Vehicles authorized to use amber colored warning lights authorized by N.H. Rev. Stat. § 266:78-h may only illuminate the warning lights when their duties require them to be stopped or parked on or immediately adjacent to a way at a road, water, or sewer line construction or maintenance site in a manner that may impede or cause a hazard to traffic, when actively engaged in the delivery of United States mail, or when actively engaged in snow removal or ice treatment or when entering or leaving a way during plowing operations, or during repair or placement of official traffic control devices, pavement marking, or escorting an oversize load or slow moving vehicle, or in the case of wreckers and emergency road service vehicles when maneuvering through congested traffic to reach a disabled vehicle, providing roadside service, or loading or unloading a vehicle.