(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.