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Literature Review 17 It was concluded that meaningful data to support heavy truck accident causation studies existed in a variety of sources. However, being able to locate, verify, and collate such data was considered challenging. Two important areas of data deficiency were noted as (1) the role of economic factors in truck operations and driving practices and (2) coarse categorization of truck accident data. 2.2.5 Accident Models for Two-Lane Rural Roads: Segments and Intersections Accident Models for Two-Lane Rural Roads: Segments and Intersections (Vogt and Bared 1998) describes the collection, analysis, and modeling of accident and roadway data pertaining to seg- ments and intersections on rural roads in the states of Minnesota and Washington. A compre- hensive review of data quality was performed as part of this effort. This included comparisons of values of multiple variables for consistency and flagging unusually large values of variables. Of particular interest were findings that there are inconsistencies in how attributes are defined in different accident databases as well as variations in reporting thresholds, making it difficult to conduct direct comparisons. The authors also note that the reliability of reported accident char- acteristics depends on the acumen of the report officer/official and witnesses. 2.2.6 The Human Factors Analysis and Classification System--HFACS With human error cited as the cause of the vast majority of civil and military aviation accidents, an argument is made that a more comprehensive accident analysis and classification framework for collecting data investigating human error is needed. HFACS was developed with this objective in mind. HFACS describes the following four levels of human failure: 1. Unsafe acts, 2. Preconditions for unsafe acts, 3. Unsafe supervision, and 4. Organizational influences. Unsafe acts are comprised of errors (mental or physical activities of individuals that fail to achieve their intended outcome) and violations (willful disregard for the rules and regulations that govern safe operations). Errors are further subdivided into those that are skill based, decision oriented, and perceptual, while violations are segmented into routine and exceptional. Preconditions for unsafe acts are based on the premise that unsafe acts are often symptoms of a deeper problem. Preconditions are divided into substandard conditions of operators and the prac- tices they commit, respectively. Substandard conditions of operators are subdivided into adverse mental states, adverse physiological states, and physical/mental limitations. Substandard practices of operators are categorized as crew resource management and personal readiness. Unsafe supervision traces causation of events back to the supervisory chain of command. Four subcategories of unsafe supervision are defined as 1. Inadequate supervision, 2. Planned inappropriate operations, 3. Failure to correct a known problem, and 4. Supervisory violation. The final category, organizational influences, addresses the institutional culture and how the organization is structured to perform. Organizational influences are subdivided into the following three groups: 1. Resource/acquisition management, 2. Organizational climate, and 3. Organizational process.