See how little time you actually save — and how much further you'd travel before stopping. A digital Paceometer, referred to regularly by Rory Sutherland.
Why 0.67s? It's the alert-driver figure baked into the Highway Code stopping-distance tables (20 mph → 6 m thinking distance = 0.67 s). Real reactions are usually slower — ~1.5 s alert, 2 s+ when tired or distracted.