Unusual road signs

[Haydnjones]

I've recently come across two signs at a motorway slip road - circular solid green with a yellow band surround and about 18 inches diameter on the exit road, and the equivalent red circle with a yellow band surround on the joining road.

[BillyNoMates]

If these are on a yellow rectangular background they may be to indicate directions for contract vehicles if there is a large construction project ongoing in the area.