Evaluating txdot's safety improvement index - a prioritization tool
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In accordance with the federally mandated Highway Safety Improvement Program (HSIP), 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 and decreasing the potential for crashes on all highways” (FHWA, 1979). The federal government via the HSIP provides a significant amount of funding that allows every state to improve the safety of their highway network. With such large amounts of federal funds involved, it becomes essential that state transportation agencies take appropriate measures to utilize these funds in the most cost effective manner. As part of this program, the Texas Department of Transportation (TxDOT) uses a formula known as the “Safety Improvement Index” (SII) for identification, ranking and selection of eligible projects. The SII is in essence used to rank potential projects by giving priority to projects that have a higher benefit-cost (B/C) ratio. Since the SII has not been updated within the last two decades, there is a need to determine whether the current formulation needs to be revised or updated. This concern has been reported in the literature. The objective of this thesis is to evaluate the SII in its current functional form and its usefulness to rank and prioritize projects for safety improvement. The evaluation procedure proposed in this thesis uses sensitivity analyses to study the effects of different input variables on the SII. The sensitivity analysis is performed with respect to five critical variables chosen on the basis of a literature review. The five variables studied are the Interest Rate, Removal of PDO Crashes, Crash Reduction Factors, Crash Rates, and Crash – Flow Relationship. The focus of the evaluation is to compare the ranking of projects with respect to changes in the value of these input variables. The ranking are evaluated using various statistical methods, such as the Spearman Rank Order Correlation Test and Kendall’s Tau Test. The results of the analysis indicated that, although changes in the value of input variables affect the SII output, the ranking of projects is usually not affected, with the exception of the crash reduction factor variable. Hence, the same projects will be selected for safety improvement, even if different values are used in the SII. Therefore, it is recommended that the current formulation of the SII and the value of input variables used in the formula be retained by TxDOT for prioritizing safety improvement projects. However, it is suggested to examine the accuracy and uncertainty associated with reduction factors, since in some cases they were found to affect the ranking of projects.
SubjectSafety Improvement Index
Singi Reddy, Giridhar Reddy (2007). Evaluating txdot's safety improvement index - a prioritization tool. Master's thesis, Texas A&M University. Available electronically from