Section 7B. The driver of a vehicle of a fire, police or recognized protective department and the driver of an ambulance shall be subject to the provisions of any statute, rule, regulation, ordinance or by-law relating to the operation or parking of vehicles, except that a driver of fire apparatus while going to a fire or responding to an alarm, or the driver of a vehicle of a police or recognized protective department or the driver of an ambulance, in an emergency and while in performance of a public duty or while transporting a sick or injured person to a hospital or other destination where professional medical services are available, may drive such vehicle at a speed in excess of the applicable speed limit if he exercises caution and due regard under the circumstances for the safety of persons and property, and may drive such vehicle through an intersection of ways contrary to any traffic signs or signals regulating traffic at such intersection if he first brings such vehicle to a full stop and then proceeds with caution and due regard for the safety of persons and property, unless otherwise directed by a police officer regulating traffic at such intersection. The driver of any such approaching emergency vehicle shall comply with the provisions of section fourteen of chapter ninety when approaching a school bus which has stopped to allow passengers to alight or board from the same, and whose red lamps are flashing. Source