To add to the pedantry -- the Scottish system re-uses the number with an "M" instead of an "A" when a new motorway replaces the old road on a new alignment, but

To add to the pedantry -- the Scottish system re-uses the number with an "M" instead of an "A" when a new motorway replaces the old road on a new alignment, but if the existing alignment is upgraded then (as in the rest of Great Britain) the A road sometimes keeps its number with a "(M)" suffix. And it's not necessarily the case that the old road is renumbered, either. So we have the M74 and A74(M) which are different pieces of the same road. There are bits of the A90 interspersed with bits of the M90, although the bulk of the A road is north of all the motorway and the motorway has in many places a different alignment to the original road. And the M8 is the main road connecting Glasgow and Edinburgh, replacing the A8, which is a different road with only segments remaining.