About The braking-distance rule of thumb: (speed:10)² : 2: which statement is WRONG?
If speed doubles, braking distance also doubles (a linear relationship).
The rule of thumb literally is '(speed(km/h):10) x (speed(km/h):10) : 2', with the book's own worked examples 50 km/h = 12.50 m, 80 km/h = 32.00 m, 130 km/h = 84.50 m.
trap: doubling error. Book p.126 - 'braking distance increases progressively: if you drive 2x as fast, your braking distance becomes 4x as long.' At 40 km/h braking distance is 8.00 m; at 80 km/h (2x as fast) it is 32.00 m — 4x, not double