The grade of approaches to an overhead bridge shall be such as to keep the cost as low as practicable consistent with the standard design practice of the State Highway Commission and in no event shall the approaches to creosote timber overhead bridges be considered to extend more than five hundred feet from the outside track. The overhead bridge shall be so constructed as to give a vertical clearance of not less than twenty-two feet, six inches above the base of rail of railroad track and a horizontal clearance of not less than eight feet, measured from the center line of such track, and at right angles thereto, due allowance being made for curvature of track.