From Roader's Digest: The SABRE Wiki
The A12 has been an important road from London to East Anglia ever since it was given the number. The road as far as Colchester is one of the oldest main roads in Britain, and the original route used the course of the Roman Road. Since the 1950s, various portions have been bypassed or improved so there is now continuous dual carriageway as far as Woodbridge, with other improvements beyond this up to Great Yarmouth.
London - Brentwood
The A12 originally started on the A11 in Stratford, and ran along the historic Roman road through Ilford and Romford up to what's now Gallows Corner. In the mid 1940s, Eastern Avenue (then the A106), took over the A12 route, with the old road becoming the A118. This moved the start point up to the Green Man roundabout in Wanstead. Eastern Avenue was gradually dualled in stages from the 1940s to the 1970s, eliminating various local bottlenecks, particularly at Newbury Park.
From Gallows Corner to the M25, the A12 follows the original route. It was dualled some time after the M25 was completed.
Brentwood - Ipswich
Many of the current problems with the A12 stem from the fact that it is mainly a series of bypasses built at different times, with each constructed to differing specifications depending upon predicted traffic flows and design standards of the day. The section between Brentwood and Chelmsford in particular had been marked for improvements as early as the 1930s, but like many plans the war stopped progress. During the 1960s and early 1970s, it was expected that the M12 would be built, rather than the A12 improved online. When these plans were put on hold, it became obvious that completing the remaining single carriageway sections was the most practical answer.
Starting from the current junction with the M25 at Brook Street roundabout, itself originally opened in 1966 with the rest of the Brentwood bypass, the M25 viaduct was built over the top in 1981, finally opening in 1983 and linking the A12 to the motorway network. This junction has been remodelled in recent times to cope with the large traffic flows using it, and the Brentwood bypass was improved in 2001, adding lighting and resurfacing the concrete carriageway.
Past Junction 12 is the Mountnessing bypass, opened in 1973 and was originally 3 lanes in both directions. This is now reduced to two lanes eastbound in order to reduce queueing. After junction 13 is one of the oldest of the bypasses, Ingatestone which was constructed in 1958. It does show its age with tight corners and poor sight lines, although it was improved recently, adding lighting and removing the bushes from the central reservation to improve visibility.
The Margaretting bypass was built in 1973 and the road opens out to 3 lanes again, albeit for a short distance before the start of the Chelmsford bypass. The original route up Three Mile Hill was dualled in the 1950s, though it is no longer part of the A12 mainline.
Chelsmford has been bypassed twice. The first opened in 1931 and ran from Widford to Springfield. It is now the A1114 and A138. By the 1980s, it was the largest bottleneck on the route between the M25 and Colchester as it was the last remaining single carriageway section, and so a second bypass was required. It opened in November 1986 but was built to a fairly low standard with concrete carriageways and low capacity junctions. At Boreham interchange we meet half of the Boreham bypass, the other half extending into Chelmsford. Opened in 1971 this is another 3 lane section, again having been improved in 2000.
The Hatfield Peverel bypass was completed in 1965, though the completion of the second Chelmsford Bypass means that the western section is now part of the A138. The current road drops back to two lanes after Hatfield Peverel to run on the original Roman Road. Plans are afoot to provide a turning lane between these junctions as it is an accident blackspot.
The Witham bypass opened in 1963 and was remodelled recently, one of its more interesting features is for some distance the carriageways are split level. Beyond Witham at Rivenhall the road again returns to its original alignment. The next bypass is around Kelvedon, which was opened in 1967 and has to be upgraded to more modern standards.
The A12 was dualled online between Kelvedon and Marks Tey in the 1960s, and originally ended at an at grade roundabout with the A120. The original roundabout is still there, and was reused as part of the modern grade separated junction with the current route. This leads onto the Stanway bypass, which opened in 1971 to 3 lanes. It has since been upgraded and now has a motorway style concrete barrier in the central reservation.
The original route of the A12 through Colchester was via Lexden Road, Crouch Street and Head Street, where it met the A134 at the top end of the High Street. It ran down the High Street to East Hill and East Street and onto Ipswich Road. Like Chelsmford, Colchester has two bypasses. The first opened in 1933 and was a wide single carriageway with at grade roundabouts. It is still used as part of the main route into the town centre from the current A12. The second bypass opened in 1974, while the A120 extension towards Harwich opened in 1980. Towards Ipswich the road is mainly a series of online improvements, with Capel St Mary being bypassed in the mid 1990s.
The original route though Copdock was dualled online, and remains as such, though it is no longer possible to drive it end to end as the A14 Ipswich Bypass only has a foot and cycle underpass. The A12 originally ran through the centre of Ipswich, but a bypass was opened in 1930, that ran to the north of the town. The current route of the A12 was originally part the A1091, but the A12 was moved to multiplex with the A14 over the Orwell Bridge in the early 1990s. The old route (including the original bypass) is now the A1214.
Ipswich - Great Yarmouth
The section from the A14 to Falcon Park was originally a rerouted A1093, which already ran in the area, but it was decided later to reroute A12 traffic this way to avoid using the old Ipswich Bypass. The Woodbridge Bypass is one of the oldest on the A12, and opened around 1933.
The Great Yarmouth and Gorleston Relief Road was built in the 1980s on the alignment of the former Lowestoft - Yarmouth Railway, which closed in 1970. It involved a significant amount of engineering, including replacing the original swing bridge over Breydon Water because the old bridge's foundations were in the wrong place. It eventually opened to traffic in 1993.