The A12 runs along the southern side of Ronaldsway Airport, although it was once considerably longer.
It starts at a roundabout on the A5 at Janets Corner on the edge of Castletown and heads south along Shore Street. At the end, it TOTSOs left onto the Promenade and enjoys a scenic run along the north shore of Castletown Bay with the large buildings of King William College to the left. As the bay slowly curves to the south, so the road cuts across the neck of Langness to Derbyhaven. Just after entering the village, a right turn leads out to the car parks and golf course on Langness and also to St Michaels Island, a fascinating area to explore with many ruinous buildings and some spectacular coastal scenery. The A12, meanwhile, continues ahead to the shore where it turns hard left and heads north along the coast, curving with the bay at first before turning inland a little. An unexpected narrow section soon widens out, with the white line reappearing shortly before the route comes to an end at Gate 14 for the airport.
The road from Castletown to Derbyhaven was originally part of the B32, upgraded to become the A12 in the 1930s. At this time, the current B53 which runs south east from Ballasalla, and also ends at a gate into the airport, was also numbered as part of the A12, although the route appears to have been severed during the war when the airfield was used by the RAF. Mapping evidence suggests that the two parts of the route were not rejoined for long, if at all. By 1963 the runway had been extended east of the natural line of any connection, and it has been further extended since, removing any hope of connecting the two parts of the A12 and so leading to the northern section being renumbered as the B53.
The Manx Government Circular 1814 of 17/08/1937 defines this route as: Derbyhaven Road, Ballasalla to Castletown