The A1243 follows the original line of the A16 into Grimsby.
The road starts at a roundabout on the western edge of New Waltham, as the A16 turns off its old route to join what was a railway line running into Grimsby. The A1243 bears left and soon enters Scartho. It runs along a straight, residential road through the village, detouring slightly to meet the B1203 at a roundabout.
The road continues north along Scartho Road, now running into the suburbs of Grimsby. This road is wide and tree-lined with a bus lane for much of its length. Presently a roundabout is met on the A46 and there is a short D2 multiplex to the right, with neither road mentioned in brackets on the road sign. The multiplex ends with the A46 turning right at traffic lights. The A1243 continues north along Bargate to end at traffic lights on the A1136 to the south of the town centre. That road continues along the ex-A16.