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.