(1) Special fuel will not be taxed when consumed in the operation of motor vehicles upon roads, or the rights-of-way of which are owned by the United States department of agriculture, under the following circumstances:

(a) When said roads are maintained by any person on their own volition without compensation from any state, county, municipality, or other governmental agency of Montana; and

(b) When such roads are being built or maintained by any person or contractor under an agreement for such construction or maintenance involving one of the parties of the United States department of agriculture.