- A421 between A43 and Northamptonshire boundary, Northamptonshire (Grid ref SP586335)
- A421 between Oxfordshire boundary and A4421, Oxfordshire (Grid ref SP605333)
- A421 between A4421 and A413, Buckinghamshire (Grid ref SP691330)
- A421 between A413 and A413, Buckinghamshire (Grid ref SP706329)
- A421 between A413 and B4033 Nash Rd, Buckinghamshire (Grid ref SP720333)
- A421 between B4033 Nash Rd and Buckinghamshire boundary, Buckinghamshire (Grid ref SP800325)
- A421 between Milton Keynes boundary and B4034, Milton Keynes (Grid ref SP830328)
- A421 between B4034 and Rhoscolyn Drive, Milton Keynes (Grid ref SP840334)
- A421 between Rhoscolyn Drive and Grafton St, Milton Keynes (Grid ref SP850346)
- A421 between Standing Way and A5(T), Milton Keynes (Grid ref SP863360)
- A421 between A421 split and A4146, Milton Keynes (Grid ref SP880372)
- A421 between A4146 and Larsborough Rd/Bransworth Ave, Milton Keynes (Grid ref SP900380)
- A421 between Larsborough Rd/Bransworth Ave and A5130, Milton Keynes (Grid ref SP905380)
- A421 between A5130 and Milton Keynes boundary, Milton Keynes (Grid ref SP924384)
- A421 between Central Bedfordshire boundary and M1, Central Bedfordshire (Grid ref SP950376)
- A421 between Central Bedfordshire boundary and A421 North before M1, Central Bedfordshire (Grid ref SP950376)
- A421 between A507 and Central Bedfordshire boundary, Central Bedfordshire (Grid ref SP956375)
- A421 between Roundabout and Roundabout, Central Bedfordshire (Grid ref SP956373)
- A421 between Roundabout south of M1 oand on ramp and Rounabout south of M1 and flyover, Central Bedfordshire (Grid ref SP957373)
- A421 between Roundabout south of M1 oand on ramp and Rounabout south of M1 and flyover, Central Bedfordshire (Grid ref SP957373)
- A421 between Rounabout south of M1 and flyover and Roundabout south of M1, Central Bedfordshire (Grid ref SP957376)
- A421 between Roundabout south of M1 and A507, Central Bedfordshire (Grid ref SP959375)
- A421 between M1 and A5134, Central Bedfordshire (Grid ref TL000425)
- A421 between M1 and A5134, Central Bedfordshire (Grid ref TL000425)
- A421 between A507 and Central Bedfordshire boundary, Central Bedfordshire (Grid ref TL000425)
- A421 between A507 and Central Bedfordshire boundary, Central Bedfordshire (Grid ref TL000425)
- A421 between LA boundary and A5134, Bedford (Grid ref TL019449)
- A421 between A5134 and A421 split, Bedford (Grid ref TL034463)
- A421 between A5134 and A421 split, Bedford (Grid ref TL036463)
- A421 between A5134 and A421 split, Bedford (Grid ref TL036463)
- A421 between A421 split and A6, Bedford (Grid ref TL040464)
- A421 between A6(T) and A600, Bedford (Grid ref TL063473)
- A421 between A600 and A603, Bedford (Grid ref TL077479)
- A421 between A603 and A428, Bedford (Grid ref TL089500)
- A421 between A428(T) and A1(T), Bedford (Grid ref TL125535)
- A421 between A428(T) and A1(T), Bedford (Grid ref TL145540)