The A870 was the main road on the Isle of Colonsay. This road took a western loop around the island, starting at the top of the pier in Scalasaig, alongside the A871. After a mile or so, the A869 branched off to the left, heading south for Oronsay, while the A870 continued to the west coast, before doubling back to Kiloran where it met the other end of the A871. Strangely, the road then continued for nearly a mile to stop short of the junction providing access to the north of the island.
Unclassified in 1922 the A870 came into being in the mid-1920s. Whereas the road survived until the 1970s all the Class I roads on Colonsay have now been downgraded and the A870 is now numbered B8086.