50 A SUMMARY OFACCESSIBILITY STANDARDS FOR OUTDOOR DEVELOPEDAREAS accessibility standards for Federal outdoor developed areas Surface, Slope, and Openings [1011 2 2, 1011 2 3, and 1011 2 4] The surface of the clear ground space must be firm and stable. Additional infor- mation on firm and stable surfaces is provided in the trails section of this guide. When the surface of the clear ground space is constructed of concrete, asphalt, or boards, the slope of the clear ground space must be no steeper than 1:48 (2 percent) in any direction. When the surface of the clear ground space is con- structed of materials other than concrete, asphalt, or boards, slopes no steeper than1:20 (5 percent) are allowed when necessary for drainage. Openings in the surface of the clear ground space must be small enough so that a sphere more than one-half inch in diameter cannot pass through. Where possible, drainage grates should be located outside the clear ground space and elongated openings should be placed perpendicular, or as close to perpendicu- lar as possible, to the dominant direction of travel. Operable Parts [1011 3] The operable parts of outdoor constructed features, such as handles, levers, and latches, must comply with the technical requirements of sections 309.3 and 309.4 of the ABA Standards, unless an exception applies. The technical requirements for operable parts and excep- tions to the technical requirements are discussed under the applica- ble outdoor constructed feature.