Mont. Code Ann. § 61-9-201

Every vehicle upon a highway within this state at any time from one-half hour after sunset to one-half hour before sunrise and at any other time when due to insufficient light or unfavorable atmospheric conditions persons and vehicles on the highway are not clearly discernible at a distance of 500 feet ahead shall display lighted lamps and illuminating devices as hereinafter respectively required for different classes of vehicles, subject to exceptions with respect to parked vehicles.