What the exam asks here: Many learners calculate only the braking distance and forget that, before you even brake, a stretch of road has already passed.
Many learners think: The reaction distance is so small that in practice it can be ignored next to the braking distance.
The rule is: At 50 km/h the reaction distance is 13.75 m, against a braking distance of 12.50 m: almost equal, not negligible. Together they form the 26.25 m stopping distance
Many learners calculate only the braking distance and forget that, before you even brake, a stretch of road has already passed. That distance is not small: at 50 km/h it is almost as long as the braking distance itself. The book has its own rule of thumb for it — separate from the braking-distance formula.
The reaction second lasts on average 1 second (Book p.125) — the time between 'I must brake' and actually pressing the brake pedal. In that second, at 80 km/h you already cover 22.00 m, before the braking distance even starts.
The reaction second is a time (1 second on average), not a distance: it is the period between the moment you perceive the hazard and the moment you actually press the brake. The reaction distance is what you cover in that second, and has its own rule of thumb: ((speed:2) + 10%) : 2. At 80 km/h: ((80:2)+10%):2 = (40+4):2 = 22.00 m. At 130 km/h: ((130:2)+10%):2 = (65+6.5):2 = 35.75 m. This formula differs from the braking-distance formula (which squares speed); reaction distance grows roughly in direct proportion to speed, not quadratically. Double the speed, and the reaction distance roughly doubles too — very different from braking distance, which then becomes 4x as long.
Ignoring the reaction distance means systematically underestimating your own stopping distance: at 50 km/h the reaction distance (13.75 m) is almost as large as the braking distance (12.50 m) — together almost double the braking distance alone. On the N36 near Vriezenveen, where wildlife regularly crosses, that difference is exactly the margin between stopping in time and a collision.
At night on the N36 near Vriezenveen you are driving 100 km/h. What is your reaction distance per the book's rule of thumb?
Explain: what is the difference between the reaction second and the reaction distance, and why is the reaction distance not a negligible extra?
1 second reaction time ≈ ((speed:2)+10%):2 metres. 50→13.75m · 80→22.00m · 130→35.75m. Not a negligible extra.
One evening on the N36 near Vriezenveen a deer suddenly jumps onto the road. You are driving 80 km/h. Before you even press the brake pedal, how many metres has your car already covered?
The reaction second lasts on average 1 second (Book p.125) — the time between 'I must brake' and actually pressing the brake pedal. In that second, at 80 km/h you already cover 22.00 m, before the braking distance even starts.
Correct answer: 27.50 m
'1 second' is the reaction second (time); the reaction distance (metres) is ((100:2)+10%):2 = 27.50 m
Why learners get this wrong: The reaction second is a distance in metres, just like braking distance.
Correct answer: 11 m
Correct: 11.0 m (±1.5).
Correct answer: 35.75 m
Why learners get this wrong: The reaction second is a distance in metres, just like braking distance.
Correct answer: False
Why learners get this wrong: The reaction distance grows quadratically with speed, just like braking distance.
Correct answer: About 14 metres, more than half of your stopping distance
False — you think the reaction distance is negligible. At 50 km/h you travel about 13.75 m during your reaction time, almost as much as the braking distance of 12.50 m
Why learners get this wrong: The reaction distance is so small that in practice it can be ignored next to the braking distance.
Correct answer: 22 m
Correct: 22.0 m (±2.0).
Correct answer: True
Why learners get this wrong: The reaction second is a distance in metres, just like braking distance.
No. At 50 km/h the reaction distance is 13.75 m, against a braking distance of 12.50 m: almost equal, not negligible. Together they form the 26.25 m stopping distance
No. The reaction second is a TIME (1 second on average); the reaction distance is the METRES you cover in that time, via the rule of thumb ((speed:2)+10%):2. At 100 km/h that is 27.50 m, not '1 second'
No. The reaction-distance rule of thumb ((speed:2)+10%):2 is linear in speed: double the speed from 50 to 100 km/h, and the reaction distance goes from 13.75 to 27.50 m — also doubled, no quadratic jump like the braking distance