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 Highway, it must meet these construction requirements or obtain a waiver from the Federal Highway Administration. Standards Standardization helps keep road design consistent, such that drivers can learn the consistent features and drive accordingly. Standardization can therefore decrease accidents and increase driver safety. These standards are, : Controlled access: All access onto and off the highway is to be controlled with interchanges and grade separations, including all railroad crossings. Interchanges are to provide access to and from both directions of the highway and both directions of the crossroad. Interchanges should be spaced at least apart in urban areas and apart in rural areas; collector/distributor roads or other roadway configurations that reduce weaving can be used in urban areas to shorten this distance. In urban areas, there should be no driveways or other access points to adjacent properties along the crossroad for at least from entrance and exit ramps, in both directions, and for at least in rural areas. In urban and suburban areas, consideration should be given to accommodating bicycles and pedestrians along crossroads. Minimum design speed: A minimum design speed of is to be used, except in mountainous and urban areas, where the minimum is . The sight distance, curvature and superelevation of the highway should follow the current edition of AASHTO's A Policy on Geometric Design of Highways and Streets for the chosen design speed. Maximum grade: The maximum permissible vertical angle, or grade, along the highway is determined from terrain and design speed, with up to 6% generally allowed in mountainous areas, 5% in rolling terrain, and 4% on level terrain. An additional 1% is allowed in urban areas. Minimum number of lanes: There are to be at least two lanes in each direction, unless more are necessary for an acceptable level of service, according to the current edition of AASHTO's A Policy on Geometric Design of Highways and Streets. Climbing lanes and emergency escape ramps should be provided where appropriate. Minimum lane width: The minimum lane width is , identical to most US and state highways. Shoulder width: The minimum width of the left paved shoulder is , and of the right paved shoulder . With