Moving sidewalk to replace train?

Architecture company NBBJ thinks it can replace the 17-mile Circle Line loop running beneath central London with a moving sidewalk that would have three lanes, with the slowest moving at around 3 miles per hour and the fastest track moving at about 15 miles per hour right in the middle.

I kind of like the moving walkway (or Travelator) at SFO.

At 15 mph, what could go wrong?

http://bgr.com/2015/09/08/london-subway-circle-line/

This scientist thinks that moving walkways actually slow you down.

http://www.telegraph.co.uk/news/science/science-news/5836445/Using-the-airport-moving-walkways-actually-slows-you-down.html