The A421 is an important road for east/west journeys across south central England. Together with the A428, the A43 and A34, it forms the route from Cambridge through Milton Keynes to Oxford. The section between the A1 near St Neots and the A5 in Milton Keynes is a national primary route.
The road begins at the A1, just south of St Neots (and the junction with the A428 from Cambridge), 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 local grid road H8 Standing Way. During this time it crosses the A5 (and connects to it via a short spur which is part of the V6 Grafton Street).
Continuing westwards as the route approaches Buckingham the 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 (Northampton — M40 junction 10) near Brackley. (On crossing the A43, the route due westward becomes the B4031 through Croughton, Aynho and Deddington to join the A361 to Chipping Norton).
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 section of single carriageway from the new flyover (beside Junction 13) 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.
In conjunction with the M1 widening schemes and the widening of the A421 between M1 Junction 13 and Bedford that is already finished, proposals have been put forward to also 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. The scheme was given in 2005 an estimated total cost of £33 million. In 2014, funding of GB£23.5 million was announced by the government for the dualling work 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.
The A421 Tingewick bypass, near Bicester 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 157 miles per hour (253 km/h) on a motorcycle. His friend Neil Bolger, 30, of Gaydon, was clocked at 148 miles per hour (238 km/h). Both were convicted of dangerous driving, imprisoned for 28 days and banned from driving for two years (with a compulsory re-test).
Blind Martine Brooks drove along the newly constructed A421 Great Barford Bypass (From the A1 Black Cat Roundabout to Bedford) 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 (Mayor of Bedford), Steve Clarke (Teacher and Navigator), and Denise Hubbard (Driving Instructor and Car Owner).
She reached a speed of 65 miles per hour (105 km/h) before doing a flawless three point turn, and returning to the A1.