It's a balancing act I suppose. People still find that it's worth paying to take the car, but not to the same extent as without the tolls (it only costs between $1 and $4 depending on the time of the day). I'd guess there's still some headroom for increasing the tolls without decreasing the car traffic too much.

If it were too expensive there'd be less money to finance public transportation which, I guess, creates a strange dependency. It somehow works, but there's still of course far too much car traffic.