The reaction distance is the distance covered by the vehicle:

1. From the moment the driver eases off his step on the accelerator until the vehicle stops.
2. Correct answer: From the moment the driver notices the danger until he starts to apply the action he decided to take.
3. From the moment the driver steps on the brakes until the vehicle stops.
4. From the moment the driver notices the danger until the vehicle makes a complete stop.

WhyReaction distance runs from the moment the driver notices the danger until they start carrying out the action they decided on.