The A842 originally bisected the Isle of Arran from the A841 on the northern edge of the island capital of Brodick to Blackwaterfoot on the west coast, following the road known locally as 'The String'. At Blackwaterfoot it met the A843 south coast road and then continued on to the Quay. The road was originally built c1810 by Thomas Telford as part of his commission on Highland Roads and Bridges.
However, it was soon realised that having three A-roads on Arran was unnecessary, and while the A841 was ultimately extended in the late 1920s to form a complete circuit of the island, the A842 was downgraded at the same time to become the B880, as it still is today (apart from the westernmost section which became the A841). The only slight difference between the modern and original routes is that the A842 originally extended to the shore in Blackwaterfoot, while the B880 still stops where it used to meet the A841 island ring road, before that was truncated. The original westernmost section of the A842 is therefore now unclassified.
For a description of the route, please see the B880.