70 mph Study

In July and August 2014, the Pennsylvania Department of Transportation (PennDOT) and Pennsylvania Turnpike Commission (PTC) raised the posted speed limit on rural sections of Interstates 80, 380, and 76 from 65 to 70 mph. The purpose of this study was to assess the speed and safety performance of these “pilot” sections. This was done by comparing the operating speeds and crash frequencies before and after the posted speed limit increase. Additionally, operating speed data in several work zones were collected to assess how drivers comply with posted speed limits in work zones on the pilot sections. An inferred design speed method and pavement friction degradation method are proposed as methodologies to assess site conditions on rural Interstate roadways with 65 mph posted speed limits. Collectively, the operating speed, safety, inferred design speed, and friction information can be used by PennDOT and the PTC to identify candidate locations for 70 mph posted speed limits. The findings suggest that mean and 85th-percentile operating speeds increased after increasing the posted speed limit from 65 to 70 mph; however, the increases were less than 5 mph. A framework was developed to estimate the safety effects of the posted speed limit increase, because only 12 to 16 months of after period crash data were available for the analyses included in this study.