(1) It shall be deemed a single purpose to: (a) acquire a right-of-way for and construct a public highway, including any bridge or bridges thereon; (b) contribute to the cost of a federal-aid bridge; (c) contribute to the cost of a federal-aid highway project on a highway leading to a federal-aid bridge. (2) Construction of two or more bridges not forming a part of the same public highway shall be deemed separate purposes.