That does not necessarily follow. If you offer free mass transit, the people who take advantage of it may not be the same people who would otherwise be clogging up the roads in their personal cars.
It does not need to be a 1:1 relationship for my statement to be true. If every 10 rides reduces 1 car trip it's still reducing congestion. The important thing to remember about congestion is at maximum capacity new trips simply increase wait times so a 30 min trip can add more than 30 minutes of wait time spread among all drivers. And removing a 30 minute trip can save people more than 30 minutes of wait time.
Granted as delays reduce people my decide to simply take more trips, but a combination of a car congestion tax, free public transit, and walk-able / bike-able city's are far more efficient say trying to stack highways.
