I have a nav system that talks, but I still have to glance down occasionally. I'd much rather have it in my peripheral vision.

It doesn't matter where it's displayed, looking at a map while driving takes attention away from the road.

In the days of paper maps everyone knew it was silly and dangerous to try to unfold and read one while driving. I don't know how this common sense has disappeared just because of LCD screens.

Not if the map is displayed in your field of vision. This is why airplanes have a display projected onto the glass.

I drove a Corvette with a heads up display once, and it was great not having to glance down at the gauges.

And also, you still have to glance down while driving to see how fast you're going, so clearly glancing off the road isn't that big of a deal.

A HUD does steal from outward-facing attention. It's ok in aircraft because the forward environment for aircraft is far less cluttered than for a car. That's also why self-driving cars are so much harder to do well than self-flying aircraft.