In 2005, the federal government put in place the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) aimed at significantly improving the operation and safety of the U.S. surface transportation system. This act, which set forth eight core emphasis areas, established the Highway Safety Improvement Program (HSIP). Under this 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.” The HSIP is used by the federal government to provide a significant amount of funding that allows every state to improve the safety of its highway network.
The Texas program
To comply with the act, the Texas Department of Transportation (TxDOT) has implemented the Hazard Elimination Program (HES) to identify, rank and select eligible projects to reduce the negative impacts associated with motor vehicle crashes. The formula used for this program is known as the “Safety Improvement Index” (SII). This index was first established in 1974 and revised in 1984. The SII is used to rank potential projects by prioritizing those that have a higher benefit-cost (B/C) ratio.
The formula documented in the index determines the ratio between the expected benefits (based on an anticipated crash reduction following the implementation of a proposed improvement) and the costs associated with putting the project into execution, including operating and maintaining costs of the project over its design life. The formula in its current form also contains terms or variables related to the exposure (i.e., traffic flow), life of the project, interest rate, crash costs and reduction factors.
The procedure to select the projects is as follows:
- Each TxDOT district (25 in total) sends a list of sites that have been identified as having safety problems as well as the proposed project characteristics to improve the safety of these sites;
- TxDOT’s Traffic Operations Division then compiles all the projects and ranks each project using the SII index; and
- The projects are individually funded starting with the most important project and ranked sequentially down until the states allotment of funds is depleted.
Given the significant changes in highway safety research that occurred within the past two decades and the necessity to stay in accordance with the HSIP, there was a need to determine whether the equation should still be used in its current form to rank and prioritize projects for safety improvement. TxDOT asked the Texas Transportation Institute (TTI) to review the equation used in the SII to determine whether the index needed to be updated.
In order to answer the request from TxDOT, the TTI researchers collected information about 230 projects that were classified as safety-improvement projects and were qualified to receive HES funding over a span of 9 years—1990 to 1998. The collected data spanned a three-year period of accident history for different severity levels comprising the following categories:
- Before and after, traffic flow values;
- Present and future;
- Crash costs;
- Maintenance costs;
- Total estimated costs; and
- The originally calculated SII value.
The data were assembled in a large electronic database and the projects were grouped for the five program years that were examined. The number of projects for each program year varied from 32 to 79 projects.
Researchers assessed the Index by performing sensitivity analyses with respect to different variables in the SII equations. Five scenarios were examined:
- Changing the interest rate;
- Removing property damage-only crashes since this type of crash is significantly under-reported;
- Evaluating different values for the reduction factors;
- Changing the accident rate to simulate the effects for the regression-to-the-mean (a common phenomenon observed when crash data are used for various types of analyses); and
- The non-linear relationship between the number of crashes and traffic flow.
Each of these variables was modified in different ways (by keeping other variables constant) and the effects on the ranking of projects were evaluated. Various statistical tests, such as the Spearman Rank Order Correlation Test and Kendall’s Tau Test, were used.
SII still performed well
The results documented in the study showed that, although the SII is influenced by changes in the value of each of the five variables examined, the ranking of projects usually remained the same or changed slightly. Researchers found the change in ranking with respect to the modification for each variable (except for the reduction factors) statistically insignificant. The analysis showed, however, that up to 11% of the original projects may not be funded in extreme cases.
The variable associated with the reduction factor was the only one that seriously affected the ranking of project. This meant that using unreliable reduction factors may lead to the selection of projects for improvement that should not have been selected in the first place. Hence, this could lead to a waste of funds and the loss of opportunity to reduce injuries and societal costs associated with motor vehicle collisions. Thus, selecting accurate reduction factors was found to be very important when applying the SII.
Despite the limitations related to the reduction factors, TTI recommended keeping the current formulation for prioritizing safety-improvement projects, since changes made to any variables in the index would not affect the ranking of the projects submitted by the various districts. Although the ranking was not affected by the change in interest rate, the researchers recommended that the most current value available be used to better reflect the true costs associated with the life-cycle of the project.
The contents of this paper reflect the views of the authors, who are responsible for the facts and accuracy of the results presented herein. The contents do not necessarily reflect the official view or policies of TxDOT.
Note: This research study was based on a master’s thesis written by Giridhar Reddy Singi Reddy in the Zachry Department of Civil Engineering at Texas A&M University. The original document can be found here: http://repository.tamu.edu/bitstream/handle/1969.1/85814/Singi.pdf?sequence=1.