More than 30% of trips made in cars in Europe cover distances of less than 3 km and 50% less than 5 km.

That's walkable.

Thi I agree with and this is what I see as the low hanging fruit. It's not necessarily about the fuel efficency of the car, but the fuel consumption of the driver measured as a total volume over time, not distance travelled.

We already have fleet of cars that if they were driven much less than they are now, would still be going in 10,15 or 20 years time. With some serious rethinking about how we use the cars, rather than the type of cars we use, we could achieve that same or better fuel consumption over that period without having to spend an enormous of capital (energy) to replace a perfectly good working fleet of cars. The oil savings that are made could be redeployed into making our walkable neighbourhoods places that are worth dwelling in, as opposed to the rather dead lifeless palces that most sprawl uburbs are.

Kaishu,
Thanks for posting that data again, its very valuable to help understand how we use oil now. You are correct that in an emergency(either long or short) we could drive a lot less than we do now. I live only 1Km from shops, but usually drive, because my arms tend to stretch carrying home dog food, milk, potatoes,wine, beer etc. I wouldn't stop driving these short trips even if petrol was $3/L, because it really doesn't use that much. Now commuting 15 Km to work by train without any transfers, and 1Km walk at each end, that's easy. In an emergency though I could easily buy smaller amounts and shop more often or go by car less frequently and buy a freezer or a larger fridge.
Social activities often involve weekend and late night trips and usually 2 or more in the vehicle, so again mass-transit not a good option. Sydney trains stop running between 1-4 am ? each morning.
Voluntary activities such as dropping off groceries, taking elderly shopping, picking up kids for soccer; if mass-transit worked for these trips wouldn't need be volunteering.
So I really see the best role of mass-transit to replace single passenger trips, over longer distances such as commuting to work and of course interstate trips between capital cities.
You are correct that some cities are designed to work on mass-transit, such as Paris, Venice, London and other European capital cities that were build by great empires, well before cars. We could re-build our cities to be like that buts its going to take several life-times, and most of our mineral wealth, meanwhile if BEV's become widely available, a lot of people are going to use them for that 65% of "non-essential" trips including those "walkable" less than 3km trips that Europeans drive now even though they have a good mass-transit system.