Excuse me, but the road youre describing is the one racing people is always doing up and down, so that you said about t-shirts its wrong.
Apart from that, the road from Luarca to Foz (N-634) there have heavy traffic all the year, and have very poor corners. There are much better roads in this region than that.
Take the route through the mountains via Navia. This is a stunning route through to La Corunna. Locals will look at people who ride bikes in more than t shirts and sandals with curiosity, but all part of the fun. There are a few blind bends on the way, but mostly the country sweeps along. Wife didn't whine too much about being thrown around!