Vehicles shall not be parked on public streets, roads, and places, where they obstruct traffic; and shall not be allowed to remain on the above named places longer than is necessary for loading and unloading, unless special permission is obtained from the police authorities. When such special permission is obtained, they shall remain in the places allotted them. When horses and carts are drawn up on public highways, they shall be as far as possible to the side of the road.