Re: Wrong question
You're quite correct. With the prevalence of roadside tax collectors dressed up as police officers, people need to watch their speedo constantly so which is safer? A HUD or a control panel?
The answer is cruise control :). When there is a motorway restriction and traffic flow is reasonable (not stop/start), I tend to enable cruise control and blip my speed up and down with its controls. That way, I can spend more time observing traffic and the heightened danger of a road narrowed by construction than on maintaining a speed limit, and the system goes offline as soon as I touch clutch or brakes.
Road safety went IMHO off a cliff after speeding fines were automated via cameras instead of putting people on the road. I'd rather have some people taken off the road for dangerous manoeuvres than for speeding on an empty road at 3am..