The most common equipment in buildings to facilitate emergency evacuations are fire alarms, exit signs, and emergency lights. Some structures need special emergency exits or fire escapes to ensure the availability of alternative escape paths. Commercial passenger vehicles such as buses, boats, and aircraft also often have evacuation lighting and signage, and in some cases windows or extra doors that function as emergency exits. Commercial emergency aircraft evacuation is also facilitated by evacuation slides and pre-flight safety briefings. Military aircraft are often equipped with ejection seats or parachutes. Water vessels and commercial aircraft that fly over water are equipped with personal flotation devices and life rafts.