From Pistonheads and written by and engineer on the tunnel:
40mph has been used as the speed limit for the first 3 hours of tunnel running this morning (few days ago now), to allow cars and traffic to adapt to the new road layout.
Once it is clear that traffic is running safely, the limit will be raised to 70mph, which is where it will stay.
The 40mph signs are only to be activated when a single tunnel bore is operated in contraflow, allowing maintenance works in the other bore.
For 99% of the time, it'll be 70mph.
There are average speed check cameras at each end of the tunnel, set for 70mph traffic (what speed they will catch you at we are not told, I'd imagine 79-80)
The average speed check cameras are variable, so are able to catch you speeding when it's 70mph OR 40mph in contraflow.
Slots fifth down from sixth with a delicate touch of throttle to balance the revs, slots fourth, third, then second, check mirrors, nails it... BWAAAARRRRRRRRRRRRP!
Myself and few Astonauts are heading down about 2:15. Anyone fancy a look? Will post the meeting point if so