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Evaluation of Texas Department of Transportation Safety Improvement Index as a Prioritization Tool

In accordance with the federally mandated Highway Safety Improvement Program, every state is required to “develop and implement, on a continuous basis, a highway safety improvement program which has the overall objective of reducing the number and severity of crashes.” As part of this program, the Texas Department of Transportation (DOT) uses a formula known as the Safety Improvement Index (SII) for identifying, ranking, and selecting eligible projects. The SII is used to rank projects for potential funding by giving priority to projects that have a high benefit–cost ratio. Because the SII has not been updated within the past two decades, there is a need to determine whether the current formulation should be revised or updated. The objective of this study was to evaluate the SII in its current functional form and in its usefulness to rank and prioritize projects for safety improvement. The evaluation procedure proposed in this study used sensitivity analyses to study the effects of input variables on the SII. The results of the analysis indicated that although changes in the value of input variables affected the SII output, the ranking of projects usually was not affected, with the exception of the crash reduction factor variable. Thus, the same projects would be selected for safety improvement, even if different values were used in the SII. It is therefore recommended that the current formulation of the SII and the value of input variables used in the formula be retained by the Texas DOT to prioritize safety improvement projects.