a HUD would be more in your field of vision in terms of the X and Y dimensions, but in the Z dimension, depth, in the worst place of all.
Focusing your eyes an inch away from your face is, unfortunately, taking your eyes off the road. The closer something is to you, the greater and greater focal length change is required. Switching between road vision and 1-inch off your face is much much harder than switching between road vision and 24-inches away from your face (where conventional speed dials are, on the handlebars).
It’s not just about the time and effort it takes to do this. When your eyes are focused 1inch away, everything on the road will be super blurry. When your eyes are focused 24 inches away, the road will not be as blurry, and it’s easier for your eyes to jump back onto the road.