A culvert is a tunnel structure constructed under roadways or railways to provide cross drainage or utilities passage. Culverts are commonly made of concrete, steel, plastic, aluminum, or high-density polyethylene. Concrete culverts are often preferred and can be reinforced, non-reinforced, or cast in place. Common culvert types include pipe, pipe arch, box, arch, and bridge culverts. Culvert location and size depends on factors like water flow, cost, and drainage needs.