It shall be unlawful to park or leave standing upon any street, any vehicle, whether attended or unattended, behind any other vehicle parked at an angle to the curb, or at the side of any parked vehicle where such vehicle is parked parallel with the curb, except that an operator may stop temporarily provided the operator does not leave the vehicle, during the act of actually loading or unloading passengers or when necessary in obedience to traffic regulations, or traffic signs, or signals of a police officer.