Mo. Rev. Stat. § 227.060
The commission shall determine the width of the right-of-way and of the surface roadway of state highways and shall also determine the type and character of construction. In making decision as to widths, types and character of construction, there shall be taken into consideration the probable volume and weight of intercounty and intracounty vehicular traffic over such state highways, the density of population and the character of the territory through which such state highways are to be constructed. Durability and low maintenance cost shall be considered in connection with the probable volume and weight of such traffic; provided, that in the construction of state roads, no width of less than twelve feet of hard surface shall be considered.
§ 227.060, RSMo