Evaluating the Impacts of Freeway Speed Limit Increases on Various Speed Measures: Comparisons Between Spot-Speed, Permanent Traffic Recorder, and Probe Vehicle Data

Abstract

The state of Michigan increased the maximum speed limit for passenger cars from 70 to 75 mph on 614 mi of rural freeways between May and June of 2017. The maximum speed limit for trucks was also increased from 60 to 65 mph on all freeways where the passenger car limit was 65 mph or above. This study examined the effects of these changes on traffic speed characteristics. Speed data were collected before the speed limit increases were introduced, as well as at several intervals afterward using three different data sources. Free-flow speed data were collected from spot-speed studies using light detection and ranging (LIDAR) devices whereas aggregate-level traffic speed data were obtained from both permanent traffic recorder (PTR) stations, as well as from probe vehicle data. These data were integrated with associated roadway and traffic characteristics and the impacts of the speed limit changes were evaluated using seemingly unrelated regression equations, which provide insights into how these increases affected the 15th, 50th, and 85th percentile speeds, as well as the standard deviation in speeds. Regardless of data source, the results consistently showed increases in each speed metric. However, the magnitude of these increases varied from 1.1 to 3.2 mph depending on the specific metric and data source. Marginal changes in speeds were observed at sites where speed limits were not changed, suggesting limited spillover effect. Several site-specific characteristics were also correlated with speed selection, with these effects being less pronounced when considering free-flow and probe vehicle data as compared to PTR.