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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== These standards are, , as follows: *Controlled access: All access onto and off the roadway is to be controlled with interchanges and grade separations (including railroad crossings). See List of gaps in Interstate Highways for the few cases that violate this rule. Interchanges should provide full access; ramps are to be designed with the appropriate standards in mind. Minimum interchange spacing should be in urban areas and in rural areas; collector/distributor roads or other configurations that reduce weaving can be used in urban areas to shorten this distance. * *Access control (from adjacent properties) should extend at least in urban areas and in rural areas in each direction along the crossroad from the ramps. *Minimum design speed: In rural areas, a minimum design speed of should be used, with acceptable in rolling terrain, and as low as allowed in mountainous and urban areas.〔 Speed limits as low as are, however, occasionally encountered and generally assigned to pre-existing freeways that were grandfathered into the system. * *Sight distance, curvature and superelevation according to the current edition of AASHTO's ''A Policy on Geometric Design of Highways and Streets'' for the design speed.〔 *Maximum grade: Maximum grade is determined by a design table based on terrain and design speeds, with up to 6% generally allowed in mountainous areas and hilly urban areas with exceptions for up to 7% grades on mountainous roads with speed limits below . *Minimum number of lanes: At least two lanes in each direction, and more if necessary for an acceptable level of service in the design year, 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. Brief one-lane ramps that either "deviate" off of multi-lane freeways to connect and merge with another freeway (or one-lane bottlenecks for less than a mile before another merging freeway) are exempt from this requirement, these exceptions are mostly in rural areas. *Minimum lane width: Minimum lane width of . Also for most US and state highways. *Shoulder width: Minimum outside paved shoulder width of and inside shoulder width of . With three or more lanes in each direction, the inside paved shoulder should be at least wide. If truck traffic is over 250 Directional Design Hour Volume, shoulders at least wide should be considered. In mountainous terrain, outside and inside shoulders are acceptable, except when there are at least four lanes in each direction, in which case the inside shoulders should also be wide. *Pavement sloping: Pavement cross slope of at least 1.5% and preferably 2% to ensure proper drainage on flat sections. This can be increased to 2.5% in areas of heavy rainfall. Shoulder cross slope should be between 2% and 6% but not less than the main lanes. * *Land slopes within the clear zone should be at most 4:1 and preferably 6:1 or flatter. Roadside barriers should be used for slopes of 3:1 or steeper, in accordance with the current edition of AASHTO's ''Roadside Design Guide''. *Median width: Minimum median width of in rural areas, and in urban or mountainous areas. To prevent median-crossing accidents, guard rail or Jersey barrier should be installed in medians in accordance with the current edition of AASHTO's ''Roadside Design Guide'', based on traffic, median width and crash history. When possible, median openings between parallel bridges less than in width should be decked over; otherwise barriers or guard rails should be installed to exclude vehicles from the gap. Recently, cable median barriers are being installed on busier Interstate Highways in rural areas regardless of median width. *Recovery areas: No fixed objects should be in the clear recovery area, determined by the design speed in accordance with the current edition of AASHTO's ''Roadside Design Guide''. When this is not possible, breakaway supports or barriers guarding the objects shall be used. *Curb slope: Vertical curbs are prohibited. Sloping curbs are to be at the edge of the paved shoulder, with a maximum height of . The combination of curbs and guard rail is discouraged; in this case the guard rail should be closer to the road than the curb. *Vertical clearance: Minimum vertical clearance under overhead structures (including over the paved shoulders) of in rural areas and in urban areas, with allowance for extra layers of pavement. Through urban areas at least one routing should have clearances. Sign supports and pedestrian overpasses must be at least above the road, except on urban routes with lesser clearance, where they should be at least higher than other objects. *Horizontal clearance: under or along a bridge shall be the full paved width of the rest of the road. Bridges longer than can be narrower, with a minimum of on both sides of the travel lanes. *Bridge strength: New bridges are to have at least MS 18 (HS-20) structural capacity. Weaker bridges that can continue to serve the route for 20 more years are allowed to remain. * *Additionally, existing bridges can remain if they have at least lanes with outside and inside shoulders. Long bridges are to have at least on each side of the travel lanes; bridge railing should be upgraded to current standards if necessary. *Tunnel clearance: Tunnels should in theory be equivalent to long overcrossings, but because of cost the standards can be reduced. Vertical clearance is the same as under bridges, including the provision for alternate routing. Width should be at least , which consists of two lanes, outside and inside shoulders, and safety walkways on each side. If necessary to meet the dimensions of the approach, this can be shifted left or right. A reduced width is acceptable due to high cost. In this case, the minimum width is , with at least more than the approach for the sum of the shoulder widths, but at least total, and at least on each side for a safety walkway. If there is no safety walkway, a offset with a "safety shape" in the wall is acceptable. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Interstate Highway standards」の詳細全文を読む スポンサード リンク
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