The true potential of a Prius can't be unlocked without a scangauge / ultragauge or something similar. It's highway sweet spot is at TPS 18/ ignition advance (IGN) 14. This is informally called the 'Super Highway Mode'.
There's another technique called Warp Stealth; the engine is in fuel cut, but the electric motor is spinning the ICE to prevent engine braking. When combined with the HS P&G technique, this is the best way to gain fuel economy on the highway.
BTW, 17-18 is very low for a Prius. I got that with mild hypermiling in my 270k driven car. If it was maintained better, and I had a scangauge, 21-22 was very much possible. Keep in mind that I'm talking about a non-hybrid 1600cc engine.