Except cases in which the public utilities commission finds that such construction is impracticable, bridges, viaducts, overhead roadways, footbridges, wire, or other structure built over the track of a railroad after April 16, 1900, by a county, municipal corporation, township, railroad company, or other corporation or person, shall be not less than twenty-one feet in the clear from the top of the rails of such track to such wire or other structure, or to the bottom of the lowest sill, girder, or crossbeam, and the lowest downward projection on the bridge, viaduct, overhead roadway, or footbridge.