Notwithstanding any other provision of this Code to the contrary, no township road district may continue in existence if the roads forming a part of the district do not exceed a total of 4 centerline miles in length as determined by the county engineer or county superintendent of highways. On the first Tuesday in April of 1975, or of any subsequent year next succeeding the reduction of a township road system to a total mileage of 4 centerline miles or less, each such township road district shall, by operation of law, be abolished. The roads comprising that district at that time shall thereafter be administered by the township board of trustees by contracting with the county, a municipality or a private contractor. The township board of trustees shall assume all taxing authority of a township road district abolished under this Section.