accessibility standards for Federal outdoor developed areas 15 Surface  The surfaces of trails, passing spaces, and resting intervals must be firm and stable. A firm trail surface resists deformation by indenta- tions. A stable trail surface is not permanently affected by expected weather conditions and can sustain normal wear and tear from the expected uses between planned maintenances. Paving with concrete or asphalt may be appropriate for highly developed areas. For less developed areas, crushed stone, fine crusher rejects, packed soil, soil stabilizers, and other natural materials may provide a firm and sta- ble surface. Natural materials also can be combined with synthetic bonding materials to provide greater stability and firmness. These materials may not be suitable for every trail. DESIGN TIP—Building a firm and stable surface A firm and stable surface does not always mean concrete and asphalt. Some natural soils can be compacted so that they are firm and stable. Other soils can be treated with stabilizers without drastically changing their appearance. Designers are encouraged to investigate the options and use surfacing materials that are consistent with the site’s level of development and that require as little maintenance as possible.