I've been along the A55 by Aber today and took the opportunity to see what the drainage is actually like. As far as I can see, it's rubbish. Considering the massive area of land that water drains off on the higher side of the road, there appears to be only a small french drain and the original 1960s road drains to cope with all the water - no surprise that the road floods then! The solution is not difficult (or particularly expensive) - a large drainage channel running along the field side of the hedge that adjoins the A55, that collects the water running off the fields and sends it under the road.
The section that caused the largest problem is in a dip by Tal Y Bont, a stream that usually flows under the road became obstructed and decided to flow onto the road. However, the River Ogwen flows under the A55 less than a hundred yards away - again, not difficult to construct a large culvert that would drain away flood water into the Ogwen in extreme weather conditions.
It's probably impossible to construct a drainage system that will deal with all eventualities. However, it not difficult to construct one that will ensure the road is clear again within, say, 30 minutes of the rain stopping.