The A812, less than a mile and a half long, acts as a western bypass route for Dumbarton town centre, connecting the A814 to the A82. However, it has a complicated history, interlinked with successive changes to the A82.
Commencing at a junction with the A814 near to Dalreoch Station on the west side of Dumbarton, the A812 heads north as Renton Road, immediately crossing the railway line to Helensburgh. The road itself then follows the Alexandria branch line north through open countryside.
After about three quarters of a mile, a slip road from the westbound A82 joins from the right, and after passing under the trunk A82 itself the A812 reaches an end-on junction with the B857 which is the northward continuation of Renton Road. The road to the right at this point is a slip to and from the eastbound A82 (there is no access here to the westbound A82).
The original A812
The original route of the A812 is Bank Street (now mostly the B857) and across Bonhill Bridge in Alexandria. In 1935 the A82 was rerouted across the bridge and through Alexandria, moving the A812 onto that road's vacated route south to Dumbarton. This included the current route and also Renton Road and Main Street as far north as the Bank Street junction. Further improvements to the A82, including the construction of the Alexandria Bypass have truncated the A812 to its current route; the northern section into Alexandria is now numbered B857. The southern bypass to Alexandria was built first, in the early 1970s, and so for a brief period the section through Renton appears to have become the A82 once more, before the western bypass was completed.
For more information on the history of the A812, see the A82/Great Western Road page, as the two routes have been inextricably linked.
The 1922 MOT Road List defines this route as: Alexandria - Bonhill An official document from 16-5-1934 details the following changes: Wholly renumbered A82. Route number A812 is re-allocated to the Dumbarton - Alexandria section of A82. A812 (Dumbarton - Alexandria)