A road is more than a smooth, flat surface. A road that's designed for cars has to have an extensive roadbed of gravel and soil laid down, as well as a thick base of pavement on which to lay the surface, because of the weight of vehicles. A bike path, or a street where children can play, is comparatively speaking just some asphalt.

Definitely don't need streets to play. A couple paved areas for basketball, skating, etc is nice - but that's just a park.

ish, but without cars you don't need a 20 lane highway. and streets can also less lanes,

roads designed for cars are massively larger than roads that aren't made for cars, a standard modern car-centric road would be a pretty major and significant thing for most of human history.

A single standard car lane is a generously wide bike path.