Bicycle facilities on roadways are typically paved in asphalt or concrete. Brick, paving stones, or other materials are occasionally used in select situations. Trails may also be paved in a soft surface such as crushed stone, or a natural surface. The cost model provides the user a selection of the three most common trail surfaces; concrete, asphalt, and crushed stone Depth of pavement, and aggregate base will vary at the project and at the regional level.