On or before January 1, of each year the state road commissioner shall ascertain and determine the total amount of available funds for expenditure in the whole state for the construction, reconstruction and maintenance of feeder and state local service roads during such annual period. Of the amount so ascertained the commissioner may set aside as a reserve fund not to exceed twenty percent thereof, to be used and expended by him in his discretion in such manner as will best serve the interests of the state and the convenience of its inhabitants.

The remaining eighty percent, or, if such reserve fund is not set aside, then all the funds shall be appropriated in the following order and preference:

(1) Maintenance funds. -- To be allocated to the various counties on the basis of the mileage of various types or road surfaces:

(a) Paved surfaces of all types, such as, brick, cement, concrete, bituminous, etc.

(b) Traffic-bound surfaces, such as, slag, crushed stone, gravel, chert, red dog, shale, etc.

(c) Unimproved earth roads; by applying certain weights or percentages, based on past experience, to the various types of road surfaces.

(2) Construction and reconstruction. -- To be allocated to the various counties on the basis of the ratio of the unimproved mileage of feeder and state local service roads in the county to the total unimproved mileage of feeder and state local service roads in the state.

For the purposes of this section, the words "unimproved mileage" are defined and shall be construed to mean roads which are not passable for all-weather travel by motor vehicle.

All moneys received from the federal government for road construction or reconstruction shall be expended as provided, or as may hereafter be provided by act of Congress.