2 + 1 Roadways
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A 2+1 road design has a continuous three-lane cross section with alternating passing lanes. A review of European safety and operational experience with 2+1 roads shows that they can be an attractive alternative to two- or four-lane roads in some cases.
 
References
KTC 2+1 Brief
This briefing developed by the Kentucky Transportation Center describes capacity, LOS, lane length, and safety benefits of using the 2+1 roadway design
Ktc 2+1 Brief​
Application of European 2 + 1 Roadway Designs
This digest summarizes the results of NCHRP Project 20-7/Task 139, "Application of European 2+1 Roadway Designs." The great majority of U.S. rural roads have only two lanes. Under NCHRP Project 20-36, researchers conducted a scan of geometric design practices and identified the 2+1 roadway design as an alternative design used by some European countries.
Application of European 2 1 Roadway Design.pdfApplication of European 2 1 Roadway Design.pdf

Missouri Alternate Four-Lane
Trying to pass a slower moving vehicle on a two-lane highway can be both frustrating and dangerous. In order to help make Missouri highways safer and reduce the number of fatalities and disabling injuries that occur each year, MoDOT has embraced the concept of an alternate four-lane design on certain routes. An alternate four-lane consists of an additional lane added to one side of a conventional two-lane highway to provide motorists an opportunity to pass another vehicle without having to find gaps in the on-coming traffic.
Missouri Alternate Four-Lane

Alternating Passing Lane Lengths
This paper reports on a study of passing lane operations, with a focus on continuous three-lane cross sections with alternating passing lanes (three-lane alternate passing or 2+1 segments in Arkansas.
Alternating Passing Lane Lengths.pdfAlternating Passing Lane Lengths.pdf