N.H. Rev. Stat. Ann. § 266:78-g

Police, fire, ambulance, state department of corrections, and rescue vehicles, and other vehicles authorized under RSA 266:78-c shall only illuminate emergency lights and warning lights when responding to but not returning from an actual or reported emergency, transporting a patient to a hospital for emergency treatment, or when parked at the scene of an emergency for purposes of warning approaching traffic. Fire apparatus and ambulances may also illuminate emergency lights and warning lights while parking or backing the vehicle to access a garage housing the apparatus or when serving as a mobile command post to designate the location of the command post. No persons other than emergency response personnel shall drive a vehicle with red emergency lights in operation.