Safety Effectiveness of High-Speed Expressway Signals
Principal investigator: Reginald Souleyrette
Other authors: Todd Knox
Student researcher: Todd Knox
Start date: 01/01/05
End date: 08/31/05
Report: August 2005, http://www.intrans.iastate.edu/reports/expressway_signals.pdf 428 kb (*pdf)
Related publications: Safety Effectiveness of High-Speed Expressway Signals (Tech transfer summary) November 2005
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Sponsor(s):Iowa Department of Transportation
About the research
Abstract: The safety benefit of signalizing intersections of high-speed divided expressways is considered. Analyses were conducted on 50 and 55 mph and on 55 mph only intersections, comparing unsignalized and signalized intersections. Results of the 55 mph analysis are included in this report. Matched-pair analysis indicates that generally, signalized intersections have higher crash rate but lower costs per crash. On the other hand, before-and-after analysis (intersections signalized between 1994 and 2001) indicates lower crash rates (~30 percent) and total costs (~10 percent) after signalization. Empirical Bayes (EB) adjusted before-and-after analysis reduces estimates of safety benefit (crash rate) to about 20 percent. The study shows how commonly used analyses can differ in their results, and that there is great variability in the safety performance of individual signalized locations. This variability and the effect of EB adjustment are demonstrated through the use of innovative graphics.