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5 C H A P T E R 1 Introduction Background The Highway Safety Manual (1st Edition) (AASHTO 2010) has revolutionized the transportation engineering practice by providing crash modification factors and functions (CMFs), along with methods that use safety performance functions (SPFs) for assessing and quantifying the safety consequences of planning, designing, and operating highway facilities. It recognizes that access management is an effective component in the operation and safety of roadways. For example, at the site level, the safety performance of an intersection depends on factors such as the type of turning movements, presence and length of turn lanes, and the number and proximity of driveways. At the corridor level, the safety performance depends on factors such as the density and spacing of intersections, driveways, and median openings. One challenge to using quantitative methods to support safety-related access management decisions is that information is relatively limited. The Highway Safety Manual (1st Edition) includes few opportunities to quantify the safety effects of access management strategies. For example, the Part C Predictive Method includes a limited number of variables or adjustment factors related to access management strategies (e.g., median type). Further, Part D includes a limited number of CMFs for access management strategies. While there are additional CMFs related to access management strategies in the Federal Highway Administrationâs (FHWAâs) CMF Clearinghouse, several of these were passed over for inclusion in the Highway Safety Manual (1st Edition) due to quality concerns. There are multiple reasons for the limited quantity and quality of information on the safety effects of access management strategies. Part of the reason is the difficulty in developing high quality CMFs and SPFs for access management strategies due to challenges related to data availability, sample size, and statistical methods. These difficulties have been compounded by the stringent criteria used to decide which CMFs are included in the Highway Safety Manual (1st Edition). The validity of applying access management CMFs to existing SPFs is also in question since most of the SPFs in the Highway Safety Manual (1st Edition) were established without consideration of base conditions for access management features. For example, the same SPF from the Part C Predictive Method would be used to estimate the safety performance of the two roadways shown in Figure 1 (i.e., chapter 12 SPF for urban/suburban, four-lane, median-divided arterial). The question remains as to whether or not these SPFs, along with the corresponding CMFs, adequately capture the differences in access management features. To do so correctly would require knowledge of the access management features in the data used to develop the base models, knowledge that is not available. There is a need to better understand the use of existing CMFs and SPFs for quantifying the safety effects of access management strategies. First, there is a need to identify ways to group and organize them into various categories related to roadway geometry, land use, geography, and other identifying conditions or features. There is also a need to identify CMFs and SPFs that are useful and reliable, and to clearly identify knowledge voids for strategies at the site, segment/intersection, and corridor levels. Finally, there is a need to fill critical knowledge gaps by developing new CMFs (factors and/or functions) and SPFs. Of particular interest in addressing these needs is the effect of features such as medians, corner clearances, access density, access spacing, turn lanes, and turn restrictions. Once reliable CMFs and SPFs are established, there is a need to provide guidance for the application of this information, considering the applicability and quality. The guidance should address the use of CMFs
6 and SPFs to estimate the effects of individual strategies as well as the cumulative and interactive effects of features where several strategies are applied simultaneously. For example, current practice is to apply CMFs for driveways and median type individually as though they are independent, and the combined effects are presumed to be the product of the two CMFs. This may change as a result of this research. Related to these issues is an additional need for guidance on the calibration and application of CMFs and SPFs to estimate the safety performance of sites as site characteristics change from one location to another. What is the difference in safety performance for these two multilane, median-divided facilities? Figure 1. Illustration of the differences in access management features Overall Study Objective The objective of NCHRP Project 17-74 was to develop and refine CMFs and SPFs for access management features and develop guidance to assist transportation agencies in quantifying the safety impacts of their decisions related to access management. Specifically, this project: ï· Verified the reliability of using existing SPFs from the Highway Safety Manual (1st Edition) to quantify the safety performance of urban and suburban arterials, ï· Quantified the safety performance of access management features by developing and refining CMFs and SPFs for various levels of analysis (site, segment/intersection, and corridor), ï· Provided guidance for the use of access management related CMFs from the Highway Safety Manual (1st Edition) and the CMF Clearinghouse, and ï· Identified opportunities for future research. In developing the guidance, this project: ï· Recommended and prioritized CMFs for individual access management strategies at various levels (site, segment/intersection, and corridor) for various categories, ï· Assessed and recommended methods to quantify the cumulative and interactive effects of access management features, and ï· Recommended procedures to aid with evaluating and documenting access management planning and design decisions. To accomplish these objectives, there was a need to identify existing high-quality research, define existing gaps, and conduct new research to fill those gaps. There was also a need to develop concise, practice-ready guidance and vet the guidance with potential end users to ensure it is understandable and
7 useful to the target audience. Finally, there was a need to develop an effective implementation plan that ensures the research results are adopted and applied in practice. Overview of Approach The research plan was divided into two phases, comprised of 13 technical tasks as summarized in Table 1. Phase 1 focused on Task 2 through Task 5, culminating in an updated Phase 2 work plan. Phase 2 implemented the work plan to collect and analyze the data and develop the final deliverables. Table 1. Summary of research plan. Phase / Task Description Phase 1: Preliminary Analysis and Refined Work Plan Developed a refined work plan and guidance outline based on the literature review, data reconnaissance, and gap analysis. Task 1: Amplified Work Plan and Kickoff Meeting Met with the Panel to discuss the amplified work plan, addressed comments and questions, and confirmed initial research priorities. Task 2: Literature Review Reviewed/summarized current information on the safety effects of access management strategies at three levels: site, intersection, and corridor. Task 3: Data Reconnaissance and Assessment Identified and reviewed existing data sources to determine the availability of data to support this research, and identified current policies/practices, challenges, and case studies to quantify the safety effects of access management decisions. Task 4: Gap Analysis and Study Design Summarized current knowledge and identified gaps to establish research priorities and appropriate study design(s), considering schedule, budget, and data. Task 5: Guidance Outline Developed outline of guidance based on literature review and data reconnaissance. Task 6: Interim Report and Panel Meeting Summarized progress and presented plan for Phase 2 in an interim report. Met with the Panel to discuss the progress, guidance outline, and Phase 2 work plan. Phase 2: Execute Work Plan and Prepare Final Deliverables Executed the approved work plan to develop and refine the guidance based on existing information and new data collection and analysis. Task 7: Data Collection Collected crash, traffic volume, roadway, and other relevant data to support the study design developed in Task 4 and refined based on Task 6 efforts. Task 8: Data Analysis Conducted analysis to fill priority gaps in the guidance on estimating the safety effects of access management strategies. Task 9: Draft Guidance Fleshed-out annotated outline and developed draft guidance on estimating the safety effects of access management strategies. Task 10: Presentations and Focus Groups Presented the research results and draft guidance to transportation professionals for comments on the practicality and usability. Task 11: Implementation Plan Developed a stand-alone technical memorandum titled âImplementation of Research Findings and Productsâ to identify and describe the critical steps, action items, and champions to implement the research results in practice.
8 Task 12: Draft Report and Panel Meeting Developed draft report to document the project, including the guidance document and recommendations for future research needs and priorities. Task 13: Final Report and Deliverables Developed final deliverables. Scope of Final Report This report documents the research process and presents the results of each task. Chapter 2 presents a review of the relevant existing information, identified through a review of literature and practices. Chapter 3 presents the results of a survey as well as a summary of the data reconnaissance and assessment. Chapter 4 presents the gap analysis. Chapter 5 presents the study design. Chapter 6 presents a summary of the data collection. Chapter 7 presents the analysis methodology and results of the data analysis. Chapter 8 presents the process for developing the Practitioner Guide, which is delivered under a separate cover. Chapter 9 describes the effort to convene a focus group to review and provide comments on the Practitioner Guide. Chapter 10 presents the implementation plan. Chapter 11 provides a summary of the report. Chapter 1 References AASHTO. 2010. Highways Safety Manual. 1st edition, Washington, DC.