 The old route between Didcot and Harwell was then downgraded to become the B4493.Until 1990 the road started at a junction with the A423 west of Nuffield, and the stretch between that point and Crowmarsh Gifford was designated a trunk road.The road between Burchetts Green (Maidenhead) and Nuffield was part of the A423 trunk road from Maidenhead (via Oxford) to Coventry.