The A877 is a very short A-road, following the residential Renfrew High Street, while the A8 dips south along Glebe Street. The road is dual-carriageway throughout, while the A8 ceases to be at the roundabout where the two meet. Could this, therefore, be part of an unrealised scheme to build a new A8, with a new bridge across the Black Cart Water?
Back in 1922, this road was the original route of the A8. The route was moved before 1930, and working on the assumed dating of the A878 in conjunction with the A82 Great Western Road extension, it would seem that the A8 was realigned in 1926-7. However, the fact that the dualling continues on the A877, and that pre 1930 buildings only line one side of the High Street at any one time, it seems that perhaps the A8 was always meant to come this way, and as suggested above, continue westwards. The big problem is that at the end of the road stands Renfrew Town Hall (above), which would have been a very big problem for any future 'expressway' like the Great Western Road to the north of the Clyde.
The other problem to this thought is that taking a new expressway down the high street of a county town would seem unlikely when there was open land not far to the south, and quite possibly along the banks of the Clyde as well.