This chapter shall in no respect be considered as a repeal of the provisions of the motor vehicle laws but shall be construed as supplemental thereto. Its provisions shall not apply to a vehicle owned by, or under lease to, the federal government nor to a member of the national guard when engaged in military duty pursuant to orders from proper authority under existing state and federal laws, nor to a permanent, call, or volunteer fireman of a municipal fire department, nor to a city or town police officer, nor to a municipal public works or highway department employee while on official duty, nor to a state police employee or highway enforcement officer while on official duty, driving a vehicle owned by the state or said fire or police department or said municipality, and shall not apply to said vehicle while so driven.