The B778 is a rural B-road in north Ayrshire.
It starts on the A737 to the east of Kilwinning town centre. The road heads east past the cemetery and soon enters open country. It continues across the undulating terrain of this part of the country, passing few buildings and no settlements of any size. Eventually the road reaches the bleak Auchentiber Moss which is crossed to reach Auchentiber itself and a T-junction on the A736.
Originally the B778 ended here. However, it had been extended by 1932 and so there is now a multiplex to the right for nearly half a mile. Immediately after a bridge over the Lugton Water the B778 regains its number by turning left. The land is fairly flat until the Glazert Burn is crossed, after which it becomes hillier. This section of road is still quite empty and so it comes as a bit of a shock when the road climbs into a housing estate, on the edge of Stewarton. The B778 enters town along Dalry Road. It goes under a railway line before reaching a mini-roundabout on the A735.
There is then a short multiplex east. As it happens the B778 is not the only subordinate number at this point: the B769 exists here as well. At the next set of traffic lights the three roads go their separate ways. The A735 turns left, the B769 goes straight on and the B778 turns right. It crosses the Annick Water before having to TOTSO left to reach open country once more.
The road continues east and climbs up into the hills. It continues across undulating terrain, still with few buildings en route. Eventually the road descends again to cross a valley and then reaches a roundabout on the A77. Logic would assume that the B778 ends here - but it doesn't. It continues ahead and goes under the M77, after which it reaches the two sliproads at junction 7 for the southbound carriageway. Soon afterwards the road reaches another mini-roundabout where it does end. To the right is the B7061 (former B751) and the other direction is unclassified.