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7  Chapter 2 defines terms related to quantitative safety performance. Refer to Appendix B for 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. Introduction to Quantitative Safety Analysis One of the most basic components of evidence-based safety is quantifying the safety per- formance of an existing or planned facility in terms of the estimated frequency and severity of crashes. Traditional approaches rely on short-term average crash history to represent the safety performance of a given location. More reliable approaches to quantitative safety performance focus on long-term crash frequency and severity, incorporating the crash history for the location of interest when applicable, but also incorporating crash history from other, similar locations and adjusting for changes in traffic volume and other factors that influence safety over time. Quantifying safety performance supports decisions throughout the project development pro- cess from planning and design to operations and maintenance. Agencies can use the methods presented in this guide to estimate and compare safety performance with and without a given access management strategy or to select the preferred alternative based on the estimated safety performance of multiple alternatives. For example, transportation planners could use these methods to quantify and compare the safety impacts of different median alternatives alongside other factors such as operational and environmental impacts. Permitting departments could use these methods to quantify and consider the safety impacts of adding one or more drive- ways along a roadway segment as well as locating driveways with respect to the functional area of an adjacent intersection. Highway designers could use these methods to quantify and compare the safety performance of a median opening with and without a turn lane. While crash-based methods, such as those presented in the Highway Safety Manual (1st Edition) Part C Predictive Method (AASHTO 2010), are preferred for quantifying safety performance, these methods are not currently available for all facility types and access management strategies of interest. When crash-based methods are not available for the scenario of interest, there is an opportunity to consider alternative performance measures such as alignment with Safe System principles. A Safe System approach to access management might consider the ability to remove or reduce the number of higher-angle and higher-speed conflict points (such as left-turning conflict points), reduce vehicle speeds or speed differentials (such as removing turning vehicles from through lanes), and reduce user workloads (such as simplifying turning movements). For example, consider a proposal to convert an existing four-lane undivided corridor with a high density of driveways to a median-divided facility. Using the Safe System approach, one could compare the safety performance of the two scenarios based on the number of conflict points, C H A P T E R  2 Definitions for Quantitative Safety Analysis
8 Application of Crash Modification Factors for Access Management the types of conflict points (crossing, merging, and diverging), typical severity distributions of types of conflict points, and exposure at each conflict point (e.g., turning movements or con- flicting volumes). The proposed median-divided facility would remove crossing and left-turn conflicts, except at median openings, and these types of conflicts tend to result in the most severe crashes. Definitions The following are basic definitions related to quantitative safety performance that are used throughout this guide: Alternative: The scenario of interest such as existing conditions or proposed conditions. The following are examples of alternatives (or scenarios of interest): ⢠Existing roadway and traffic conditions. ⢠Existing roadway conditions under future traffic conditions. ⢠Proposed alternative that incorporates some changes to the roadway geometrics and/or operations. Calibration factor: A multiplicative factor used to adjust a safety performance function (SPF) to reflect local conditions and changes over time. Calibration factors greater than 1.0 indicate the SPF is under-predicting crashes for local conditions. Calibration factors less than 1.0 indicate the SPF is over-predicting crashes for local conditions. Refer to Appendix C of this guide, the Highway Safety Manual (1st Edition), or FHWAâs user guide for The Calibrator (Lyon et al. 2018) for more information on how to develop and apply calibration factors or functions. Crash modification factor (CMF): A multiplicative factor used to compute the long-term average crash frequency after implementing a given strategy at a specific site. A CMF of 1.0 indicates no expected change in crashes. A CMF less than 1.0 indicates an expected reduction in crashes, and a CMF greater than 1.0 indicates an expected increase in crashes. Throughout this guide, the term CMF includes both factors and functions. Refer to Appendix B of this guide, the Highway Safety Manual (1st Edition), or FHWAâs CMF Clearinghouse (http://www. cmfclearinghouse.org/) for more information on how to select and apply CMFs. Crash modification function (CMF): A formula used to compute the crash modification factor for a specific site based on its characteristics. It allows the factor to change over the range of a variable or a combination of variables. Throughout this guide, the term CMF includes both factors and functions. Crash type: The manner of collision (e.g., right-angle, left-turn, rear-end, head-on, and run-off-the-road). Crash severity: The greatest level of injury in a crash, which may be categorized as fatal, injury, and property damage only (PDO) or by the KABCO scale (a definition of the KABCO scale follows). Expected crashes: An estimate of long-term average safety performance based on the weighted average of observed and predicted crash frequencies using the Empirical Bayes (EB) method. Refer to Appendix B of this guide or the Highway Safety Manual (1st Edition) for more informa- tion on how to compute expected crashes. Facility type: A group of similar segments or intersections, typically defined by geometric and operational characteristics such as traffic control (signalized, stop-controlled, or uncontrolled), area type (urban or rural), number of lanes, and number of approaches.
Definitions for Quantitative Safety Analysis 9  KABCO scale: The Model Minimum Uniform Crash Criteria (MMUCC), 5th edition, provides the following definitions for the KABCO scale (NHTSA 2017): K (fatal crash): fatal injury where death occurs within 30 days after the motor vehicle crash in which the injury occurred. A (suspected serious injury): any injury other than fatal which results in one or more of the following: severe laceration resulting in exposure of underlying tissues/muscle/organs or resulting in significant loss of blood; broken or distorted extremity (arm or leg); crush injuries; suspected skull, chest, or abdominal injury other than bruises or minor lacerations; significant burns (second and third degree burns over 10% or more of the body); unconsciousness when taken from the crash scene; or paralysis. B (suspected minor injury): any injury that is evident at the scene of the crash, other than fatal or suspected serious injuries. Examples include lump on the head, abrasions, bruises, minor lacerations (i.e., cuts on the skin surface with minimal bleeding and no exposure of deeper tissue/muscle). C (possible injury): any injury reported or claimed which is not a fatal, suspected serious, or suspected minor injury. Examples include momentary loss of consciousness, claim of injury, limping, or complaint of pain or nausea. Possible injuries are those which are reported by the person or are indicated by his/her behavior, but no wounds or injuries are readily evident. O (no apparent injury): a situation where there is no reason to believe that the person received any bodily harm from the motor vehicle crash. There is no physical evidence of injury and the person does not report any change in normal function. Observed crashes: The number of reported crashes at a specific location during a defined time period. Performance measure: An indicator of how well a particular segment, intersection, or project is performing. The typical measure of safety performance is the estimated number of crashes, but this could be in terms of specific crash types or severities. Predicted crashes: The estimated number of crashes from an SPF. Safety performance function (SPF): An equation that represents the statistical relationship between safety and roadway characteristics for a given facility type. Refer to Appendix B of this guide, the Highway Safety Manual (1st Edition), or FHWAâs CMF Clearinghouse (http://www. cmfclearinghouse.org/) for more information on how to select and apply SPFs. Strategy: A term to describe an actual or proposed change in the geometric or operational conditions to achieve a specific performance outcome of the roadway. This term is synonymous with countermeasure or technique. Study area: The location of interest, which may be defined as an individual roadway segment or intersection or as an entire corridor or network. Study period: The timeframe of interest, which is typically defined in terms of years, and may include a single year or multiple years.
