A blue sign with a diagram showing a branch off the road ahead and the following text.
Forward destinations: SOUTH WALES; Bristol; Bath.
Exit destinations: Chippenham; A350; Cirencester; A429.
Junction number: 17.
If you wanted to go to Cirencester, the A419 would have got you there much more quickly, but it doesn't appear on the signs back at junction 15 because... well, I've complained about that already.