Evaluating the Impacts of Speed Limit Increases on Rural Two-Lane Highways Using Quantile Regression

Abstract

Understanding speed selection behavior of drivers following speed limit increases is critically important. To date, the literature has largely focused on freeways and the effects of speed limit changes on two-lane highways remains under researched. Prior research has generally focused on changes to mean speeds, although the speeds of both the highest and lowest drivers are also of great interest. This study investigates trends in free-flow travel speeds following 2017 legislation that increased the posted speed limit from 55 to 65 mph on 943 mi of rural highways in Michigan. Speed data were collected for over 46,000 drivers at 67 increase segments where speed limit increased and 28 control segments where speed limits remained unchanged, before and during each of the two successive years following the speed limit increases. Site-specific traffic, geometric, and cross-sectional information was also collected. Impacts of the speed limit increases on the 15th, 50th, and 85th percentile speeds were evaluated using quantile regression. Separate analyses were conducted for passenger cars and heavy vehicles. Locations where the speed limits were raised experienced increases in travel speeds ranging from 2.8 to 4.8 mph. The control sites experienced marginal changes in speeds, which suggests that any spillover effects of the higher speed limits have been limited. Significant differences were observed across the quantiles with respect to the effects of the speed limit increases, as well as numerous site-specific variables of interest. The results provide important insights about the nature of driver speed selection and the impacts of speed limit increases.

Driver Response to a Dynamic Speed Feedback Sign at Speed Transition Zones Along High-Speed Rural Highways

“…or 18-in. display) to be effective in speed reduction in other contexts such as interchange ramps and rural highway curves ( 5 , 18 ). To that end, future evaluations should also assess the amount of time that the portable DSFS, such as those utilized in this study, should be left on site before moving elsewhere.…”

“…This includes Michigan, where, in 2017, speed limits were increased from 55 to 65 mph on greater than 900 mi of rural state highways across the northern portions of the state. Recent research found that travel speeds increased by 2.8 to 4.8 mph in a sample of rural highway segments where the speed limit was raised to 65 mph ( 18 ). However, this prior research did not consider impacts to travel speeds within speed transition zones along those routes.…”

Driver Response to a Dynamic Speed Feedback Sign at a Freeway Exit Ramp Considering the Sign Design and Installation Characteristics

“…A recent safety evaluation of freeway exit ramps also found crashes to increase as the speed difference between mainline and ramp speed increased ( 9 ). This speed difference is expected to increase with the trend of increasing speed limits, which often results in an increase in average speed, and an even higher increase in speed by the faster drivers ( 10 , 11 ).…”

Evaluating Driver Response to a Dynamic Speed Feedback Sign at Rural Highway Curves

“…This includes Michigan where, in 2017, speed limits were increased from 55 to 65 mph on more than 900 mi of rural state highways across the northern portions of the state ( 22 , 23 ). Recent research found that travel speeds increased by 2.8 to 4.8 mph along a sample of rural highway segments where the speed limit was raised from 55 to 65 mph ( 24 ). However, prior research has not explicitly considered the impact on travel speeds on curved portions of highway segments, where safety performance issues may be particularly exacerbated by increased travel speeds.…”