After WWII, public transit use peaked at 23 billion unique trips, meaning that 20% of the US population took public transit twice a day. Public transit usage then quickly declined, hitting a low of 6.6 billion unique trips in 1972. In 2014 ridership probably hit 10.8 billion trips, which, while the highest level since 1956, still means that just 4% of the population now uses public transit twice a day.