accessibility standards for Federal outdoor developed areas 23 Tread Obstacles  A tread obstacle is anything that interrupts the evenness of the tread surface. The vertical alignment of joints in concrete, asphalt, or board surfaces, as well as natural features, such as tree roots and rocks, within the trail tread can be tread obstacles. The limit on the height of tread obstacles on trails, passing spaces, and rest- ing intervals is based on the surface material used. When the trail surface is constructed of concrete, asphalt, or boards, tread obstacles cannot exceed one-half inch in height at their highest point. When the trail surface is con- structed of materials other than concrete, asphalt, or boards, tread obstacles are permitted to be a maximum of 2 inches high. Frequent tread obstacles and tread obstacles that cross the full width of a trail tread can make travel very difficult for people using mobility devices. Where possible, separate tread obstacles by at least 48 inches, particularly when the obstacles cross the entire tread width. This separation allows people using mobility devices to fully cross one obstacle before confronting another.