Anything longer than 30 minutes each way is probably beyond what many people would like to spend commuting. So, that's about 4-7 miles (depending on variety of conditions). Which, I gotta say, is not very far for most Americans. Most of us commute around 15 miles each way.
The really crappy part is how much of that 15 miles is caused by car infrastructure. Imagine how much closer one might be to work if the distance were compressed, removing about 80% of all street and highway width, the entirety of all road buffer zones (highways often have several times as much of this as they have actual road), front yards (= mostly to keep house back from busy road and allow space for cars on driveway), parking lots (often several times as large as the place they're meant to serve), and so on. I'd be surprised if such compression didn't shave about half the distance off.
Just take a couple more steps, and stack things into big buildings. You'll save even more space, and get things even closer together. It would certainly be more efficient (although, also more similar to a lot of places I've chosen to avoid living in...)
I commute about 4.5km by bike(each way) twice a day, in London. It's much quicker than public transport. 12-17minutes by bike, with traffic + stop lights. I'd be looking at 30~ minutes by public transport. Even a taxi/car is likely 20 or more. Cities designed before cars are (imo) - so much nicer.
