How does a High Quality Dual Carriageway differ from Motorway?

In terms of physical standards, motorways are designed to a geometry that facilitates a travel speed of 120kph.  High Quality Dual Carriageways can be designed to either a geometric standard that facilitates 120 kph or can be designed for the lower general national roads speed limit of 100kph. If designed to the higher limit, there is unlikely to be any physical difference between a motorway standard road and the 120 kph standard High Quality Dual Carriageway except for signage.

There are, however, a number of other differences between motorways and dual carriageways.  Use of motorways is restricted under the Road Regulations 1994 and certain users such as learner drivers cannot use a motorway.  No pedestrian use is permitted on a motorway but is on dual carriageways.  One of the more fundamental differences is that the legislation prevents any future access being developed onto a motorway but does, subject to planning controls, allow future accesses to be developed onto dual carriageways.