I was watching Chris Froome on the mountain time trial in this years TdF and he'd look up to see the road, then look down to be more aero, then look up, then look down etc, etc.
So why don't time triallists wear belay glasses e.g.
http://cubelayglasses.co.uk/ and they could see where they're going and have their head down in an aero position at the same time???