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Construction Timeline
When the various parts of the M4 were built, listed in chronological order. An asterisk (*) indicates a temporary terminus close to a present-day junction.
| Open | Jct | Section |
|---|---|---|
| 1959 | J1 | Chiswick Flyover |
| 1961 | J7-9 | Maidenhead Bypass |
| 1963 | J5-7 | Slough - Maidenhead Bypass |
| 1965 | J1-5 | Chiswick - Slough |
| 1966 | J18-20 | Tormarton - Almondsbury |
| 1966 | J39-41* | Port Talbot Bypass |
| 1966 | J20-21 | Almondsbury - Aust |
| 1967 | J24-28 | Newport Bypass |
| 1967 | J23*-24 | Crick - Newport Bypass |
| 1971 | J14-18 | Wickham - Tormarton |
| 1971 | J10-14 | Winnersh - Wickham |
| 1971 | J9-10 | Holyport - Winnersh |
| 1972 | J44-46 | Morriston Bypass |
| 1977 | J46-49 | Pontardulais Bypass |
| 1977 | J28-29 | Tredegar Park - Castleton |
| 1977 | J37*-39 | Pyle Bypass |
| 1977 | J32-35 | Coryton - Pencoed |
| 1980 | J29-32 | Castleton - Coryton |
| 1980 | J35-37* | Bridgend Bypass |
| 1994 | J41*-44 | Baglan - Lon Las |
| 1996 | J21-23 | Second Severn Crossing |
