The Pacific Crest National Scenic Trail as defined by the National Trails Systems Act, 82 Stat. 919, shall be administered primarily as a footpath and horseback riding trail by the Forest Service in consultation with the Secretary of the Interior. The use of motorized vehicles may be authorized by the Federal Agency administering the segment of trail involved when use of such vehicles is necessary to meet emergencies or to enable landowners or land users to have reasonable access to their lands or timber rights.