The frequency and severity of crashes on rural two-lane roadways have increased in the US relative to other road types. This trend can be explained by the growing number of vehicles, higher speeds, narrow shoulders, and vehicle mixes. One solution for improving traffic flow and safety outcomes on rural two-lane roadways is to adopt a 2+1 design, which confers the benefits of four-lane highways but at a lower cost. Transportation agencies throughout Europe — and increasingly the US — have seen good results from 2+1 layouts. Crash data from Sweden, Germany, Finland, and Denmark reveal better safety outcomes following the implementation of 2+1 designs, with reductions in fatal and fatal and injury crash rates of 25 – 80 percent. Studies in the United States have found crash declines of 35 – 44 percent following the transition to 2+1 layouts. Over the past 10 years, the Kentucky Transportation Cabinet (KYTC) has built several 2+1 roadways. Evaluations of three 2+1 segments in the state found lower crash rates on two segments, however, not enough crash data are available to draw definitive conclusions. Despite this lack of confirmatory data, there is consensus among practitioners that 2+1 designs hold considerable promise for improving rural roadway operations. Building off of 2+1 guidance originally issued by KYTC in 2013, this report outlines updated policies that account for lessons learned at the agency during the design and construction of 2+1 roadways as well as best practices adopted by other states.
Digital Object Identifier
Jasper, Jeff; Wolfe, Jeff; Baskette, Robin; Ross, Paul; and Van Dyke, Chris, "2+1 Roadway Design Guidance Update" (2023). Kentucky Transportation Center Research Report. 1772.