(a) State aid roads in the several counties shall receive state aid in the manner and under the terms and conditions set out in this subchapter.
(b) The state aid system shall be allocated to the several counties of the state on a mileage basis in the following proportions:
(1) Fifty percent (50%) to be divided equally among the seventy-five (75) counties;
(2) Twenty-five percent (25%) to be divided in the proportion that the area of each county bears to the area of the state; and
(3) Twenty-five percent (25%) to be divided in the proportion that the rural population of each county bears to the rural population of the state as shown by the most recent federal decennial census.