When any such through traffic routes are established and designated by appropriate signs, it shall be unlawful for any operator of any vehicle mentioned in Section 9.32.020 to drive on any other route in the City except when it is impractical to do otherwise, or except when it is necessary for such vehicles to traverse another street to a destination for the purpose of loading or unloading, but in such case only by such deviation from the nearest truck route as is reasonably necessary. (Prior code § 9-7-3)