Not exactly and expert but here goes.
SMA is a different from 'surface dressing' which I think Simon is referring to. This is where small stone 'chippings' are applied to a bitumen applied top an exsisting road surface and then rolled. Quite a lot have to be removed with road sweepers but until the surface settles down those that are loose get flung about in all directions , hence the signs and the 10 or 20 mph signs (which no one adheres to), hence the stones get flung around etc.
SMA is a road surface which has a smoother texture than the 'traditional' hot rolled asphalt road surafce which had large 'pre coated' chippings pressed in with a roller. This gave a much more uneven surface texture and hence noise , ride etc.
Hope this helps (a bit)
