You have to dig up and destroy hundreds of miles of walkways.

You have to replace brick, concrete and tiles with hundreds of miles of very, very expensive electromechanical technology.

You have to do this in TWO directions.

You have to hire thousands of workers, technicians and engineers.

You have to fund the development of the technology because it doesn't exist for this application.

You have to provide power, lighting, etc.

If this is in snow country, you have to provide a small army of snow removal trucks and crews to keep the darn things clean. Even then you'll still pay to move tons of snow, which won't be cheap.

You'll have to widen sidewalks in lots of places in order to accommodate all forms of traffic and work around existing infrastructure (subway station entrances?).

You'll have to have medical services available because you will have people getting hurt as they fall off the thing and do stupid stuff at 10 miles per hour.

Liability is likely to be huge.

And, of course, there is no practical way to charge for usage so we are probably going to sock everyone with yet more taxes to build yet another bullshit project nobody is going to use.

If the context is to replace "crosstown buses" you are talking about tens to hundreds of miles of sidewalks and all of the organization, infrastructure, cost and support to take something simple (buses and bus routes) and turn it into something complex, pointless, questionable and unlikely to really work.

The cost of such a ridiculous system would be staggering. Not sure why anyone who reads HN would need this spelled out.