The A878 is little more than a spur of the A814, which heads north east to meet the A82 at the junction with the A898 at the northern end of the Erskine Bridge. The Old Kilpatrick end, on the A814, is a simple Give Way junction, but at the other end it splits into slip roads amongst the GSJ of the Old Kilpatrick Interchange. However, while traffic from the A878 can head either way along the A82 (with the through route taking traffic to the west), only westbound A82 traffic can access the A878, and there is no access to or from the A898.
Although the A878 number is clearly marked on most of the signs, many maps claim the road to be a spur of the A814.
Before the construction of the Erskine Bridge, and the connected GSJ between the A82 and A898, the A878 would have left the A82 at an at-grade junction with full access. This belies the history of this road as part of the original project that saw the A82 Great Western Road Dual Carriageway route built in the late 1920s/early 1930s (and opened as the A876).
However, going back even further, the A878 was originally the western end of the A810. Between here and Duntocher where the A810 currently starts, its former route was taken over by the new Great Western Road route of the A82 in the later 1920s, leaving the short spur at Old Kilpatrick which was evidently given a new number to avoid any confusion.