Passenger-miles of travel are estimated on a yearly basis by adding together
estimates for each mode, which are derived from separate sources. Passenger-miles
of travel for large air carriers and intercity trains are estimated from tickets
and are very accurate. A variety of methods are used to estimate travel in other
modes, each with different strengths and weaknesses. For more information see
the Accuracy Profiles in BTS’s National Transportation Statistics 2001 (www.bts.gov).
