230.250.  County highway commission may designate certain roads as part of supplemental state highway system — state highways and transportation commission to approve. — Any county adopting sections 230.200 to 230.260 may, through their county highway commission, designate not less than twenty-five miles nor more than fifty miles of roads within the county which are connecting roads between present supplementary farm to market state highways.  Upon designation and approval by the state highways and transportation commission, these roads shall become a part of the permanent supplementary state highway system.