The A66(M) is a tiny little spur from the A1(M) to connect it to Darlington, which disappointingly ends on a single-carriageway road. Its full name is the Blackwell Spur.
It's quite odd that it was given its own number as it doesn't last much longer than a mile, and there are many other little spurs on the motorway network just like this one that don't have their own number and aren't considered motorways in their own right.
This road is actually a link from the new A1 (or rather A1(M)) to the old A1: the road's original course is now the unclassified road to Barton, and the A66 onwards towards Darlington is also part of the original Great North Road.
