It shall be unlawful for any person to operate a motor vehicle with gross vehicular weight rating in excess of 18,000 pounds, or 18,000 pounds gross cargo weight rating in the case of a truck trailer or tractor trailer unit, to use, travel upon, or drive over any street or highway within the city not designated as a truck route, except that if the point of origin or destination, for commercial purposes only, for any such motor vehicle shall be off such truck route, then such vehicle may proceed by the shortest possible route to and from such destination to the nearest truck route.