The Marin County Board of Supervisors last week voted unanimously to restrict the length of buses on the steep and winding Muir Woods Road to 30 feet, tightening a 36-foot restriction implemented three years ago. The new rule, which applies to the stretch between the entrance to Muir Woods National Monument north to Panoramic Highway, will go into effect later this year after improvements are made to Franks Valley Road, where longer buses will be…