SG: The book of road signs is very clear that speed limits have to have a red circle around the numbers. It is permitted, however, to have that mounted onto a yellow reflective background (even square) where the local highways authority think it would be beneficial to the motorist. Speed limit signs with a black circle are not legally enforceable - neither are the ones painted on the road surface.
SG: As above, the background can be reflective yellow and square. Any other speed sign (like the matrix signs on most motorways) are simply "advisory", although some motorwy matrix signs - like the M25 - have red circles and are mandatory.
The subject of legalality of signs gets quite a regular airing on the IAM forum and there are loads of examples of incorrect ones in use. There is a mini roundabout in Bristol that has 3 roads joining it. Depending on which road you're on you either see a normal roundabout sign with a give way line, a mini-roundabout sign with a give way line or a mini-roundabout sign with a stop line - they all have the triangular give way signs on the approach :-)
Regards Steve G
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