26 A SUMMARY OFACCESSIBILITY STANDARDS FOR OUTDOOR DEVELOPEDAREAS accessibility standards for Federal outdoor developed areas CONSTRUCTION TIP—How is running slope measured? Running slope is often described as a ratio of vertical distance to horizontal distance, or rise to run (figure 9). For example, a running slope of 1:20 (5 percent) means that for every foot of vertical rise, there are 20 feet of horizontal dis- tance. The technical require- ments specify running slope as both a ratio and percentage. Figure 9—Determining the slope ratio. Cross Slope  Cross slope is the side-to-side slope of a trail tread. Some cross slope is necessary to provide drainage and to keep water from ponding and damaging the trail surface, especially on unpaved or natural surfaces. When the trail surface is constructed of concrete, asphalt, or boards, the cross slope must be no steeper than 1:48 (2 percent). When the trail surface is constructed of materials other than asphalt, concrete, or boards, cross slopes no steeper than 1:20 (5 percent) are allowed when necessary for drainage.