In transportation engineering, the operating speed of a highway is the speed at which motor vehicles generally travel on that highway.

The precise definition of "operating speed", however, is a matter of debate. Some sources, such as the American Association of State Highway and Transportation Officials (AASHTO), have recently modified their definitions to conform to common use of the term. In 1994, the AASHTO Green Book[1]​ defined operating speed as "the maximum speed at which a driver can travel on a given highway under favorable weather conditions and with prevailing traffic conditions, without at any time exceeding the safe speed established by the design speed for each section", a definition still used by most states in the United States[2]​ and countries such as Mexico.[3]​ However, in July In 2001, AASHTO revised its definition for the new edition of the Green Book [4]​ and defined it as "the speed at which drivers are observed operating their vehicles under free-flowing conditions."