The petition shall designate the road or roads to be improved by name and by terminal points and other specially fixed locations, the lands within the proposed benefit district, the type or types of permanent improvement, and width or widths of roadway desired, which types or widths may be different for different sections of the road, and the number of annual assessments to be levied upon the lands in the benefit district in payment thereof, which shall be not less than ten nor more than twenty. The petition may specify more than one type or width of road, and may improve any types that may be acceptable for federal aid, and a maximum and minimum number of annual assessments, leaving the final choice to the board of county commissioners, subject to the approval of the secretary of transportation: Provided, That where the petition specifies a certain type of road, if such petitions be allowed, the type of road shall be of the type in said petition set forth: Provided, however, That said petition may request gravel alone, in which event only gravel shall be used.