The charity Plantlife says “An astonishing number of wild plants grow on our road verges, some of which are threatened or near threatened.  Proper management of verges is critical if these species are to avoid extinction. Includes a list of known plants found on a road verge in the UK.”

https://www.plantlife.org.uk/uk/our-work/publications/road-verges-last-refuge-for-some-of-our-rarest-wild-plants.

“Of the 1,596 species we looked at 724 or 45.3% grow on verge habitats.  If we add in hedgerow and ditch habitats, the total rises to 809 species or over 50.7% of our flora.”