|South end||Milton Keynes|
|Kettering, Milton Keynes|
The A509 is a short A-class road (around 30 miles (48 km) long) for north–south journeys in south central England, forming the route from Kettering in Northamptonshire to the A5 in Milton Keynes, via M1 J14.
From north to south, the road begins at Wicksteed Park in the outskirts of Kettering. It then crosses the A14 (where it becomes a primary route) and goes through Isham and Great Harrowden. After this it goes on to form the Wellingborough western bypass before leaving Northamptonshire to cross into the Borough of Milton Keynes (ceremonial Buckinghamshire). From there, it crosses the A428 at a roundabout and cuts through the centre of Olney. South of Olney the road passes Emberton, meeting the A422 just north-east of Newport Pagnell, where the routes multiplex to form the Newport Pagnell eastern bypass. South of Newport Pagnell, the routes diverge at a roundabout with the A509 turning south and the A422 continuing westbound. Here the A509 is a single carriageway once more until it crosses the M1 at Junction 14, where it enters Milton Keynes proper and (at another roundabout) the existing southbound route becomes part of the A4146 (here, the A4146 takes over the 'primary route' designation and the A509 loses it). Continuing westwards from here, the road once more becomes a dual carriageway, running for a further 4 miles (6.4 km) past the edge of Central Milton Keynes (the central business district) and the Network Rail National Centre ('Quadrant:MK'), to link up finally with the A5. Through Milton Keynes, the road is known as the H5 Portway.