A blue sign with the following text.
POLICE; Unmarked cars; operating.
This bit of road is notorious for drivers - er - picking up a bit of extra speed.
From here to the next junction, the road was widened to three lanes each way in a very economical way in the 2010s, resulting in a very twisty, undulating alignment. From here you can see the dual carriageway bouncing around the hills ahead of us like a rollercoaster.