As with everything each example has to be taken on its own merits. That is why I said 'likely' :-)
All standing stones could have originally marked routes now long forgotten, but that is 'unlikely'.
All roads could have originally just linked up ceremonial standing stones that are now long lost, but that is 'unlikely'.
Obviously, a stone at the edge of the Newbury Bypass is by a road/track, but is 'unlikely' to be a route marker.
I would say that a stone at a junction is more 'likely' to be older than the road system, because this would indicate people coming from several directions and using it as a landmark to converge upon. However, as with all things, each one must be considered by itself and generalisations are 'unlikely' to apply.
