I have another data point that supports your modern day transportation constraint of 10-20%: skyscrapers. Engineers can build skyscrapers that are more than a mile high, much taller than our current skyscrapers. However, the taller a skyscraper gets, the more of its footprint is devoted to elevators and and associated equipment. At a certain point, your skyscraper just becomes a useless bundle of elevators. Right now, developers stop adding floors to a skyscraper when elevators are 30-40% of its footprint and 2-10% of its energy costs. The true transportation costs are probably slightly higher because they also include the costs of elevator installation and maintenance, and are probably roughly in line with your transportation constraint.
This makes me imagine a city-skyscraper, where an inhabitant can do all their work, living, and shopping within the skyscraper, across a limited range of floors. If they only rarely ever need to go to ground level, then that can cut down on how much of the building is dominated by elevators