"Yokel" wrote in message
...
While this is true, the road signs should show both numbers with the
"secondary" route in brackets to show that one can be reached by a turn
off the other. This is the general convention in the UK for all such
cases. A case I saw recently is near Colchester, where the A120 to / from
Harwich crosses the main A12. Both roads share the Colchester by-pass and
the road is shown as A12 (A120).

Whilst it should be true that the number of the "senior" road is used where
the route is shared, it is surprising how often this is not the case:

Newmarket By-pass - shared A11/A14, designated "A14";
Rhayader-Llangurig - A44/A470, designated "A470";
The use of A55 for the last part of the London to Holyhead road (rather more
excusable);
The use of A303 for the first few miles west of Popham to Micheldever
Station (which should be A30).

There are opposite examples, too. The short stretch from the end of the M50
to Ross-on-Wye should be A449 but is marked "A40", with three legs of that
road apparently converging on one roundabout.

Regards

Jonathan