"Yokel" wrote
Roy Badami wrote:
Ivor The Engine wrote:
In some places, roads merge then separate but keep their numbering
before and after.

That happens here, too.
The difference is what happens during. In the UK, the common section has
the number of the most major road (only), leading the minor road(s) to be
discontinuous. In the US (and I think in many parts of Europe?) the
common section bears multiple designations.

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), with the first junction you encounter
with the A120 shown as A120 east or A120 west to help those on the A12
know which one they need to turn at.

There are some examples of poor signage where roads merge and then separate.
The A57 - the main route from Lincoln to Sheffield - merges with the A1
between Markham Moor and Apleyhead, but drivers approaching Markham Moor
from the Lincoln direction are given no clue whatsoever as to the correct
route to take, and it is not at all obvious. The A57 just disappears.