The A5160 was proposed to be reserved in 1935, in order to form part of a Swindon - East Anglia route, together with the A4160 which was proposed to be reserved for an elevation of the B489 to Class I, with the last three digits the same to give the number continuity. As the tonnage wasn't enough on the B489 to elevate it to Class I, and never looked likely to, the proposal fell through, and the original plan of simply extending the A505 over the A601 happened instead.
