In this chapter, "bridge" means a structure, having a clear span of 10 feet or more measured along the center line of the roadway at the elevation of the bridge seats, spanning a watercourse or other opening or obstruction, on a public highway to carry the traffic across, including the substructure, superstructure and approaches to the bridge. For purposes of qualification of bridge aid under this subdivision, but not applicable to RSA 234:4 or RSA 234:13, the term bridge shall include a combination of culverts constructed to provide drainage for a public highway with:
I. An overall combined span of 10 feet or more; and
II. A distance between culverts of 1/2 the diameter or less of the smallest culvert.