[Tagging] Is a sidewalk always adjacent to a traversable road?

[Nathan Edgars II]

Getting this whole thread out of the black hole it's gone into: was it 
the intention of those creating the footway=sidewalk and sidewalk=* tags 
that:
*They be consistent with each other: if a footway would be tagged as a 
sidewalk, then the parallel highway would have a sidewalk tag (at least 
if the footway is not separately mapped) and vice versa?
*They include or not include informal desire lines created by walking on 
the grass strip on the side of the road?

If there's no agreement on the latter, there's no point in using the 
sidewalk tags.