With fewer cars it's a lot easier and cheaper to get quicker and better public transit going. I don't know anything about Portland, but in general going by bus anywhere is mostly waiting in traffic (i.e. cars) instead of actually driving. This is why some places have bus lanes, which do help, but in a limited fashion.
I don't know how it compares exactly, but it's quite complex and there are a lot of factors and I'm not so sure public transport ends up being significantly more expensive in the end.
Trimet is competitive in downtown once you account for time spent looking for parking. The max is also pretty good when compared to rush hour traffic on the freeways.
Intercity transport really needs HS2 pushing through and increase electrification of other routes. This will speed up commuter routes and increase freight capacity.