I reckon JiK has a good theory going there for new growth.

I suspect, however, that MrH may be talking about the expansion of existing vegetation [probably a hedge or similar] disproportionately around roadsigns. I reckon this is due to the industrial revolution having almost extinguished the agricultural labourer.

When hedges were maintained by hand, knowlegeable men walked the hedgelines trimming, planting, and laying, keeping them stockproof, clearing ditches and maintaining sightlines.

The job is now done, if at all, by a man on a tractor fitted with a mechanical flail. This does a fast, not very effective, job of trimming the hedge to shape. It does nothing to keep the hedge stockproof [wire mesh can see to that] and fills the ditch with crap so that the roadway will be more likely to flood. One of the few things the operator knows is that he will catch an earfull from his bosses if he touches a roadsign, the flail will as happily chew that up as it will a hedge, or the nest of a threatened species bird, and roadsigns cost money. He therefore gives all roadsigns a wide berth, and the hedge grows thick and high around each succeeding sign.

I am happy to report that in dales country dry stone walls do not misbehave in this fashion.