Radar
Side by side, with no surprises: you always know where the other car is
What it is for
The most frustrating crashes come from a car your mirrors didn't show. The radar lights up the side it's on: amber while it's nearby, red once you're side by side.
There is nothing on it to read, no numbers, only a side and a color. That is what lets it be noticed at the edge of your vision while your eyes stay on the apex.
Where you change it
Each change takes effect at once — there is no apply button.