As someone living (or rather studying) in Cambridge
It seems like a silly city. As someone previously mentioned, the bus services are awful. They seem to shut ridiculously early, so if you do live slightly out of the town center, and don't want to cycle, your choices are either a massively expensive taxi or to walk.
As for people driving in from outside Cambridge, I'm not really sure. Park & Ride seems to have worked well, but I know my parents usually prefer to drive into the centre for convenience - but this is usually off peak.
It's not really big enough to merit a congestion charge - the traffic is usually quite bad on the arterial routes in and out of the centre, but fine otherwise. I actually think there's more traffic where I live, in Watford - and there's never been talk of a congestion charge. Then again, the Cambridge council may not be useless like the Watford council are.
There might also be something in the actual layout of Cambridge - it seems that most locals commute in from nearby villages or towns - there's no easy way to get in without driving. That might be a very difficult thing to change.