The funds which are allotted by the commission to the construction or acquisition of supplementary state highways and bridges in each of the counties of the state shall be apportioned to the several counties as follows: One-fourth in the ratio that the area of each county bears to the area of the state, one-fourth in the ratio of the population, and two-fourths on such basis as the commission may deem to be for the best interest of highway users; provided the areas and population of cities having a population of 150,000 or more shall not be considered in making such apportionment, and the latest available United States decennial census shall be used; provided further, that if traffic on any supplementary state highway becomes such that a higher type than ordinary supplementary highway construction shall be required, then the commission may construct such higher type and charge such extra cost to unallotted state highway funds. Supplementary state highways shall be selected by mutual agreement of the commission and the local officials having charge of or jurisdiction over roads in the territory through which such supplementary state highways are to be constructed.