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Page 20
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Page 21
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Page 22
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Page 23
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
×
Page 23
Page 24
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Page 25
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Page 25

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20 Chapter 1. Introduction 1.1 Background In May 2010, the American Association of State Highway and Transportation Officials (AASHTO) published the first edition of the HSM. This was an important step forward in providing quantitative safety analysis tools to inform decisions made by transportation agencies. HSM Part C includes predictive methods that can be used to anticipate the safety performance of new facilities, assess the safety performance of existing facilities, or estimate the expected effectiveness of proposed improvements to existing facilities. The HSM has become a key safety prediction tool, and state transportation agencies are gaining experience using the HSM in different planning and project contexts. In preparing the first edition of the HSM, decisions that determined which facility types would be addressed by the predictive methods chapters were made based on availability of data, funding limitations, and highway agency priorities. Since the preparation and publication of the first edition of the HSM, several National Cooperative Highway Research Program (NCHRP) projects have been funded to expand the safety knowledge and improve the crash prediction methods provided in Part C of the first edition of the HSM. The HSM Part C presents predictive methods for estimating the expected average crash frequency for specific intersection configurations and traffic control types. The HSM Part C methods can also be applied to multiple roadway segment and intersection sites to estimate the safety performance at the project, corridor, or network levels. The estimation of expected average crash frequency with the Part C methods uses a combination of SPFs, crash modification factors (CMFs), calibration factors, and (when applicable) observed crash data. Table 1 shows the intersection configurations and traffic control types for which models are available in the HSM Part C chapters (Chapters 10, 11, and 12) and the 2014 Supplement to the HSM (Chapter 19) (AASHTO, 2014), which includes crossroad ramp terminal models. In summary, the first edition of the HSM provides the capability to analyze the safety performance of approximately thirteen intersection configurations and traffic control types. Many intersection and traffic control types such as all-way stop-controlled intersections and rural three-leg signalized intersections are not addressed within the first edition of the HSM. Table 1. Intersection types addressed by predictive methods in the first edition of the HSM Intersection Type HSM Chapter 10 11 12 19 Three-leg intersections with stop control on minor approach X X X Four-leg intersections with stop control on minor approaches X X X Three-leg intersections with signal control X Four-leg intersections with signal control X X X Diamond ramp terminals at crossroad X Parclo ramp terminals at crossroad X Free-flow ramp terminals at crossroad X The science of crash frequency prediction and crash severity prediction has evolved as the HSM was developed. The earliest HSM Part C chapter—Chapter 10 on rural two-lane, two-way roads—was developed as a prototype with no new research beyond what was available at the

21 time of its development. It contains SPFs for all crash severities combined, with tabulated severity distributions in the form of proportions available to separate the total crash frequency predictions into crash frequencies for individual crash severity levels. HSM Chapters 11 and 12 contain separate SPFs by crash severity level. HSM Chapter 19 for ramp terminals contains SPFs to predict the frequency of all FI crash severity levels combined and then implements crash SDFs to separate the FI crash frequency into frequencies by individual severity level as a function of ramp terminal characteristics. Table 2 summarizes the treatment of crash severity in the various HSM Part C chapters. Table 2. Treatment of severity in current HSM models HSM Chapter Treatment of Severity 10 Tabulated crash severity distributions 11 Separate SPFs for KAB and KABC crashes 12 Separate SPFs for KABC and PDO crashes 19 SPFs for FI crashes with crash SDFs and SPFs for PDO crashes Crash severity levels: fatal (K), incapacitating injury (A), non-incapacitating injury (B), possible injury (C), and PDO. In terms of addressing crash types within the HSM predictive methods for at-grade intersections, Chapters 10, 11, and 19 use tabulated collision type distributions that can be applied to crash predictions of all collision types combined, while HSM Chapter 12 contains separate SPFs for predicting single- and MV crashes at intersections. CMFs are used in the existing crash prediction models to account for the effects of intersection skew angle, presence of left- and right-turn lanes, signal phasing, right-turn-on-red, red light cameras, and lighting. This report presents research conducted to develop crash prediction models for additional intersection configurations and traffic control types not currently addressed in the first edition of the HSM. 1.2 Research Objective and Scope The objective of this research was to develop new intersection crash predictive models for consideration in the second edition of the HSM that are consistent with existing methods in HSM Part C and comprehensive in their ability to address a wide range of intersection configurations and traffic control types in rural and urban areas. The main focus of the research was on: • Developing SPFs for intersection configurations and traffic control types not currently addressed in the HSM Part C. • Developing SDFs to be used in combination with SPFs to estimate crash severity as a function of geometric design elements and traffic control features. The crash prediction methods developed in this research include SPFs, CMFs, and SDFs as applicable in a format consistent with the predictive models in the existing HSM Part C and the 2014 Supplement to the HSM (Chapter 19). Data and methodologies used to develop the predictive models consider traffic volumes on all intersecting roads and streets as well as design

22 elements and traffic control features considered by engineers and planners during the project development process. Roundabouts are not addressed in this research, as new crash prediction models were recently developed for possible inclusion in the second edition of the HSM as part of a separate study (NCHRP Project 17-70, Development of Roundabout Crash Prediction Models and Methods). Crash prediction models were developed for the following intersection configurations and traffic control types for consideration in the second edition of the HSM: • Intersections with all-way stop control - Rural four-leg intersections with all-way stop control - Urban and suburban three-leg intersections with all-way stop control - Urban and suburban four-leg intersections with all-way stop control • Three-leg intersections with signal control on rural highways - Three-leg intersections with signal control on rural two-lane highways - Three-leg intersections with signal control on rural multilane highways • Intersections on high-speed urban and suburban arterials - Three-leg intersections with minor road stop control - Three-leg intersections with signal control - Four-leg intersections with minor road stop control - Four-leg intersections with signal control • Urban and suburban five-leg intersections with signal control • Three-leg intersections where the through movements make turning maneuvers at the intersections - Three-leg intersections on rural two-lane highways - Three-leg intersections on urban and suburban arterials • Crossroad ramp terminals at single-point diamond interchanges • Crossroad ramp terminals at tight diamond interchanges NOTE: A crash prediction model for three-leg all-way stop-controlled intersections on rural two- lane highways was developed as part of this research and is included in this report, but due to limited sample size the model was not recommended for consideration in the second edition of the HSM. 1.3 Overview of Research Methodology In Phase I of the research, the research team reviewed and summarized literature related to current HSM intersection crash prediction methods; protocols, best practices, and emerging approaches for predictive model development; and current knowledge related to intersection safety. The research team also surveyed transportation agencies to gain knowledge about their experience with the current HSM intersection predictive methods and assess their needs and priorities as they relate to additional (new) intersection models and/or expanded capabilities of

23 existing models. Based on the results of the literature review and survey, the research team identified and prioritized the types of intersection configurations and traffic control types not currently addressed in the HSM for further consideration in this research. The research team then developed work plans for creating crash prediction models for the higher priority intersection configurations and traffic control types, including: • Intersections with all-way stop control • Three-leg intersections with signal control on rural two-lane and multilane highways • Intersections on high-speed expressways • Three-leg intersections where the through movements make turning maneuvers at the intersections • Three-leg intersections with a commercial driveway forming a fourth leg • Five-leg intersections • Single-point diamond ramp terminals • Indirect left-turn intersections (i.e., U-turns or J-turns) The work plans addressed site selection, data collection, database development, and model development. Other intersection configurations and traffic control types considered for model development but for which work plans for possible execution in Phase II were not developed included: intersections with yield or no control, six-or-more-leg intersections, and diverging-diamond ramp terminals. Work plans were not developed for these intersection configurations and traffic control types due to a combination of priorities from the HSM user survey and the likelihood of successful model development with a sufficient number of sites, exposure, and crash data. For example, because diverging-diamond ramp terminals are relatively new in the United States, limited years of crash data were available for model development at the time of this research. Therefore, the research team did not create a work plan for developing crash prediction models for diverging-diamond ramp terminals. In Phase II of the research, based on a combination of priorities from the HSM user survey, the likelihood of successful model development, and cost, the research team executed the approved work plans to develop crash prediction models for intersections with all-way stop control, three- leg intersections with signal control on rural two-lane and multilane highways, intersections on high-speed expressways, five-leg intersections, and single-point diamond ramp terminals. The research team updated existing spreadsheet tools to include the new crash prediction models developed as part of this research, conducted sensitivity analyses to check that the results were reasonable, and updated/revised the crash prediction models as necessary. In addition, the research team developed recommended text for consideration in the second edition of the HSM and prepared portions of this report that document Phases I and II of the research. In Phase III of the research, the research team executed the approved work plan to develop crash prediction models for three-leg intersections where the through movements make turning maneuvers at the intersections and adapted and executed the work plan for crossroad ramp

24 terminals at single-point diamond interchanges to address crossroad ramp terminals at tight diamond interchanges. Similar to Phase II, the research team updated existing spreadsheet tools to include the new crash prediction models developed in Phase III of this research, conducted sensitivity analyses to check that the results made sense, updated/revised the crash prediction models as necessary, developed recommended text for consideration in the second edition of the HSM, and prepared portions of this report that document Phase III of the research. Throughout the course of the research, the research team kept abreast of other ongoing research related to the HSM that could potentially impact the direction of this research. 1.4 Outline of Report This report presents an overview of research conducted to develop new intersection crash predictive models for consideration in the second edition of the HSM, for intersection configurations and traffic control types not addressed in the first edition of the HSM. The remainder of this report is organized as follows: Chapter 2. Literature Review and Survey of Practice Chapter 3. Development of Models for Use in HSM Crash Prediction Methods: Intersections with All-Way Stop Control Chapter 4. Development of Models for Use in HSM Crash Prediction Methods: Three-Leg Intersections with Signal Control on Rural Highways Chapter 5. Development of Models for Use in HSM Crash Prediction Methods: Intersections on High-Speed Urban and Suburban Arterials Chapter 6. Development of Models for Use in HSM Crash Prediction Methods: Five-Leg Intersections Chapter 7. Development of Models for Use in HSM Crash Prediction Methods: Three- Leg Intersections where the Through Movements Make Turning Maneuvers at the Intersections Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges Chapter 9. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Tight Diamond Interchanges Chapter 10. Conclusions and Recommendations Chapter 11. References Chapter 12. Abbreviations, Acronyms, Initialisms, and Symbols

25 Appendix A—Draft Text for the Second Edition of the HSM HSM Chapter 10—Predictive Method for Rural Two-Lane, Two-Way Roads HSM Chapter 11—Predictive Method for Rural Multilane Highways HSM Chapter 12—Predictive Method for Urban and Suburban Arterials HSM Chapter 19—Predictive Method for Ramps

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The first edition of the Highway Safety Manual (HSM), in 2010, included Safety Performance Functions (SPFs) for roadway segments and intersections. However, not all intersection types are covered in the first edition of the HSM.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 297: Intersection Crash Prediction Methods for the Highway Safety Manual develops SPFs for new intersection configurations and traffic control types not covered in the first edition of the HSM, for consideration in the second edition of the HSM.

Supplemental to the Document is recommended draft text for the second edition if the HSM, a worksheet for Chapter 10, a worksheet for Chapter 11, a worksheet for Chapter 12, a worksheet for Chapter 19, and a presentation.

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