I've toured in Andalucia, Catalonia, Galicia, Asturias and have just come back from a coast to coast Bilbao - Malaga.
Roads are pretty much deserted, even the northern N coastal roads, the vast % of traffic is on the autovias and the authorities have taken to taping over (hiding) destinations on road signs hoping to to funnel even more folk onto them, really they have!

In one 25-30km journey only one vehicle passed me, a Google Earth car.
Drivers are very friendly and very aware, often they'll give a little toot to let you know of their approach from behind and always give plenty of room. In 3 weeks I didn't have one "incident", nearly every day in the UK I have one.
Helmets, I don't wear one and having to would spoil my touring enjoyment. In the 3 weeks I must have asked the Guardia Civil for directions upwards of 30 times (I did it all the time on previous tours too), not once was my helmetless bonce commented on. It's a convoluted law and IMO you'd have to meet a right niggardly cop to fall foul of it, unless they have been specifically told to enforce it, however, that was not the case in my trip, I talked to them from north to south and was of course, passed and seen by many more.
Average Spanish "B" road traffic. Very enjoyable.