The B178 is a cross-country road in north County Down. It was originally unclassified but gained its number in the late 1920s.
The road starts on the B177 to the east of Hillsborough opposite the wall of Hillsborough Forest Park. This junction has been upgraded and the original western end of the B178, still with its central white line, now only provides access to a house. We pass two large car parks, for a sports centre and church, and then enter open fields. The road winds to the northeast and presently crosses the River Ravernet to reach Legacurry, where we cross an unclassified road at a staggered crossroads; this was originally the A49 so could explain why it has priority.
We cross the present route of the A49 about a mile further on and soon afterwards cross the B6 at a staggered crossroads; field boundaries suggest we once went straight across. We continue northeast across undulating fields and presently reach Carryduff. After entering town along Hillsborough Road traffic lights are reached on the A24. There's a short multiplex to the left to the next set of lights, where the B178 regains its number by turning right onto Church Road. We pass a retail park and a school to reach another set of lights on the A7; the modern church is opposite. A right-left zigzag (controlled by the same set of lights) takes us through an industrial estate and back into open country.
The road continues eastwards across undulating fields before skirting the southern edge of Moneyreagh. The settlement is only accessed by a single road heading north and no houses front onto the B178 although plenty are visible. There's then a glimpse of open fields once more before we cross the A23 at a staggered crossroads. Continuing east, we go through a wood before descending to journey's end at a T-junction on the A21. Comber is a short distance to the left.