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: