From Roader's Digest: The SABRE Wiki
Stratford to M25
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 1950s, Eastern Avenue (then the A106), took over the A12 route, with the old road becoming the A118.
From Gallows Corner to the M25, the A12 was dualled and improved online.
M25 to 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. 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, going coast bound this is now reduced to 2 lanes to reduce the queues where the 3rd lane ended. 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.
Onto the Margaretting bypass built in 1973 and the road opens out to 3 lanes again, albeit for a short distance before the start of the Chelmsford bypass. One of the newer bypasses, the Chelmsford bypass was opened in November 1986 and 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 recently, in 2000. We lose a lane at Hatfield Peverel and meet the next bypass which was constructed in 1965, again resurfaced recently. Between Hatfield Peverel and the Witham turn off is one of the few sections to follow the original alignment and there is a row of houses facing onto the dual carriageway. Plans are afoot to provide a turning lane between these junctions as it is an accident blackspot. The Witham bypass was constructed in 1965 and was remodelled recently, one if it's more interesting features is for some distance the carriageways are split level. Beyond Witham at Rivenhall the road again returns to it's original alignment with side turns and also service facilities. The next bypass is around Kelvedon, this was opened in 1967 and is one of the most recent to be upgraded to more modern standards.
Past Marks Tey and the junction with the A120 we meet the Stanway bypass opened in 1971 and 3 lanes wide, this has been upgraded and now has a motorway style concrete barrier in the central reservation. We lose a lane which heads into Colchester and we bypass it on the route opened in 1974. On the Northern side of Colchester is the junction with the A120 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 1990's. The A12 joins the A14 to bypass Ipswich to the south over the impressive Orwell bridge, opened in 1982.