The A404 is a road in the United Kingdom that starts at Paddington in London and terminates near Maidenhead in Berkshire. It is 44.6 mi(71.7 km) long.
The road initially follows a course through north-west London via Harlesden, Wembley, Harrow, Northwood and Rickmansworth. During this stage, it is known as Harrow Road.M25 Junction 18 is at Chorleywood. The A404 crosses into Buckinghamshire and continues towards Little Chalfont and Amersham.
Between Paddington and Amersham, the road closely follows the route of the London/Harrow-on-the-Hill/Aylesbury railway lines, The Chiltern Line, from Marylebone, and runs near several stations along that line.
At Amersham Common, the road turns south-west and continues in that direction joining the Amersham by-pass (A413) for a short distance, and then proceeds towards High Wycombe. After passing through the town centre, and crossing the A40, it changes to a dual carriageway up the hill to the M40 Junction 4, and continues as a dual carriageway past Marlow and Bisham.
The road eventually becomes the A404(M) at Maidenhead Thicket. It then runs a short distance towards the M4 where the A404(M) finally terminates at Junction 8/9.