjacent or nearby sites (e.g., access spacing and density). The existing Part C Predictive Method may even produce counterintuitive results (e.g., fewer estimated segment crashes with an increase in the number of intersections along a corridor). As such, the corridor-level predictive method presented in chapter 5 is more appropriate for considering interactions among access management features and estimating the safety effect of variables related to access spacing and density. Chapter 5: Corridor-Level Analysis Chapter 5 presents two methods for estimating corridor-level safety performance and provides guidance on when to use each method. The first method combines predictions for individual locations based on the predictive method in chapter 4. Again, the method does not account for the potential interactions among adjacent or nearby sites and should not be used to estimate the safety effect of variables related to access spacing and density. The second method is based on corridor-level prediction models, which provide a more reliable method to account for interactions among adjacent sites and among multiple access management strategies. The corridor-level prediction models help to account for situations where safety performance is influenced more by corridor-level characteristics rather than the specific characteristics of an individual location. For example, some access management strategies may shift turning traffic from one location to another (e.g., converting an undivided road to a physically-divided road). In other cases, detailed access design (e.g., intersection or median opening spacing) will impact the safety performance of the corridor beyond the simple presence of the feature. In such cases, corridor-level crash predictions are more appropriate than aggregating crash predictions for individual sites.
187 Chapter 6: Communicating Results Chapter 6 explains how to communicate analysis results and document access management planning and design decisions. While the methods in this guide can help to quantify and compare the safety performance of alternatives, the analysis effort is futile unless decision-makers use the results to inform decisions. Chapter 6 describes how the guide can serve the needs of both technical and non-technical audiences and acknowledges the potential wide range of expertise and interest within these audiences. The chapter then identifies communication methods and explains how to select and employ appropriate communication methods to target and effectively reach various audiences. Several different measures and formats are presented for conveying the key results to technical and non-technical audiences. Measures include technical safety and economic factors such as the expected number of crashes and the return on investment (benefit-cost ratio) as well as more human-centric safety indicators such as lives saved and injuries prevented. The formats provide a balance of simple tabular information and creative graphical displays to present multiple dimensions of the analysis. Regardless of the audience, method, measure, and format, one common theme is the need to engage communication experts throughout the process.