Dave wrote:

Can anyone explain why some road signs have those triangle, square and
circular emblems on them? Sometimes they are empty and sometimes they
are solid colour. I would imagine that they are there for the emergency
services, but I can't understand what is what.

No, they're not there for the emergency services, they are there for
motorists who have been diverted from another (usually trunk road or
motorway) route. The symbols don't have any meaning in themselves, i.e.
there's no difference between filled or open square, triangle or circle.
However each symbol marks a different alternative route.

When a pre-planned diversion is in operation signs will be placed which
have messages specifying which classes of vehicle are to follow the
diversion routes, some examples a

DIVERSION
AT NEXT JCT
FOLLOW (CIRCLE)

HGVS
FOLLOW (SQUARE)

WINDS - HGVS
FOLLOW ( OPEN SQUARE)

HIGH SIDED VEHS
FOLLOW (OPEN TRIANGLE)


Another use is for Holiday Routes. When they are in operation drivers
will be instructed to follow the appropriate symbol.


WTF this has to do with DIY, I cannot imagine.