Apparently, photographs show at least one rear tyre deflated.
Here is a clip from GoogooMap of a refuge (though not the one where the crash occurred - this is from the opposite carrigeway).
http://maps.google.co.uk/maps?q=Sier...,,0,19.48&z=19
I believe the crash occurred on a right-hand curve.
I wonder if the driver was 'drawn-in' to the lay-by by the green lighting thinking it was the rear of another vehicle, then swerved out when he realised and then lost control due to a rear-tyre being deflated.
Speculation of course . . .