Edit: Used wrong numbers for Portland, it's metro area is over 2Million people. That's close to the ~5M population for a metro area which really starts causing major issues.

Portland Oregon is a vary low population city .6M in the middle of no where. It would have been fine without any form of public transit. The real issues show up at around 10x the population where sprawl is to low density to support office buildings and people at the edge can't reasonably commute to the center of the city even with public transport.