Sometimes it is necessary to have the water flow from one side of the road to the other or have the road cross a small stream. You do this with cross drains. They are called culverts if they are 10 feet or less in width. Over 10 feet wide, they are called bridges. Culverts are made of many materials, such as corrugated metal, reinforced concrete, concrete pipe, timber, logs, and even open-ended oil drums. The type of material selected is dependent upon various factors including, in part, the type and life expectancy of the road. For permanent roads and highways with concrete or asphalt paving, the most durable of materials, such as reinforced concrete or concrete pipe, should be used. Concrete pipe is one of the strongest and most durable materials used in making culverts. The shell thickness and length depend on the pipe diameter. (The larger the diameter, the thicker the shell and longer the section.) Pipe diameters are nominal inside dimensions. For semipermanent and temporary roads, the design engineer may choose to use materials such as those shown in figure 3-13.