|Distance:||1.4 miles (2.3 km)|
|Now part of:||A818|
|Route outline (key)|
The B831 formed part of the access to Helensburgh from the A82 near Loch Lomond. It was more heavily used than the signed route via the A814 and Dumbarton Town Centre, and so was upgraded to become part of the A818 on 7 January 2005.
The B831 originally started on the A82 just to the south of the Arden Roundabout and headed west for about half a mile before looping to the south to cross the Fruin Water. This loop was subsequently removed, with a new bridge constructed further north to take the new alignment. The surviving old alignment does, however, reveal that this road was still single-track. Perhaps surprising, it was not replaced until the 1980s, with the 1981 OS Routemaster map still showing the old route.
Beyond the bridge, the road remains reasonably straight until it reaches the roundabout with the B832. Here the old route ended, while the new A818 turns sharp left to take over the former B832 into Helensburgh. Straight ahead is the old Glen Fruin Road, now superseded by the modern A817 a little to the north.