Summary: GAO discussed on the National Transportation Safety Board's (NTSB) recommendation that the Federal Aviation Administration (FAA) limit air traffic at some of the busiest airports until it can improve its traffic flow control program. GAO noted that: (1) it recently made essentially the same recommendation to FAA; (2) its survey of the FAA traffic control work force showed that controllers thought that they were understaffed and might not be able to maintain an appropriate margin of safety; (3) FAA believes that its existing flow control measures are adequate; and (4) its review of FAA safeguards in one area revealed numerous flow control deficiencies, including severe understaffing and a reliance on controllers' judgments about how much traffic they could handle. GAO believes that: (1) the NTSB recommendation was valid; and (2) FAA should solicit its controllers' views on system improvements, establish criteria for maximum safe traffic density, and develop means for predicting dangerous traffic levels.