|Length:||2.8 miles (4.5 km)|
|Meets:||A1114, A1060, B1009, B1137, A130, A12, B1137|
|Route outline (key)|
The A138 runs along one of the old alignments of the A12 (the first bypass of Chelmsford, dating to the 1930s) to the north of Chelmsford city centre. It gained that number when the A12 was moved to its current route in the 1980s.
The road starts at the Army and Navy Roundabout where it meets the A1060 from the city centre and the A1114 which forms the ex-A12 from the south and then goes on to the east. The A138 heads north. It crosses a rather ornate bridge over the River Chelmer (the Chelmer Viaduct) before reaching a roundabout by a retail park.
At the next roundabout, which is residential, the A138 becomes dual carriageway. There is a GSJ with the B1137 (the original route of the A12) and then runs parallel to the railway line. It goes under the A130 but has no junction with it.
The next turn-off runs straight onto the northbound A12 and the A138 bears right to end at the dumbbell roundabout forming part of the GSJ at Boreham Interchange (J19). There is access here to the A130 seen earlier and both directions of the A12. Ahead is the B1137, still on the original line of the main road.
In 2014, the Chelmer Viaduct section of the road was trunked, so that the Highways Agency could pay for a replacement viaduct (that was originally built by the MOT as this was part of the London-Gt Yarmouth Trunk Road).