FAA Regulations part 139 certifies airports for functionality and safety. Part 139 is only one of the many regulations instituted by the federal government to oversee the safety of America's airline industry. The Federal Aviation Administration, the FAA, falls under the Department of Transportation and has created over ten thousand regulations and sub-regulations to protect citizens from dangers inherent with air travel. Some air travel regulations are dependent on the passenger. Passengers can only carry on one piece of luggage which must fit certain measurements. This is considered a safety measure because it limits what passengers can bring aboard an airline and frees up available space in the airplane cabin. Only passengers are permitted past final security check points. This regulation was instituted in order to limit the number of people congregating around gates and the amount of things passed through security checkpoints. Regulations have involved passengers in a more active role since the terrorists' attacks of September 11, 2001.