The prima facie speed limitations set forth in this Chapter shall not apply to authorized emergency vehicles when responding to emergency calls and the drivers thereof sound audible signal by bell, siren. or exhaust whistle. This provision shall not relieve the driver of an authorized emergency vehicle from the duty to drive with due regard for the safety of all persons using the street, nor shall it protect the driver of any such vehicle from the consequences of a reckless disregard of the safety of others.