Thanks to Mike Kendrick for this news:
Speed cameras incorporated in traffic signing on the M4, I have received further details of locations, etc.
The sign / camera bases are now in place, although the equipment is yet to be installed.
They will operate in pairs and are spaced half mile apart.
Eastbound, the first pair are located 1 mile west of junction 36 (Bridgend / Sarn Park ) And they continue in pairs at 2 - 6 mile intervals until the Eastern side of the Brynglas tunnels ( junction 26 ).
Westbound, the first pair are located at the Western side of the Brynglas tunnels (junction 26 ) And they continue again in pairs at 2 - 6 mile intervals until junction 36 (Bridgend / Sarn Park )
It remains to be seen whether the cameras operate solely within each pair (ie. over a half mile stretch ) or whether average speeds are recorded over the full 27 mile length.
I think it's safe to assume, the whole stretch of the M4 between Brynglas and Sarn Park is pretty much covered in both directions.
Cameras go live 14 July 2003 apparently.
Hope this of use..........