The one problem I can't quite work out is - if all roads go over
and
under each other, how on earth do you turn left or right?!

Nobody has commented yet about the idea of sacrificing everybody's
back gardens for a system of bike paths and footpaths. In Stevenage
the bike bike paths are 12' wide with an 8' pavement on one side,
plus some grass on each side to account for "shy distance" needed
near obstacles such as trees and houses. Modern bike paths are less
generous, which is the reason, one reason anyway, why most cyclists
tend to ride in the road instead.

Jeremy Parker