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Development of SPFs for Iowa’s High Speed Paved Secondary Roads

Date and Time: Monday, July 24: 1:30 PM - 3:00 PM
Location: Grand Ballroom A

 

Lead Presenter: Nicole Oneyear

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Speaker Biography

Dr. Nicole Oneyear was a research scientist at Iowa State University for over 7 years. Her research focused on a variety of safety topics including rural road and rural l intersection safety, lane departures, low cost safety improvements, automated enforcement and maintenance and safety. She worked for 5 years with the National Center for Rural Road Safety helping develop and provide training on a variety of rural road safety topics.

Co-Authors 

Presentation Description/Paper Summary

Iowa’s secondary(county) roads make up approximately 78% of the total miles of roads within the state. High speed paved secondary roads account for almost 18,000 miles total which over a five-year period saw approximately 23,000 crashes. These crashes tend to be widespread with some sections of road not seeing any crashes and therefore network screening provides a useful process to help identify candidates for safety improvements. In order to support this effort, a set of six safety performance functions (SPFs) were developed for the entire population of paved secondary roads within the state of Iowa. Models developed included (1) tangents with speed limits of 40/50 mph, (2) tangents with speed limits of 55 mph as well as segments with (3) less than 25% curves and (4) more than 25% curves. Segments with curvature were further broken into models for low volume (less than 400 vpd) and higher volume (greater than 400 vpd) segments. This was done instead of calibrating HSM SPFs as it would likely result in better predictive reliability. Empirical Bayes (EB) method, which accounts for regression to the mean, can then be conducted with SPFs. EB combines the predicted crashes for the segment from the SPF with the observed crashes. These EB estimated crashes can then be compared to the number of crashes that were predicted for the segment from the SPFs and use these two values together to calculate a Potential for Crash Reduction (PCR) value which can be used in prioritizing sites.

Presentation File

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