> that's much more efficient in terms of space than a world where 80% of trips are via car and 20% are walked.

I feel like you are trying to say that it will require less space? Like roads can be made narrower or shorter? Why though? The road width is defined by the vehicle size and the number of lanes, and you don't want to cut the number of lanes below 1 but realistically you want 2 so a broken vehicle won't block traffic for everyone. And there are already not many 3 lanes in each direction roads so you are not saving much. Also most 3+ lanes roads I see are arterials and interstates, people driving those routes usually cannot walk them physically as they are tens if not hundreds of miles.