I'm talking about having to move the focus from the road to the windshield. That still takes time. If the HUD is "at infinity" then it's ok, but would you need special glasses to make that work?

I use the term "holographic" for a reason. The HUD appears to be projected at a distance of, say, 20-30', and is positioned low on the windshield so that it essentially overlays on the road. You do not need to refocus your eyes on the windshield. Focusing on the road also focuses on the HUD projection.