> Even on a personal level, say a car lets me do a trip in 30 minutes instead of 2 hours walking. I don't relax for an hour and a half.

And that's why building highways does not reduce average commute time—people just move further from the downtown where they work. If you build a new highway that cuts commute time in half, on the population level, the sprawl increases until average population is twice as far and their commute remains the same.