Reaction time determines thinking distance

Definition

Reaction time

Reaction time is the time between detecting a stimulus and beginning a response.

Definition

Thinking distance

Thinking distance is the distance a vehicle travels during the driver's reaction time.

A reaction begins when receptors detect a stimulus, the nervous system processes it and muscles produce a response.
Typical measured human reaction times are about $s0.2\ \text{s}$ to $s0.9\ \text{s}$ , but the value varies between people and conditions.
Thinking distance is calculated using $d = v t$ , where $v$ is the vehicle's speed and $t$ is the driver's reaction time.

Practical

Measuring human reaction time

Aim: to measure reaction time, compare repeated results and evaluate the reliability of the method.
Apparatus: a $cm30\ \text{cm}$ or $m1\ \text{m}$ ruler, a chair and table, and a results table; an online reaction timer may be used as a second method.
Variables: the tested condition is the independent variable, reaction time is the dependent variable, and the same participant, catching hand, ruler, release height, posture, instructions and surroundings are controlled.
Method, ruler drop:
- Seat the participant with their forearm resting on the table and their thumb and first finger level with the ruler's $cm0\ \text{cm}$ mark.
- Hold the ruler vertically with the $cm0\ \text{cm}$ mark between the participant's open fingers without touching them.
- Release the ruler without warning and without giving any visual or verbal cue.
- Record the distance fallen at the point where the participant catches the ruler.
- Repeat at least five times, allow brief rests, identify any anomalous result and calculate the mean distance.
- Convert each distance $s$ from centimetres to metres and calculate reaction time using $t=2sgt=\sqrt{\dfrac{2s}{g}}$ , with $m/s2g=9.8\ \text{m/s}^2$ , or use a supplied conversion table.
Alternative electronic method: use a computer reaction timer that changes the screen after a random delay, click as soon as the signal appears, repeat at least five times and calculate the mean.
Expected pattern: healthy alert participants commonly obtain reaction times between about $s0.2\ \text{s}$ and $s0.9\ \text{s}$ , with ruler-drop values often clustered near the lower part of this range.
Quality: random release timing prevents anticipation, repeats reduce random uncertainty, the same operator improves consistency, and comparing two methods helps identify systematic delay from the apparatus.
Uncertainty: anticipation, inconsistent finger position, reading the wrong ruler mark, practice, tiredness and screen or mouse delay can change the result.
Improvement: use more repeats, randomise the delay, discard only justified anomalies, test conditions in a random order and use several participants before drawing a general conclusion.
Safety: keep the seated participant's arm supported, keep the floor clear and do not test the effects of alcohol, illegal drugs or unsafe tiredness.

Stopping distance has two parts

Definition

Stopping distance

Stopping distance is the total distance travelled from the point a driver detects a hazard until the vehicle comes to rest.

Definition

Braking distance

Braking distance is the distance a vehicle travels after the brakes are applied until it comes to a stop.

The relationship is $distance\text{stopping distance}=\text{thinking distance}+\text{braking distance}$ .
Thinking distance ends when braking begins, so the two distances are consecutive and must be added.
At constant reaction time, thinking distance is directly proportional to speed because $d = v t$ .

Typical stopping distances

$mph20\ \text{mph}$ : $m6\ \text{m}$ thinking distance and $m6\ \text{m}$ braking distance give $m12\ \text{m}$ in total.
$mph30\ \text{mph}$ : $m9\ \text{m}$ thinking distance and $m14\ \text{m}$ braking distance give $m23\ \text{m}$ in total.
$mph40\ \text{mph}$ : $m12\ \text{m}$ thinking distance and $m24\ \text{m}$ braking distance give $m36\ \text{m}$ in total.
$mph50\ \text{mph}$ : $m15\ \text{m}$ thinking distance and $m38\ \text{m}$ braking distance give $m53\ \text{m}$ in total.
$mph60\ \text{mph}$ : $m18\ \text{m}$ thinking distance and $m55\ \text{m}$ braking distance give $m73\ \text{m}$ in total.
$mph70\ \text{mph}$ : $m21\ \text{m}$ thinking distance and $m75\ \text{m}$ braking distance give $m96\ \text{m}$ in total.
These values are general guides because attention, road conditions, weather, tyres and brake condition can change the actual distance.

Calculating total stopping distance

Example

Given: a car travels at $m/s18\ \text{m/s}$ , the reaction time is $s0.75\ \text{s}$ and the braking distance is $m32\ \text{m}$ .
Thinking distance: $md=vt=18\times0.75=13.5\ \text{m}$ .
Stopping distance: $m13.5+32=45.5\ \text{m}$ .
The total stopping distance is $m45.5\ \text{m}$ .

Reaction-time evidence must be evaluated

A mean from repeated readings is more reliable than one measurement because random variation has less effect.
A result for one person cannot represent all drivers because age, alertness, practice and individual differences affect reaction time.
A ruler-drop test measures a simple visual response and does not reproduce every decision involved in recognising a road hazard.

Exam technique

For a reaction-time method, state what is measured, explain why the release must be unexpected, include repeats and a mean, and identify a controlled variable.
For stopping-distance calculations, calculate thinking distance first and then add the braking distance.

Distance begins before the brakes act

A driver with a longer reaction time travels farther before braking even when the vehicle and road are unchanged.
Increasing speed increases the distance travelled during every second of reaction time.

Self review

Define reaction time.
State the typical range of human reaction times.
How is thinking distance calculated?
What two distances make up stopping distance?
Why should a ruler-drop measurement be repeated?

Reaction time determines thinking distance

Definition

Reaction time

Reaction time is the time between detecting a stimulus and beginning a response.

Definition

Thinking distance

Thinking distance is the distance a vehicle travels during the driver's reaction time.

A reaction begins when receptors detect a stimulus, the nervous system processes it and muscles produce a response.
Typical measured human reaction times are about $s0.2\ \text{s}$ to $s0.9\ \text{s}$ , but the value varies between people and conditions.
Thinking distance is calculated using $d = v t$ , where $v$ is the vehicle's speed and $t$ is the driver's reaction time.

Practical

Measuring human reaction time

Aim: to measure reaction time, compare repeated results and evaluate the reliability of the method.
Apparatus: a $cm30\ \text{cm}$ or $m1\ \text{m}$ ruler, a chair and table, and a results table; an online reaction timer may be used as a second method.
Variables: the tested condition is the independent variable, reaction time is the dependent variable, and the same participant, catching hand, ruler, release height, posture, instructions and surroundings are controlled.
Method, ruler drop:
- Seat the participant with their forearm resting on the table and their thumb and first finger level with the ruler's $cm0\ \text{cm}$ mark.
- Hold the ruler vertically with the $cm0\ \text{cm}$ mark between the participant's open fingers without touching them.
- Release the ruler without warning and without giving any visual or verbal cue.
- Record the distance fallen at the point where the participant catches the ruler.
- Repeat at least five times, allow brief rests, identify any anomalous result and calculate the mean distance.
- Convert each distance $s$ from centimetres to metres and calculate reaction time using $t=2sgt=\sqrt{\dfrac{2s}{g}}$ , with $m/s2g=9.8\ \text{m/s}^2$ , or use a supplied conversion table.
Alternative electronic method: use a computer reaction timer that changes the screen after a random delay, click as soon as the signal appears, repeat at least five times and calculate the mean.
Expected pattern: healthy alert participants commonly obtain reaction times between about $s0.2\ \text{s}$ and $s0.9\ \text{s}$ , with ruler-drop values often clustered near the lower part of this range.
Quality: random release timing prevents anticipation, repeats reduce random uncertainty, the same operator improves consistency, and comparing two methods helps identify systematic delay from the apparatus.
Uncertainty: anticipation, inconsistent finger position, reading the wrong ruler mark, practice, tiredness and screen or mouse delay can change the result.
Improvement: use more repeats, randomise the delay, discard only justified anomalies, test conditions in a random order and use several participants before drawing a general conclusion.
Safety: keep the seated participant's arm supported, keep the floor clear and do not test the effects of alcohol, illegal drugs or unsafe tiredness.

Stopping distance has two parts

Definition

Stopping distance

Stopping distance is the total distance travelled from the point a driver detects a hazard until the vehicle comes to rest.

Definition

Braking distance

Braking distance is the distance a vehicle travels after the brakes are applied until it comes to a stop.

The relationship is $distance\text{stopping distance}=\text{thinking distance}+\text{braking distance}$ .
Thinking distance ends when braking begins, so the two distances are consecutive and must be added.
At constant reaction time, thinking distance is directly proportional to speed because $d = v t$ .

Typical stopping distances

$mph20\ \text{mph}$ : $m6\ \text{m}$ thinking distance and $m6\ \text{m}$ braking distance give $m12\ \text{m}$ in total.
$mph30\ \text{mph}$ : $m9\ \text{m}$ thinking distance and $m14\ \text{m}$ braking distance give $m23\ \text{m}$ in total.
$mph40\ \text{mph}$ : $m12\ \text{m}$ thinking distance and $m24\ \text{m}$ braking distance give $m36\ \text{m}$ in total.
$mph50\ \text{mph}$ : $m15\ \text{m}$ thinking distance and $m38\ \text{m}$ braking distance give $m53\ \text{m}$ in total.
$mph60\ \text{mph}$ : $m18\ \text{m}$ thinking distance and $m55\ \text{m}$ braking distance give $m73\ \text{m}$ in total.
$mph70\ \text{mph}$ : $m21\ \text{m}$ thinking distance and $m75\ \text{m}$ braking distance give $m96\ \text{m}$ in total.
These values are general guides because attention, road conditions, weather, tyres and brake condition can change the actual distance.

Calculating total stopping distance

Example

Given: a car travels at $m/s18\ \text{m/s}$ , the reaction time is $s0.75\ \text{s}$ and the braking distance is $m32\ \text{m}$ .
Thinking distance: $md=vt=18\times0.75=13.5\ \text{m}$ .
Stopping distance: $m13.5+32=45.5\ \text{m}$ .
The total stopping distance is $m45.5\ \text{m}$ .

Reaction-time evidence must be evaluated

A mean from repeated readings is more reliable than one measurement because random variation has less effect.
A result for one person cannot represent all drivers because age, alertness, practice and individual differences affect reaction time.
A ruler-drop test measures a simple visual response and does not reproduce every decision involved in recognising a road hazard.

Exam technique

For a reaction-time method, state what is measured, explain why the release must be unexpected, include repeats and a mean, and identify a controlled variable.
For stopping-distance calculations, calculate thinking distance first and then add the braking distance.

Distance begins before the brakes act

A driver with a longer reaction time travels farther before braking even when the vehicle and road are unchanged.
Increasing speed increases the distance travelled during every second of reaction time.

Self review

Define reaction time.
State the typical range of human reaction times.
How is thinking distance calculated?
What two distances make up stopping distance?
Why should a ruler-drop measurement be repeated?

2.5.1 Stopping distance and reaction time

Reaction time determines thinking distance

Measuring human reaction time

Stopping distance has two parts

Typical stopping distances

Calculating total stopping distance

Reaction-time evidence must be evaluated

Distance begins before the brakes act

Review 2.5.1 Stopping distance and reaction time by teaching Genie

2.5.1 Stopping distance and reaction time

Reaction time determines thinking distance

Measuring human reaction time

Stopping distance has two parts

Typical stopping distances

Calculating total stopping distance

Reaction-time evidence must be evaluated

Distance begins before the brakes act

Review 2.5.1 Stopping distance and reaction time by teaching Genie

2.5.1 Stopping distance and reaction time

Reaction time determines thinking distance

Measuring human reaction time

Stopping distance has two parts

Typical stopping distances

Calculating total stopping distance

Reaction-time evidence must be evaluated

Distance begins before the brakes act

Review 2.5.1 Stopping distance and reaction time by teaching Genie

2.5.1 Stopping distance and reaction time Revision Guide

2.5.1 Stopping distance and reaction time

Reaction time determines thinking distance

Measuring human reaction time

Stopping distance has two parts

Typical stopping distances

Calculating total stopping distance

Reaction-time evidence must be evaluated

Distance begins before the brakes act

Review 2.5.1 Stopping distance and reaction time by teaching Genie

2.5.1 Stopping distance and reaction time Revision Guide