Whenever the issuing of special tax bills, or bonds, as herein provided, shall have been delayed for any cause for a period of at least six months subsequent to the estimate made therefor, and it shall be made to appear to the road commissioners of any such road district that the estimate of the cost of proposed improvements in said road district is excessive, or more than is needed for that purpose, they may appear before the county commission at any regular or adjourned term of said county commission, and ask for an order of the county commission authorizing the state transportation department to make and file with said county commission a new or revised estimate of the cost of such proposed improvements, and all tax bills, or bonds issued thereafter in said road district for such improvements shall be based upon the new or revised estimate so made and filed by said state transportation department, less state and federal aid, if any, and shall in all respects be the legal estimate of the cost of such proposed improvements.