On Tue, 13 Mar 2018 21:07:19 -0700 (PDT), Frank Krygowski
wrote:

The reason it's impossible to build a complete separate system is not just selfishness by motorists and shopkeepers. There are many issues that
should be obvious: Expense; Property rights, as related to right-of-way
acquisition; Road geometry; Politics; Ineffectiveness; Emergency vehicle
response; Maintenance...

speaking of geometry:

Anne McCaffrey's Pern includes a caste of messengers who have built a
network of moss-paved footpaths that connect every Hold and Weyr.
These same points are also connected by dirt paths beaten by
travellers on horseback. (Pardon me, "runner-beast"back.)

A messenger running from point A to point B never, ever crosses a
horse trail connecting point C to point D. Not so much as a cowpath!

Please draw me a map of how this works.

In "Imperial Earth", Arthur C. Clarke manages better: Since cars are
self-driving, roads are no wider than railroad tracks, and there is
only one lane in each direction. Intersections with other roads
aren't mentioned, but while being driven to his host's home, the
protagonist passes under an overpass carrying a much wider road. The
host explains to him that bicycles require more room because humans
are steering them.

--
Joy Beeson
joy beeson at comcast dot net
http://wlweather.net/PAGEJOY/