1. Keep the HUD in focus and the road out of focus. Your eyes are still seeing a lot of road information that doesn't necessarily need the high fidelity. This is relatively safer than looking away entirely. The human brain can do a lot with scarce information.
2. Keep the road in focus and the HUD out of focus. For things like navigation this will still work fairly well since all you usually need for the navigation is the shape of the road, the direction you'll need to turn in, and how many blocks (roughly) you have until the destination. This is relatively safer than looking away at your phone or gps from time to time, or even looking around for the street names.
Realistically, people will switch between the two extremes.
1. Keep the HUD in focus and the road out of focus. Your eyes are still seeing a lot of road information that doesn't necessarily need the high fidelity. This is relatively safer than looking away entirely. The human brain can do a lot with scarce information.
2. Keep the road in focus and the HUD out of focus. For things like navigation this will still work fairly well since all you usually need for the navigation is the shape of the road, the direction you'll need to turn in, and how many blocks (roughly) you have until the destination. This is relatively safer than looking away at your phone or gps from time to time, or even looking around for the street names.
Realistically, people will switch between the two extremes.