[usrusr]

The main benefit (or supposed benefit, if you insist) of the big encoder knob system is that decision and action are decoupled: you can have a short glance at the narrow/deep menu, then take your time doing the next steps of finger work with the eyes back on the road. This is incredibly slow compared to interfaces designed for full attention, but the impact on driving ability is very low. It's kind of like a garbage collector that sacrifices throughput for shorter pauses.

Touchscreens on the other hand lure the eyes into guiding the hand through the selection process, particularly when you lack a haptic reference point because the screen isn't handheld. And then the eyes are already focused on the screen when the next menu page shows up, so it's tempting to continue right there. I guess there must even be some threshold in UI slowness beyond which touchscreens are not quite as bad for safety as faster ones because one would quickly learn that there is plenty of time to focus on the road while the glacial computer's cogs are turning.

Even physical per-function controls are worse once the number of functions gets too high. Volume up/down is easy, but how do you {insert any of the hundreds of functions that you can make safely discoverable, in time, with a slow menu}? Interrupting the process of scanning a wall of buttons with tiny symbols on them for a context switch back to traffic and then continuing the scan without generous re-scanning on resume is a skill that few people would ever develop without explicit teaching, so most would simply take their chances, keeping their mind in the scan. Throughput might still be higher than a deep menu, but the effective pauses are way too long (a quick glance up just to stifle the worst fears while the mind is still on the screen/scan does not count). I would not even be surprised of the annoying inclusion of some frequently used functions in the menu wasn't just penny-pinching or a misguided interior designer striving for cleaner lines, but a deliberate decision to train users in the interface system.

[JohnFen]

> but the impact on driving ability is very low.

Are there studies to back this up? Because it sounds like that UI has a much higher cognitive load, which can be as bad as having to take your eyes off the road.