All ridership data relate to trips taken--not to people--because that is
how data are collected and reported. The heavy use of passes, transfers, joint
tickets, and cash by people transferring from one vehicle to another, one
mode to another, and from one public transportation agency to another makes
it impossible to count people. Only boardings (unlinked passenger trips) can
be counted with any accuracy. At the largest public transportation agencies,
even the number of boardings may be estimated for at least a portion of the
ridership (e.g., free shuttle vehicles without fareboxes and light rail service
using the "proof-of-payment" system).
The majority of people using public transportation take two trips per day (one to work in the morning and one home in late afternoon or evening). A small proportion--perhaps 5%--make only one public transportation trip (e.g., they ride public transportation to the airport and then fly out of town, or they ride public transportation in the morning to work, but ride home in a friend's automobile at night). A somewhat larger proportion (primarily the public transportation-dependent) take 4, 6, 8, or even 10 trips per day.
At most agencies perhaps 10% to 30% of riders must transfer to a second (and sometimes a third) vehicle to reach their final destination. Some transfer from bus to bus, from bus to train, from one agency's vehicle to another agency's vehicle, etc.; thus, there is a large amount of double-counting of people. APTA's best estimate is that the number of people using public transportation on any day is about 45% of the number of trips reported.
Saturday ridership is often about 50% of weekday ridership, and Sunday ridership may be only 25%. In many smaller cities, public transportation service does not operate on Sundays; in a lesser number, there is no Saturday service.
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