(a) Whenever, by an act of the General Assembly or by the order of a county court, the three-mill road tax collected on the property in a street improvement district and annex in a city of the first class is given to the district and annex, respectively, as an aid to the property owners on account of the extra width of the street or for any other reason, the commissioners on the board of improvement may, if they deem it fair and equitable, distribute between the district and the annex the total amount received for both the three-mill tax, in the proportion that the cost of the improvement in each bears to the total cost of the improvement in both.
(b) This section shall apply only to those counties having a population of eighty-five thousand (85,000) or more according to the latest federal census.