|Length:||2.7 miles (4.3 km)|
|Former Number(s):||A812, A82|
|Route outline (key)|
The road actually starts on the short A812 just south of Renton, at the end of the eastbound sliproad from the A82, and then runs northwards through Renton to Alexandria. After crossing the railway, the road runs parallel to it around the edge of Alexandria Town Centre. A left fork (partially blocked off) is the old route of the A812 / A82 prior to the Alexandria Bypass being built. In the centre of town we meet a large gyratory around the railway station (southbound traffic has to cross the line twice) as we bypass the southern part of Main Street. The road then curves back around the north of the town centre onto the historic route, remaining on the west side of the River Leven for its entire route. Heading north once more, the B857 passes the former Argyll Car Factory, now home to Loch Lomond Galleries, a sort of outlet shopping centre, although the north end of the building is derelict. Heading out of town now, through modern housing estates, it's not long now before the B857 terminates at a roundabout on the A811 in Balloch.
Some maps show the B857 as continuing along the line of the A812 into Dumbarton. However, West Dunbartonshire Council still list the A812 as being a current route, and signs on the ground still sign it, with the signs on the A82 having been recently updated from the incorrect (B875) to the correct (B857), suggesting that traffic uses a different route to reach the B857.