|Length:||2 miles (3.2 km)|
|Old route now:||A844|
|Route outline (key)|
The B878 runs across the Isle of Bute.
It starts on the B881 (former A845) High Street in the centre of Rothesay and heads west. After leaving town the road bends to the left to pass Greenan Loch (a reservoir) before ending at a sharp junction (the sort you'd expect to be a TOTSO but isn't) on the A844 overlooking the west coast.
The B878 today is ever so slightly shorter than the road originally numbered in 1922. Then, it continued west along what is now the A844 to Ballianlay, where it became the B879 as it turned north towards Etterick Bay. Both of these roads were taken over by the A844 later in the 1920s.