The B278 is a medium-length road on the southern edge of London.
It starts on the A239 in Merton and heads south-east as Green Lane. This road is dual-carriageway except for the bridge over the railway line.
At Rosehill Roundabout the route meets the A217, the A297, and the B2230 (although only the A217 and B278 cross the junction). It passes the St Helier Hospital and continues along Wrythe Lane through Wrythe. It then runs parallel to the B277 into Carshalton (although the two roads don't actually meet).
We meet the A232 at a mini-roundabout and multiplex south-westward to reach traffic lights where the A-road TOTSOs right and the B278 continues ahead. Left is the B271. We then proceed south along Park Hill to pass Carshalton Beeches station. Later we reach Woodmansterne Road and pass a golf course, making this section semi-rural.
The A2022 is crossed via twin roundabouts before we reach the northern edge of Woodmansterne. Arriving at a T-junction by the village green, we turn left to avoid the centre of the village. The remainder of the route is subject to a 7.5 t weight restriction. Eventually the B278 terminates at a T-junction on the B2032, Outwood Lane, Chipstead.