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On projects where a bridge will be constructed to carry a county road over a primary highway (excluding interchanges), the bridge should have a minimum width of 30 feet (9 meters) for an ADT of 1500 or less and a minimum width of 40 feet (12 meters) for an ADT greater than 1500, regardless of the county road’s surface. The 30-foot (9-meter) minimum will facilitate the movement of farm machinery. If the possibility exists that the bridge at some point in the future will also need to accommodate bicycle traffic, a 5-foot (1.5-meter) shoulder should be used, meaning a minimum 34-foot (10.4-meter) wide bridge should be used for an ADT less than 1500. The Office of Bridges and Structures has design standards for 30- and 40-foot (9- and 12-meter) two-span bridges. These standards should be used when possible and can be altered for use with other bridge widths.
Bridge widths should be discussed with the County Engineer during the design process. Proposed bridge widths of less than 30 feet (9 meters) will be examined on a case-by-case basis.
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