I have been caught travelling at 53mph in 40mph limit on the M4. This is a stretch where variable speed limits apply. However, my passenger and I both believe that the limit showing on the gantry before the camera was showing 60mph, before reducing to 40mph at the next gantry.

This belief is reinforced in my mind as other cars were travelling at similar speeds, the camera was flashing repeatedly (dozen or more times - it was dusk & we thought it was malfunctioning), and at the next gantry showing 40mph there were break lights showing suggesting other drivers were adjusting their speed downwards.

I believe the camera was catching drivers in excess of 40 while the gantry indicated 60. Is this technically possible?