The road begins at the A1, just south of St Neots, at the Black Cat Roundabout. The road bypasses both Great Barford and Bedford to the south to reach the M1 at junction 13. From there, it swings up through the southern part of Milton Keynes, doubling as the localgrid roadH8 Standing Way. During this time it crosses the A5. Continuing westwards, as the route approaches Buckinghamthe road passes close by to the 14th century Thornborough Bridge, the only surviving mediaeval bridge in Buckinghamshire which was bypassed by the new bridge in 1974. Close to here, it then forms the Buckingham by-pass before joining the A43 near Brackley.. The section from the A1 to the M1 is dual carriageway and fully grade-separated, with the section between Bedford and the M1 completed in December 2010, winning the British Construction Industry Award in 2011. There is a short 3km section of single carriageway from the new flyover to the south-eastern outskirts of Milton Keynes, where dual carriageway running resumes and continues as far as the southwestern outskirts. From here to the A43, the route is single carriageway, with the exception of the bypass around Tingewick.
Developments
M1 Junction 13 to Milton Keynes
In conjunction with the M1 widening schemes and dualling of the A421 between M1 Junction 13 and Bedford, proposals were also put forward to widen the A421 between the M1 junction 13 Bedfordshire and the Kingston roundabout in Milton Keynes. Exhibitions were held in June 2005 which rejected proposals to re-route the road in favour of widening the current road. In 2005 the project was given an estimated total cost of £33 million. Funding of 23.5 million was confirmed by the government for these works, as part of the South East Midlands Local Enterprise Partnership "Local Growth Deal". As part of the government's pinch point reduction programme, in 2014 work commenced on the A421 in Milton Keynes to improve the Kingston roundabout. That work is now complete. The upgrade work for the final part of the A421, the section running from junction 13 to Kingston, started in September 2018 and is due to be complete by the end of 2020.
The Oxford to Cambridge Expressway is a proposed grade-separated dual carriageway between the A34 near Oxford and the A14 near Cambridge, via Milton Keynes. The proposal aims to establish this route by linking existing roads and building new ones. The case for its creation is examined in a Strategic Study for the Cambridge – Milton Keynes – Oxford corridor, published by National Infrastructure Commission in November 2016. The NIC sees the road as being of national strategic importance by providing an outer orbital route around London, linking Southampton, the M3, M4, M40, M1, A1, A14/M11 and Felixstowe. This route will absorb the existing A421 and A428 roads between the A14 and the M1, with a new "greenfield" route from M1 J13 to the M40 and onwards to the A34, broadly in the East West Rail corridor., no announcement has been made on the precise route between the M1 and the A34, let alone the numbering of the route, although the M1 to M40 section is to be broadly parallel with the existing A421 and will replace it.
Notable events
Five seconds of fame
The A421 Tingewick bypass has a minor claim to fame as the location of the then fastest speeding incident ever recorded by British police, in March 2003. Andrew Osborne, 31, of Leamington Spa, was filmed by a mobile speed camera while travelling at on a motorcycle. His friend Neil Bolger, 30, of Gaydon, was clocked at. Both were convicted of dangerous driving, imprisoned for 28 days and banned from driving for two years.
Blind driver
Martine Brooks drove along the newly constructed A421 Great Barford Bypass and back to raise money for charity. The drive took place two days before the official road opening, and she was accompanied by Frank Branston, Steve Clarke, and Denise Hubbard. She reached a speed of before doing a flawless three point turn, and returning to the A1.
