To answer a question on Sustainability.SE, I am interested in finding data on how many people in the U.S. utilize public transit in a given year.
Most of the data I can find in the National Transit Data covers ridership, defined as rides taken during a time period. So if the period is one year and a person takes the bus twice a week, that counts as 104 rides. I want to know how many unique individuals use any form of public transit in a given year. A reasonable approximation with a defined methodology would be sufficient.