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161  Introduction This chapter describes the objective of NCHRP Research Report 974: Application of Crash Modi- fication Factors for Access ManagementâVolume 1: Practitionerâs Guide, identifies the benefits of using the guide, and provides an overview of each chapter (Gross et al. 2021). Objective of the Guide The objective of the Practitionerâs Guide is to assist transportation planners, designers, and traffic engineers in quantifying the safety impacts of access management strategies and make more informed access-related decisions on urban and suburban arterials. To achieve this objec- tive, the guide presents methods to quantify the safety performance of individual locations (i.e., intersections or segments) and 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 manage- ment strategies. The safety performance can then be used in the decision-making process to make comparisons with other quantitative measures (e.g., costs, operational efficiency, and environ- mental impacts) or perceptions (e.g., fairness, convenience, and competitiveness related to prop- erty access and businesses). By quantifying safety performance and considering safety alongside other factors, agencies can 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: ⢠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. ⢠Understanding of complex projects. Quantitative safety analysis can help in understanding the comprehensive costs and benefits of projects and in comparing safety performance with other factors such as mobility, environmental impacts, and regional economies. ⢠Return on investment. Quantitative safety analysis supports economic analysis, which can contribute to more informed decisions in planning, programming, and implementing trans- portation programs, which can improve the rate of return for a given budget. ⢠Documentation of the 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. C H A P T E R  8 Practitionerâs Guide
162 Application of Crash Modification Factors for Access Management 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) 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 defini- tions of key terms such as CMFs, SPFs, observed crashes, predicted crashes, and expected crashes. Appendix B of the Practitionerâs Guide 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, intersection treatments, 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 cross- roads. The roadway cross section strategies include non-traversable medians, spacing criteria for median openings, TWLTLs, 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, intersection and driveway sight distance, 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 to the observed, predicted, or expected crashes to esti- mate the magnitude of the expected change in safety. As discussed in Appendix B of the guide, the use of observed, predicted, and expected crashes results 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 predic- tive method presented 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) (AASHTO 2010). 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
Practitionerâs Guide 163  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 tem- poral 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. Chapter 4: Predictive Method for 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 that the Highway Safety Manual (1st Edition) Part C Predictive Method performs relatively well across a range of several other access man- agement features not accounted for in the existing predictive method. Specifically, the existing Part C Predictive Method (i.e., a 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 indepen- dence 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 manage- ment features and estimating the safety effect of variables related to access spacing and density. Chapter 5: Predictive Method for 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
164 Application of Crash Modification Factors for Access Management corridor-level prediction models, which provide a more reliable method to account for inter- actions 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. Chapter 6: Communicating Results Chapter 6 explains how to communicate analysis results and document access management planning and design decisions. While the methods in the Practitionerâs 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 potentially wide range of expertise and interest within these audiences. The chapter then identifies communication meth- ods 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 invest- ment (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.
