I can see why a road with a bitumen/tarmac topping could be described as paved, from when roads were topped with paving, but where did the term “metalled” come from to describe a road with a man made surface?
I think it’s because “metalled” originally meant just “surfaced with gravel or broken rock”, which substances fall under earlier definitions of “metal”, apparently meaning “stuff mined or quarried” in general, as well as the specific mineral substances that we now call “metals”.
Wow. Never heard this term before today. But, apparently, it’s because “metal” used to just refer to mining/quarrying in general, whether we’re talking about actual metals or more common non-metallic rocks:
I think it’s because “metal” used to refer to any generic mining product, not specifically those we think of as being “metals” 
the origin meaning of “metal” is “ore”, “something that was mined”.
Just a quick sort-of-hijack. In the latest Popular Mechanics (March 2006), they discuss three NASCAR tracks - Bristol, Darlington and Daytona. The Daytona track had iron filings mixed into the original paving mixture to increase adhesion. Perhaps related?
Doubt it; I think people have been talking about “metalled roads” since long before the Daytona people came up with their iron-filings idea.
Thanks all, I guess it was just laziness but I didn’t get beyond the meaning of the word as a reference to a paved road 
Just to prove that there’s never a bad time for a Thomas Hardy quote, here’s something from Jude the Obscure:
Above all, the original church, hump-backed, wood-turreted, and quaintly hipped, had been taken down, and either cracked up into heaps of road-metal in the lane, or utilized as pig-sty walls, garden seats, guard-stones to fences, and rockeries in the flower-beds of the neighbourhood.
The only thing I could offer is that typically you run a heavy, double “steel-wheel” with a watering system to put a smooth, compacted finish on the wearing surface of the road.
Maybe because it’s a steel-wheel they “metal” the road?
Tripler
That’s all I can figure. …
No, because the use of the verb “to metal” meaning “to cover or surface with broken stones” apparently long pre-dates modern uses of iron or steel in the material or equipment for paving roads.
Why are you and lawoot straining yourselves to come up with folk etymologies about road-surfacing that involve the use of substances that fall under the modern definition of “metal”?
“Metalled” just means “paved or surfaced, e.g., with broken stones” because the definition of “metal” used to include non-metallic substances such as broken stones. Simple as that.