1. For a driver, the stimulus might be brake lights ahead, a pedestrian stepping into the road, or a traffic light changing; the response is starting to press the brake pedal.
2. Reaction times vary from person to person, because people do not all process information and decide at the same speed; a typical human reaction time is in the range $0.2s$ to $0.9s$.
3. A shorter reaction time means a quicker response; this matters in driving because the vehicle keeps moving throughout the reaction time.

Factors that affect reaction time

1. A driver’s reaction time can be increased by tiredness, drugs and alcohol.
2. Distractions such as using a mobile phone, talking to passengers, or looking away from the road can also increase a driver’s reaction time.
3. These factors make the reaction time longer because they reduce the driver’s ability to notice the hazard, process what is happening, and begin the correct response.
4. A longer reaction time leads to a longer thinking distance, because the vehicle travels for longer before braking begins.

Measuring reaction time

1. A simple way to measure reaction time is the ruler-drop test.
2. The steps of the ruler-drop test are:
   1. one student holds a ruler vertically with the $0cm$ mark level with the other student’s open thumb and finger;
   2. the ruler is dropped without warning and the catcher closes their fingers as quickly as possible;
   3. the distance the ruler falls is recorded and converted to a reaction time, where a larger distance means a longer reaction time.
3. The test is repeated several times and a mean is calculated, and control variables are kept the same, such as using the same ruler and hand, the same start position, and giving no warning before the drop.
4. Only a qualitative understanding is needed: you should be able to describe the method and recall that typical reaction times are a few tenths of a second.

Reaction time and thinking distance

1. Thinking distance depends on the vehicle’s speed and the driver’s reaction time; for a given speed, a longer reaction time gives a longer thinking distance.
2. These are linked by $thinking\; distance=speed\times reaction\; time$.

Interpreting and evaluating measurements

1. You may be given results from a reaction-time test and asked to interpret them, comparing values with numbers rather than only words such as “higher” or “lower”.
2. For example, a student with a mean reaction time of $0.25s$ reacted faster than one with $0.40s$, because $0.25s$ is the smaller value.
3. Given data, you can evaluate the effect of tiredness, drugs, alcohol or distractions on thinking distance: if a factor doubles the reaction time while the speed stays the same, the thinking distance also doubles.
4. To improve the measurements, repeat and take a mean, ignore anomalous results, test a larger sample of students, control the variables, and use electronic timing to reduce uncertainty.