It’s all the “on-ramps” and “off-ramps” that would really limit the enclosed, elevated roadway.
The cable idea has some intriguing possibilities. You could route it near enough to some buildings to use their third story as an access. A ride-on, ride-off suspended cable bridge would require a lot of material and would be very expensive, but if you just suspend the passenger from a single cable, the materials would be a lot less expensive.
OTOH, you would need a drive mechanism, and everything I can think of would be pretty inefficient — unless you have purpose-built carriages. You might also imagine handicap carriages with electric motors and built-in batteries.
But I’m having a hard time seeing how it would be cost-effective. How much would you voluntarily pay per trip? A dime per mile? A quarter? You’d need something like a million (or ten) passenger trips per year to pay for it, and my estimate is that the usage would be at least an order of magnitude too low.
Oh well, maybe we could steal the money to build it.
About five years ago I had a proposal for a gondola cable car system to go from Troy Hill to Allentown, with stops at the NShore Trail, EBusway, Bedford Ave, DuquesneU/Uptown, SSide/ECarson, and Arlington Ave.
Eyeballing it on google maps, I think you’ve got way too much drop on the zipline — it’s probably around 400′ vertical from Grandview to the Point all in only 1000’+ horizontally. It might be a little too thrilling.
I rode on a zipline like system at the great wall.. It was a harrowing experience. I got the distinct impression they don’t have a great number of safety inspectors there. Probably not many liability lawsuits either.