The A1231 starts at Birtley Interchange on the A1. Eastbound traffic leaving the A1 has to merge with B1288 traffic out of Birtley before turning right at a roundabout, while westbound traffic has to go through a 270 degree loop to reach the A1 northbound. The convoluted layout here is a result of the A1231 being an add-on to the junction built much later - as such it partly uses already-existing local roads for access, and direct access to or from the A1(M) south of the junction is impossible, requiring use of the A195 or A194(M).
Dual carriageway west of the A19 junction
From here the A1231 is a high quality, NSL dual carriageway heading east towards Sunderland. After several grade-separated junctions comes a roundabout interchange with the A182, however the latter road benefits from grade-separation here while A1231 traffic has to stop for the roundabout. After this the road continues through Washington new town with several more grade-separated junctions serving its residential and commercial areas, passing Sunderland's Nissan Factory and a very large ASDA distribution warehouse. The A19 is then reached at Hylton Grange on the edge of Sunderland, A1231 traffic once again having to stop for the roundabout. Substantial widening works were completed in 2015 on the eastbound approach in order to alleviate the queues which often formed here at peak times.
Beyond the A19 the A1231 remains dual carriageway but now has to negotiate numerous roundabouts, still at NSL although the roundabouts are spaced in such a way as to make 70mph difficult to achieve. Car dealers and small industrial areas are situated off them and they also enable access to the various residential areas of north Sunderland and the retail parks on the north bank of the Wear. The A1231 then TOTSOs at a junction with the A1290, to cross the Northern Spire bridge.
The Northern Spire bridge, shown under construction in 2017.
At the southern end of the bridge the road turns left at the end of the B1405, next to Pallion station. It heads east, soon goes over the Metro again and meets the B1539 at a roundabout. The A1231 carries on ahead, passing through several more roundabouts past retail outlets up to the edge of the city centre where it passes the back of the Empire Theatre and becomes dual carriageway again. There's a brief multiplex with the A183, with the A1231 number dominant, then the A690 is met at the Priestman Roundabout.
The A1231 is not signed beyond this point but it continues some distance further, skirting the southern edge of the city centre - its standard diminishing to S4 and subsequently S2. After passing the Park Lane interchange a triangular gyratory near the Civic Centre provides access to the B1522 (original A19). After skirting Mowbray Park the road passes through two traffic light controlled crossroads before ending at a roundabout on the A1018.
The route of the A1231 into Sunderland city centre, east of Queen Alexandra Bridge was originally the B1290, When the D2 to the west of the A1290 was completed in the late 1970s the whole thing was renumbered A1231, with that road going along what is now the A1290 and B1539 (itself originally the B1290. When the Northern Spire bridge was opened in 2018, the A1231 was re-routed along that and what was previously the northern end of the B1405, with its old route receiving its current numbers at the same time.
As may be expected, the A1231 was never planned to meet the A1 at its current eastern end. The A1231 number first came into being in the early 1970s as a short road in the Washington area. Starting at Vigo Interchange the road headed east along what is now the A195 then north along what is now the A182 to end on the A194(M). It was planned to link this road up with the B1290 in Sunderland - but the growth of the New Town of Washington altered the plans, giving the road numbers used today.