FHWA Policy Statement. Access to the Interstate System, Federal Register/Vol. 74, No. 165/ Thursday, August 27, 2009/Notices.
Interstate System Access Informational Guide, U.S. Department of Transportation, Federal Highway Administration Office of Infrastructure (2010)
A Policy on Design StandardsInterstate System, American Association of State Highway and Transportation Officials, Washington, D.C., July 1991).
Wikipedia Article: 7/9/13
Standards for Interstate Highways in the United States are defined by the American Association of State Highway and Transportation Officials (AASHTO) in the publication A Policy on Design Standards - Interstate System. For a certain highway to be considered an Interstate, it must meet these construction requirements or obtain a waiver from the Federal Highway Administration.
These guidelines do not establish ramp and interchange spacing standards. Rather they provide a process and criteria for assessing spacing in a given context to assist planners and designers in considering the feasibility of new or rebuilt interchanges and ramps. Interchange spacing is defined as the distance between the centerlines of successive crossroads with interchanges on a freeway.