Raising Speed Limits on Rural Highways: Process for Identification of Candidate Nonfreeway Segments

Speed limits on rural highways have recently been on an upward trend nationwide as 25 states have increased speed limits on such roadways between 2011 and 2016. In January 2017, Michigan House Bill 4423 (Public Act 445) was signed into law, which required speed limits to be raised from 55 to 65 mph on at least 900 mi of Michigan’s state-maintained nonfreeways. The specific highway segments were to be selected by the Michigan Department of Transportation (DOT) within 1 year of enactment on the basis of an assessment of travel speeds and safety. Research was undertaken to develop a process to select candidate highway segments for speed limit increases in Michigan. The primary objective was to identify rural highway segments with comparatively low safety risks for which costly geometric upgrades, specifically horizontal or vertical realignment, would be avoided. Ultimately, 12 factors and associated selection criteria were used to identify the candidate segments. These factors included segment length, total crash rate, injury crash rate, severe (fatal and A-injuries) crash rate, horizontal curvature, speed reduction zones, no-passing zones, schools (kindergarten through eighth grade), driveway density, lane width, paved shoulder width, and signalized intersections. Application of the candidate selection criteria to the Michigan DOT nonfreeway network yielded 772.8 centerline miles along 33 candidate segments, which represented approximately one-eighth of the network mileage currently posted at 55 mph. To avoid substantial geometric upgrade costs, speed limit increases were specifically not recommended for segments that would require horizontal or vertical realignment to achieve design speed compliance with the increased speed limit.