The problem is the Greek population is heavily centered in Thessaloniki (pop. 1 million) and Athens (pop. 4 million) with not many other larger cities. Most toll roads they've built are very expensive, often exceeding $ 30 - 50 million per mile, with very low traffic volumes. You can't just spend $ 4 billion in tax money for a few thousand vehicles per day on average. So I think the choice for toll roads is legit. Toll hikes to cover government deficit isn't though.
Mountain roads are always expensive, but I didn't realize that so few vehicles use the roads. That would seem to make a difference, but in that case it doesn't make sense that so many protesters would go so far in protesting the tolls. Perhaps to them the toll roads are a metaphor for the larger issues facing Greece right now.
You have brought up something I hadn't thought of since this discussion began, though; thanks. I guess I could live with temporary
tolls used strictly to pay off the construction bonds when a roadway or structure, such as a bridge or tunnel, is of such an unusually large scale that other means of financing aren't available. I would only approve it, though, in cases where the structure would make a dramatic improvement to commerce and safety.
Knowing more about what else is going on in Greece, I would still cut some slack to the protesters. If what you say is true - that these roads are not much used - then this is clearly about more than tolls.
And given the news today from Egypt, I believe protest movements in a lot of countries might catch fire very soon. Certainly the Greeks will be influenced by what's just happened (Mubarek has resigned). Maybe they'll notice that the protesters in Egypt got what they wanted without shutting down the Suez Canal.