In plain English
Green signs guide you along the big A-roads between towns. The yellow numbers (like A61) are the route; the white words are the places.
Where you'll see it
On main A-roads and at junctions joining them — the backbone of non-motorway Britain.
The common trap
Reading the road number as a speed or a distance. Yellow-on-green numbers are always route numbers.
Memory anchor
Green for the main veins of the road network — with yellow numbers like grid references on a map.