Prevention Security measures Terrorism -- 8 Hijacking of aircraft -- 4 Aeronautics, Commercial -- 3 Airports -- 3 Merchant marine -- 3 more ... National security -- 3 Hijacking of aircraft 2 Safety measures 2 Security measures -- 2 Terrorism 2 Transportation -- 2 Astronautics and state -- 1 Boats and boating -- 1 Civil defense 1 Civil defense -- 1 Civilian use -- 1 Defenses 1 Exploration -- 1 Freight and freightage -- 1 Harbors -- 1 Hijacking of ships -- 1 History 1 History -- 1 Homeland security 1 Infrastructure -- 1 Military policy 1 National security 1 Passenger traffic 1 less ...