The original route of the A78 started at the original western end of the A8 in Gourock and then headed west round the coast on what is now the A770. The A78 was re-routed to its current route in 1985 taking over the entire length of the A742 and curtailing the A8 to show that this was the main road.
A78 historic route from 1922/3 numbering
Further south in the Three Towns, this section was bypassed between the 1970s and 2005. By this time, going though the Three Towns took around 20-25 mins. The old route from the end of Stevenson to Monkton is as follows: A738 to Kilwinning, A737 though Irvine then unclassified, A759 to Loans before rejoining the road's original route which was rebuilt online.
After the A79 roundabout (formerly a junction with the B749) the original A78 continued ahead along the now-unclassified Main Street into Monkton to meet the A77 (now B739). In the late 1950s it was diverted onto the new bypass, which allowed the old A77 to be severed by the construction of Prestwick Airport.
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