Bridge,

as used in this chapter, is defined as the following: "A structure including supports erected over a depression or an obstruction, such as water, highway or railway, and having a track or passageway for carrying traffic or other moving loads and having an opening measured along the center of the roadway, track or passageway of 20 feet or more between undercopings of abutments, backwalls, spring lines of arches or extreme ends of openings for multiple boxes or having an inside diameter of 20 feet or greater, in the case of pipes."