Motorway cantilever sign
A blue overhead sign suspended over the motorway from a post on the left, with a diagonal arrow pointing up and left, and the following text.
A4500; [Solid triangle diversion symbol] [Solid circle diversion symbol] [Hollow square diversion symbol]; Northampton; (West) .
Junction number: 16.
It will not surprise you to learn that the A4500 used to be the A45, and was given the world's least imaginative new number when the A45 was rerouted elsewhere.