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Authors: Becky White & Erin Curry
We are so excited for TDOT’s Project Safety Office. They have been recognized by the Federal Highway Administration (FHWA) and the Roadway Safety Foundation as one of nine National Roadway Safety Award winners for their contribution to improving roadway safety. TDOT was recognized in the Program Planning, Development, and Evaluation Category for using data analysis to maximize safety investments. Sain Associates prepared the Safety Project Evaluation report that documented TDOT’s progress in improving safety.
TDOT began conducting Road Safety Audits and other special safety projects in 2005 with well over 1000 projects performed to date. TDOT was interested in evaluating their road safety program and contracted with Sain Associates to analyze the effectiveness of constructed safety projects in reducing crash frequency.
The study attempted to answer five big questions:
- Has the TDOT program produced a quantifiable safety benefit?
- Were the selected sites actually over-represented in crash frequency?
- Were the applied countermeasures effective in reducing crashes?
- Could the results have been predicted using Highway Safety Manual (HSM) methods?
- Could HSM procedures have influenced countermeasure selection?
In order to conduct the evaluation, we assembled crash data for three years prior to construction of the safety countermeasures and three years after construction. A total of 261 sites had at least three years of crash data after countermeasure construction. A simple before/after comparison revealed that the majority (60%) of treated sites had experienced a decrease in total crashes and 41% had a reduction in severe injury crashes.
From the 261 projects, we selected 45 projects with good quality data to evaluate using the Highway Safety Manual. The HSM is a science-based technical approach to quantitative safety analysis that includes a method for predicting crash frequency and severity. The HSM applies a crash modification factor (CMF) for the various safety treatments or countermeasures, though not all treatments have a CMF.
Below is a case study example in Blount County of a 4-way intersection with side street stop control converted to a roundabout. The roundabout countermeasure has a CMF of 0.56 which suggests that total crashes could be reduced by 44%. Using the HSM method, we determined the expected crashes for a 4-way intersection with side street stop control were 15 crashes. Therefore, this intersection was over-represented with 21 actual crashes. In the three (3) years after the roundabout was constructed, the crashes dropped to 7 crashes, which was in line with the HSM predicted number of 8.4.
So to answer the big questions:
- Has the TDOT program produced a quantifiable safety benefit? Yes it has, but more data is needed to accurately quantify the benefit.
- Were the selected sites actually over-represented in crash frequency? Yes, 42 out of 45 sites that we examined were high crash sites before the safety studies were conducted.
- Were the applied countermeasures effective in reducing crashes? Yes for 26 out of 45 sites. The HSM will provide better tools to aid in countermeasure selection in future safety projects.
- Could the results have been predicted using HSM methods? Yes, for countermeasures that have published CMF values, but more studies are needed to develop more CMFs for use in safety projects.
- Could HSM procedures have influenced countermeasure selection? Yes, by helping reviewers target countermeasures with the best chance of success.
Tennessee has been a lead state in conducting road safety audits, and TDOT’s safety projects program has helped to reduce crashes. TDOT’s willingness to evaluate its program has led to the development of a list of ideas for improving and advancing the program. Congratulations to TDOT on your well-deserved recognition as a National Roadway Safety Award winner.