The A78 begins in Greenock, Inverclyde as a continuation of the A8 road at the Bull Ring roundabout, and runs in a southerly direction through the Spango Valley, Inverkip, Wemyss Bay, Skelmorlie, Largs, Fairlie, Seamill and then bypasses the major towns of Ardrossan, Kilwinning, Irvine and Troon amongst others before terminating near Prestwick, Ayrshire, where it forms a junction with the A77.

The rest is dual carriageway – between the Eglinton and Warrix interchanges there are in fact 3 lanes in each direction, and the road is constructed to motorway standard (with full grade separation at junctions) save for a lack of hard shoulders.

The road very rarely appears busy on this stretch – the adjacent New Town development area of Irvine, North Ayrshire (which includes Kilwinning, Dreghorn and Girdle Toll) had a population of just over 50,000 in the 2001 census;[1] the town planners designed the infrastructure for a population more in the region of 120,000.

The result was a highly congested section with numerous traffic lights making progress slow.

The new road can easily take 10 minutes off any journey which would otherwise involve travelling through the three towns and as a result has proven very popular amongst people who make such journeys – for example people from Largs and West Kilbride to the north who now have swifter access to the shopping facilities of Irvine and Ayr and the Morrisons supermarket at Stevenston, as well as any football fans who may be travelling to watch Kilmarnock FC.