American cities got very different layout, mopeds are good for certain type of more congested narrow street type of cities where people also don't care so much about mopeds zipping inbetween cars. I can't see this happening in American driving culture and if you have to just behave like another vehicle on the road you might as well just drive a car. Must say though, even countries where mopeds were popular are now transitioning to electric bikes and scooters because it is cheaper, doesn't require any license and usually good enough for the inner city. You can see it happening in China or Israel.
