The driver of a vehicle overtaking upon the right any street car stopped or about to stop for the purpose of receiving or discharging any passenger shall stop such vehicle at least ten feet to the rear of the nearest running board or door of such street car and thereupon remain standing until all passengers have boarded such car or upon alighting have reached a place of safety, except that where a safety zone has been established a vehicle need not be brought to a stop before passing any such street car but may proceed past such car at a speed not greater than is reasonable and proper and with due caution for the safety of pedestrians.