The Highway Safety Manual (HSM) procedures provide robust methods for conducting site-specific safety analyses known as micro-level analyses. These micro-level analyses can either be in reaction to evaluating alternatives to fix an identified black spot (i.e., reactive), or as part of an entirely new facility planned for an area (i.e., proactive). Micro-level analysis procedures such as the predictive method and Crash Modification Factors presented in the HSM are significant tools that highway agencies are beginning to integrate into their safety management and design procedures and practices. Micro-level analysis tools are suitable for analysis of specific intersections or roadway segments. They make it possible to consider the safety impacts of alternative design features such as the number or type of lanes, shoulder width, or intersection control. In contrast to micro-level analysis procedures for site-specific or corridor situations, macro-level analysis procedures perform analyses on an area-wide basis – entire neighborhoods, cities, and/or regions. Macro-level analysis procedures can be used to incorporate safety prediction into area-wide, long-range transportation system planning, programming, and policy development. The idea of predicting crashes within a given area, such as census tracts or traffic analysis zones (TAZs), allows agencies to identify safety concerns that may not be apparent by examining crash patterns at an intersection or segment scale. This could potentially allow planners to address safety concerns prior to their full materialization. For example, a macro-level model may indicate that changing demographics could result in a sharp uptick in fatalities among the older driver population in a particular section of a city. As a result, agencies could implement wider lane striping and larger sign lettering in that area, or work to improve transit service to older populations. Moreover, macro-level models would help quantify that uptick such that a robust cost benefit analysis could be used to justify one or several of these investments in response to this changing demographic. The ability to analyze and respond to these types of concerns is not currently accounted for in the HSM Predictive Methods. To avoid reactive expensive infrastructure retrofits, macro-level predictive models developed to a standard consistent with the Predictive Method in Part C of the HSM would make it possible to integrate area-wide quantitative safety into planning procedures and programs.
The objectives of this research were to develop validated and demonstrated quantitative macro-level safety prediction models and a quantitative safety planning chapter for the AASHTO HSM intended for use by transportation practitioners at all levels. This includes a guidance document on the development and application of these models, methods to integrate the model results into planning procedures, and electronic analysis tools for applying the models in practice. The research results are intended as a new chapter for inclusion in a future edition of the HSM. The results should address a broad range of safety planning level issues related to macro-level models such as, but not limited to, geography, demographics, transportation modes and modal interaction, existing or planned land-use and/or transportation projects, model transferability, calibration needs, and associated data limitations.
STATUS: The Final Report was published as NCHRP Research Report 1044. The guide (Appendix C) was published as NCHRP Web-Only Document 348.