The only potentially serious Motorway incidents I've seen recently have been on the M6 southbound past Preston where speed camera markings have been left on the carriageway after roadworks have long gone. The regular sight of emergency braking upon first sight of these roadmarks causing erratic and dangerous driving in an effort to get under the 70mph limit causes far more danger to everyone than a "safe" limit of 80mph.
I'd be interested to know how many accidents could be attributable to speed cameras under these circumstances...unfortunately the stats would only confirm that the camera was in the right blackspot rather than a direct causal link.