Generally accurate, especially on higher functional systems. Some inaccuracy arises from different definitions of public use and some outdated inventories. These numbers are based on 100 percent reporting of passengers and trip length by some 90 air carriers that operate aircraft with a passenger seating capacity of more than 60, or have a payload capacity of more than 18,000.
Minor errors arise from non-reporting, but amount to less than one percent of all passenger miles.
These, if added, might raise the total by roughly five percent for passenger miles. For vehicle miles, adding scheduled commuter service would raise the total by 13 percent in 1995. In 1993 about 12 percent of all registered aircraft were surveyed, with a response rate of 66 percent. For Interstates, other NHS, and principal arterials, traffic statistics are measured for every section of roadway. For other functional systems through rural major collector and urban collector systems, vehicle miles are estimated based on sample sections.