Couple of things I want to note.
Lets not forget houses. Not everyone is going to live in a flat.
We have spent the last 60 years becoming dependant on the car. Town centres are no longer the "centre" for shopping etc so public transport cannot support the travellers needs as the shops etc are dotted about predominantly out of town. People work further from home or live further from work than they used to 60 years ago. People have developed the attitude that they can go into the countryside - travel that is no longer the preserve of the rich (perhaps also the middle classes).
Radical idea is for councils to provide free parking in town centres otherwise the level of business done there will be so low that the town centre will no longer be viable. Keeping the town centres viable will ensure there is somewhere for the public transport to serve. Then look at discouraging car use WITHOUT driving shoppers etc from the town centres (carrot or stick). This is something I see as a problem with Croydon and now even the bus routes are being diverted away from near the centre by bicycle lanes. The bicycle lanes are underused which I am amazed at - there were a lot more buses than bicycles and each bus carries many passengers. I think Croydon has passed the point of no return in that respect and is now about to implode.
Most town centre car parks, if they were free, would just be parked in all day by people spending no more than they currently do in 2 hours. However with less turnover of parking fewer people would actually be able to go shopping, harming the shops and making it worse for them.
I think this is trying too hard to justify eradicating cars. None of it is an ecological problem or a problem for the economy.
There's a big difference between having fewer cars and eradicating them. The latter would be impossible. However having fewer would actually make streets a nicer place to be.
For example, try walking down a road where there's a school nearby at pick up/drop off and then walk the same street at the same time during the school holidays and it's almost certainly going to be more pleasant when there's fewer cars.
OK, that's a little unfair (I'll get accused of wanting to eradicate cars again), so how about during walk to school week when the number of parents who drive to school drops off significantly.
Trouble is the North Sea oil is heavy crude that predominantly refines down to heavier fuel (like diesel). But we need to import lighter oil from the Middle East to refine down to petrol. There is less demand for the UKs kind of crude oil.
There maybe less demand, which presumably also means from the UK, which means that we're importing (and so paying a lot to other countries) most of our fuel. Therefore, reducing our oil use would likely still keep more money within our economy rather than being sent overseas.
Developing underground has costs and where does all the dirt removed go ?. Plus flooding is a growing issue so I would not be interested in a house or flat that had space near or below ground level. As others have said - house one storey up with level access to the garage.
If we're building garages at ground floor then you're building a room which otherwise gets very little use. There's a good chance that someone would convert it into a room, especially given there's fairly few rooms in houses 6m x 3m.
On top of that councils would require a 6m setback from the highway to the garage door, so that a car waiting for the garage door to open isn't blocking the highway. That means they more land used up by cars that either restricts the building space at upper levels, or adds to the construction costs without adding building space.