Reducing density of cities actually increases a lot of problems. It means you have to spend a lot more of your time traveling, which will make all these problems worse. LA is a fairly low density city and has terrible traffic problems. It reduces economic efficiency, too.

Additionally, public transport often interferes with regular road traffic, making the problem worse. The solution is to use an elevated train (Chicago) or subways. So now you're back at 3D.

And biking in a low-density city isn't a practical solution.