Mod parent up.
Even the multi-function displays in the middle of the instrument panels on *all* cars made in the last three or four years is too much. Old fogeys like myself, at the crusty old age of 29, have gotten used to associating a particular spatial location in an automobile's console with a particular piece of information so that it's second nature.
This is how the mind is wired to absorb information from the world at a very basic level. Want to see what the weather it is? Look up. Want to see if you're walking on steady ground? Look down. Want to see if there's danger or prey out there? Look around.
Same in a car, or fighter jet for that matter: Want to see the time? Look at where the clock is. Want to see what radio station you're listening to? Look at where the tuner is. Want to see how much gas you've got? Look at where the fuel gauge is. This is constant-time lookup. If you have multifunction displays that *change* where these basic things are, now you've upped the cognitive load on the driver in that he now has to keep track of what state the display is in rather than just glancing in a well-remembered spot.
A proper heads-up display, and a proper desktop GUI, smartphone app, etc, preserve this feature so that you can see what you need by looking where you remember. Incidentally, this is a large part of what 'type rating' is on commercial aircraft, and aircraft manufacturers frequently retain large commercial customers by laying out the cockpits of their newer models the same exact way as the old one, with the selling point that pilots don't need to be retrained to figure out where they need to be looking and where their hands need to be in the new cockpit.
The point is, a good HUD for a car will show the same thing in the same place all the time. Just projections of dials and needles if I had my way. No popups, no text to read, no nothing. If there's something wrong with the car, a single idiot light that says 'check engine' will do it, because you're not going to diagnose it yourself while on the highway. That way it actually does save you time and keeps your gaze closer to the road.
But yeah. If you've got bells and whistles and distractions in your field of vision, of course it's unsafe. Most people are probably smart enough to ignore the popup message crap polluting automotive mutlifunction displays, by keeping their eyes up. If the crap follows them there, that's not an usafe display mechanism, that's unsafe human interface design. </rant>