> Adjusting for long-term ridership trends on each system, seasonal effects, and inertia (the tendency for ridership totals to persist from one month to the next), CBO estimates that the same increase of 20 per- cent in gasoline prices that affects freeway traffic volume is associated with an increase of 1.9 percent in average system ridership. That result is moderately statistically significant: It can be asserted with 95 percent confidence that higher gasoline prices are associated with increased ridership.

https://www.cbo.gov/sites/default/files/110th-congress-2007-...