A rail trail designed according to shared-use-path standards has 10 to 12 feet of firm flat surface with two-foot-wide shoulders on each side. If a rail trail is developed with partial state and federal funding, the design must conform to shared-use-path standards. In special situations, a waiver for a narrower trail may be allowed. The choice of the width and type of trail surface involves a number of considerations including types of users, number of users, construction cost, maintenance cost, cost sharing, environmental impact, and the standards set by the funding agencies. About one third of existing rail trails in the United States have paved surfaces. Others, especially in rural areas, have stone dust, gravel or other non-paved surfaces.