Gravel is stuffty for roads. Smoothstone is where it's at. Plus too, gravel for roads=no underground tunnel system.
I don't like gravel for roads, but you could easily make an underground tunnel system.
example:
g g g g g
s s s s s
a a a a a
a a a a a
s s s s s
g=gravel
s=stone, but you could use anything
a=air