(Other definitions for isobars that I've seen before include "Lines linking places of equal barometric pressure" , "Lines of equal atmospheric pressure" , "Map lines of equal pressure" , "Lines on a weather map" , "15 is strong when they are close" .)
'map lines' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for roads that I've seen before include "Tracks" , "Thoroughfares" , "Ways, routes" , "Area by shore where ships lie at anchor; streets" , "an answer to all clues, initially" .)