Browsing on the internet stumbled across this:
Otáčkoměr. Well, I know it's in Czech and very few would understand (if any), so here is uncle Google's help:
english version (google translate), and
french version (human translated)
It's an electronic device showing optimal/suboptimal/too high revolutions using series of two rotating lights. I like the idea it is visible even with one's peripheral vision.
The page itself is quite old, the contraption also - probably more than ten years. Anyway, I find it interesting idea.
What do you think?