While I'm not too familiar with the A690 east of Durham, one common reason for a dual carriageway to be limited to 50mph rather than 70mph is due to the distance between junctions.

Something in the back on my mind is saying if there's less than 1 mile between junctions then it's advised (or maybe required) to reduce the speed limit to 50mph to improve safety.

I could Google that fact and see if it's true, but I can't really be bothered.