The basic problem with aiming for fewer cars and more public transport and walking/cycling is that neither alternative is actually a viable alternative for a surprisingly large proportion of journeys made by car today.
Public transport essentially relies on having critical mass. If you have enough people wanting to travel similar routes at similar times, then you can make great economies of scale, avoiding congestion, using natural resources more efficiently, and reducing environmental damage.
Unfortunately, while megacities like London typically achieve this critical mass, smaller cities like Cambridge rarely do. You see mostly full buses for an hour or two at the start and end of each working day, or on major routes like those between the Park and Ride car parks on the edge of the city and the centre. However, the average number of passengers on a bus, taken across all times they run and all routes in the UK is only something like 7 IIRC.
At that point, buses are actually more polluting per passenger-mile than the average family car with an average of 1.5ish occupants, not so much in the carbon oxides that get all the press, but certainly in the really nasty stuff like PM10s. They don't compare so favourably on things like fuel consumption either.
This lack of critical mass also creates problems for financial viability, which in turn means that you get less frequent services and that those services that do run have to follow fewer, more circuitous routes. That makes the whole thing less efficient than personal transport, and at some point, you reach the stage where getting the bus simply doesn't save you any time over walking, and not because we're talking about a short journey where waiting for the bus dominates.
On top of all of that, buses are basically a menace to every other class of road user. They are obviously dangerous to cyclists. If you don't have space for dedicated roadside bus stops so the buses have to block the main driving lane while picking up and droping off passengers then a single bus can hold up an entire queue of traffic, slowing everything down to its stop-start pace. That is inefficient, environmentally unfriendly, and dangerous, all at once. Moreover, dedicated bus lanes and other priority features tend to violate the normal road layout rules, causing difficulties for just about everyone else at major intersections.
Basically, if you've got enough people to run frequent bus services on fairly direct routes between everywhere and everywhere else, buses have a lot going for them. Otherwise, they suck and have almost no redeeming features whatsoever.
Cycling and walking have a lot more going for them, and if we could just ban buses around here then we could probably get a lot more people cycling. They are clean and efficient methods of transport, and of course they have side benefits in terms of health and fitness. Unfortunately, they aren't much use if you're trying to get somewhere and still look presentable at the other end, it is impractical to carry large/heavy/young and wriggling loads on most journeys, and they are pretty unpleasant in bad weather. Oh, and a single cyclist can hold up a whole queue of traffic on a narrow road as well, and even on a relatively wide one the choice is between a hold-up or a stream of other vehicles passing dangerously close.
On the face of it, the solution to most of the drawbacks of cycling is to provide better cycle facilities: dedicated cycle lanes away from the main traffic, secure cycle parking, mandatory shower facilities at places of work, and so on. Sometimes, the results are inconsistent or counter-intuitive, though, particularly with the idea of separating cycle traffic. Nervous/inexperienced cyclists often prefer to ride on separate paths off the main road, but more experienced/capable cyclists typically prefer the main roads rather than having to give way to motor traffic so much when separate cycle paths need to cross main roads (or vice versa, if you prefer).
My personal feeling as a driver and cyclist is that if we could redesign the city around a good quality cycle path network from the start, and then overlay a road network for motor traffic on top with suitable under/overpasses to avoid breaking up the cycle network (since it's much harder for cyclists to stop and get going again if they have to yield priority) then a lot of these problems could be overcome. But right now, the local authorities here are proud of the fact that they have got a couple of major cycle routes from one side of the city to the other coming together after several years. I don't think it will ever be practical to implement something as comprehensive as I've described in an old city like Cambridge.
The only solution, therefore, is to give up on finding a workable transport infrastructure for the city as we have today, and change the planning policies so that over the coming years there simply isn't as much need for so many people to travel into the same small area at the heart of a small city from everywhere around on the same few roads. Urbanisation might be inevitable, but we don't have to design every city so everyone is trying to get to the same place right in the centre when the area feeding it isn't big enough for public transport to reach critical mass.