If we were starting again now from scratch with no road numbers it's hard to see that the existing system would be first choice. It's as odd as it is because it's grown up piecemeal, with an obvious reluctance to change long-established numbers.
The most obvious deficiency is that there is no mechanism for handling merged roads, or as illustrated, roads that overlap for a short distance. The result is that irritating feature when trying to follow a particular route/number is that the number can suddenly vanish, leaving one going round a roundabout searching for a road that has disappeared.
Satnav helps of course, but even there, there is inconsistency over what counts as an exit. "Take the 3rd exit" may or may not include the dummy exit for an unbuilt road, or a maintenance depot access.
|
spotted on Google maps 
spotted on Google maps 