Reversible lanes are defined as "one or more lanes designated for movement one-way during part of the day and in the opposite direction during another part of the day." The principle is to make use of "unused capacity" in one direction to carry traffic in the other direction.  

The National Cooperative Highway Research Program report NCHRP Synthesis 340 documents the current state of the practice and the history of reversible lane use. Reversible lanes have been in use for more than 70 years. More recently, reversible lanes are being used on freeways in conjunction with HOT and HOV lanes (see Freeway Lane Management).  Planning, designing, controlling, operating, and enforcement practices vary widely.