Is'nt this section of the M4 very prone to fog? There was a large multiple with great loss of life in (89?). The main contributary factor was that people continued to speed 'into' dense banks of fog.
From
http://www.learn2live.co.uk/General/News/necessity.htm
"On March 13th 1991, another of Britain's most horrific motorway accidents happened on the M4, between Membury Services and junction 14. Here, 10 people were killed and many others injured, with nearly 50 vehicles (including many large lorries) completely destroyed [in dense fog]."
The irony is that the crash site is just to the E and outside of the section (J14-J18) that will be covered by cameras.
However, 70 mph in dense fog would almost certainly guarantee a major accident