Self-healing roads? Oh, I wish! I bet the ancient Romans could have built them. Check out this video between about 9:47 and 11:24.

Even if we could get the volcanic ash, though, would it work? Maybe the ash available to the builders of the Colosseum happened to have the chemical composition that let it work the way that it did.

Perhaps there’s no guarantee that ash from any old volcano will serve; and if all the world tried to build self-healing roads using volcanic ash, said ash might go the way of the dodo within a rather short time. Then we’d have to wait for more eruptions to get the ash; and what disruption that would wreak!