The B378 is a short urban road connecting reservoirs to the south of Heathrow Airport.
It starts on the A3044 at Stanwell, just to the north of the Staines Reservoirs. It heads east into the centre of Stanwell where it TOTSOs right at a mini-roundabout. This is the original northern end of the B378; that which has gone before was originally the B379.
Continuing south, the B378 continues around the Staines Reservoirs before meeting the A30 by Ashford Hospital. The dual carriageway is crossed at traffic lights. About half a mile further on we cross a railway line by Ashford station before meeting the B377 by the war memorial in the centre of Ashford.
There is a multiplex east through Ashford with the B378 number dominant. This ends at a complicated junction involving two roundabouts, a LILO and the B3003. The first short section out of town is dual-carriageway but the road soon returns to S2 before ending at traffic lights on the A30. The embankment of the Queen Mary Reservoir is visible on the other side of the road.