Road signs

[koiman]

What are the yellow diamond, square, circle and triangle shapes for on many of the green A road signs in the UK?
I travel on the roads a great deal and see them all over the country, it bugs me not knowing.

[Buenchico]

They're potential diversionary routes, prepared in advance. So if a road has to be closed, all that will be needed will be a few signs stating (for example) "For Manchester follow < diamond >", For Buxton follow < square >", etc.

Chris

[smorodina]

They are diversion signs:

http://www.ukmotorists.com/diversion_signs.asp