Quantifying the environmental effects of speed limit policy - A case study from Michigan
Author(s): Chakraborty, Meghna; Mahmud, Md Shakir; Gates, Timothy J.
Publication Date: Aug-2023
Journal: ITE Journal
Abstract: Transportation affects human health both directly in terms of mortality and injury from traffic crashes, and indirectly through air and noise pollution, and other associated health hazards. Globally, transportation has been a major contributing factor to air pollution with more than 36 percent contribution in 2018. Tailpipe emissions, one of the main mobile sources of air pollution, include carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HC), volatile organic compounds (VOCs) and fine (PM2.5) and coarse (PM10) particulate matters. While several factors contribute to tailpipe emissions, studies have historically indicated increased speed limits and the resulting increase in operating speeds as the key contributors. The repeal of the National Maximum Speed Law (NMSL) in 1995 granted states complete autonomy to set speed limits on all classes of roadways. In 2017, the Michigan Department of Transportation (MDOT) raised the speed limit on approximately 609 miles (980 kilometers [km]) of limited-access freeways from 70 miles per hour (mph) to 75 mph (112.6 kilometers per hour [km/hr] to 120.7 km/hr) for passenger vehicles and from 60 mph to 65 mph (95.6 km/hr to 104.6 km/hr) for heavy vehicles statewide. Additionally, speed limits were also increased on about 935 miles (1,504.7 km) of state-maintained non-freeways (rural arterials with no access control) from 55 mph to 65 mph (88.5 km/hr to 104.6 km/hr) for all vehicle types. However, the impacts of speed limit increase on resulting emissions and air pollution have received little attention. The relationship between fuel consumption, tailpipe emissions, and average operating speed, especially at constant speeds, adheres to an U-shaped curve (Figure 1). This reflects higher emissions at higher speeds after passing a minimum speed threshold value. Numerous studies have also argued that vehicles travel most efficiently when the vehicle operating speed is in the range of 30 mph to 50 mph (48.3 km/hr to 80.5 km/hr). As speeds reach beyond this point, fuel consumption and emission rates increase. A comprehensive study by van Benthem following a speed limit increase from 55 mph to 65 mph in the western U.S. found a 3 mph to 4 mph (4.8 km/hr to 6.4 km/hr) increase in operating speed, a 14-24 percent increase in the concentrations of CO, 8-15 percent increase in NOX emission, and two to seven times larger social costs than the social benefits considering the increase in crashes and emissions.