I think San Franciscans might have some quibbles with your assertion that anything in The City is insignificant.
Every route* included in the system is, or is at least part of, a route where at least two states sign it, and the Federal Government recognises it in some way.
The
49-mile Scenic Drive doesn't even seem to be recognised by California as part of the state's
State Scenic Highway System.
It is insignificant. It's indisputably local. It also seems to somewhat be random nonsense going round city streets in a loop for a couple of hours (in a city where driving isn't pleasant and there's great transit). At least
this city street driving route has a theme beyond 'the city', and is part of a wider state system of tourist drives.
*Other than the Selma-Montgomery Trail. But as we have other NPS Trails, I felt it worth adding as part of that sub-system.