24 A SUMMARY OFACCESSIBILITY STANDARDS FOR OUTDOOR DEVELOPEDAREAS accessibility standards for Federal outdoor developed areas Openings  Openings are gaps in the surface of a trail. Gaps, including slots in a drainage grate and spaces between the planks on a bridge or boardwalk (figure 7), that are big enough for wheels, canes, or crutch tips to drop through or become trapped in are potential hazards. Openings in the surfaces of trails, passing spaces, and resting intervals must be small enough so that a sphere more than one-half inch in diameter cannot pass through. Where possible, elongated openings should be placed perpendicular, or as close to perpendicular as possible, to the dominant direction of travel. Figure 7—Where possible, openings in boardwalk decking should be placed perpendicular to the direction of travel. Running Slope  Running slope, also referred to as grade, is the length- wise slope of a trail, parallel to the direction of travel. Trails or trail segments of any length may be con- structed with running slopes up to 1:20 (5 percent). To accommodate steep terrain, trails may be designed with shorter segments that have a running slope and length, as shown in table 2, with resting intervals at the top and bottom of each segment.