Reaction Time and Road Safety

Reaction Distance – How far you travel before you react

Reaction distance is the distance your vehicle has to travel from the time you detect a hazard until you start braking. It is an important part of the total stopping distance and is affected by several factors, such as your reaction time and the vehicle's speed. Here you will learn all about reaction distance and how to calculate it.

What is reaction distance?

Reaction distance is the distance you drive during the time it takes for you to react to a hazard and start braking. Your reaction time is affected by factors such as:

• Fatigue: When you are tired, your reaction time increases.
• Alcohol and drugs: Affect your ability to react quickly and correctly.
• Attention: Being distracted by mobile phones or other factors can increase your reaction time.

An average reaction time is about 1 second under normal conditions.

How do you calculate reaction distance?

The formula for calculating reaction distance is:

Reaction distance = (speed in km/h ÷ 3.6) × reaction time (in seconds)

Explanation of the formula:

• Speed divided by 3.6: This converts speed from kilometres per hour (km/h) to metres per second (m/s).
• Multiply by reaction time: Reaction time is the time it takes for you to detect a danger and take action, usually 1 second in normal cases.

Example: Reaction distance at different speeds and reaction times

Reaction time: 1 second

• 50 km/h: ( (50 ÷ 3.6) × 1 = 13.9 meters )
• 70 km/h: ( (70 ÷ 3.6) × 1 = 19.4 meters )
• 90 km/h: ( (90 ÷ 3.6) × 1 = 25 meters )

Reaction time: 2 seconds

• 50 km/h: ( (50 ÷ 3.6) × 2 = 27.8 meters )
• 70 km/h: ( (70 ÷ 3.6) × 2 = 38.9 meters )
• 90 km/h: ( (90 ÷ 3.6) × 2 = 50 meters )

Reaction time: 3 seconds

• 50 km/h: ( (50 ÷ 3.6) × 3 = 41.7 meters )
• 70 km/h: ( (70 ÷ 3.6) × 3 = 58.3 meters )
• 90 km/h: ( (90 ÷ 3.6) × 3 = 75 meters )

A quick method to estimate reaction distance

To quickly calculate reaction distance, you can use this rule of thumb:

1. Remove the zero from the speed (in km/h).
2. Multiply the remaining number by 3.

Example:

• At 50 km/h: ( 5 × 3 = 15 meters )
• At 70 km/h: ( 7 × 3 = 21 meters )
• At 90 km/h: ( 9 × 3 = 27 meters )

It is a simple method that gives an approximate estimate of the reaction distance at 1 second reaction time.

How is the reaction distance affected by speed?

The reaction distance increases linearly with speed. This means that if you double the speed, the reaction distance also doubles.

Example:

• At 50 km/h and 1 second reaction time: 13.9 meters
• At 100 km/h and 1 second reaction time: 27.8 meters

The faster you drive, the further your vehicle travels before you even start braking.

The relationship between reaction distance, braking distance and stopping distance

The reaction distance is part of the total stopping distance, which consists of:

• Reaction distance: The distance you travel before you start braking.
• Braking distance: The distance your vehicle travels from when you start braking until it stops.

Formula for stopping distance: Stopping distance = Reaction distance + Braking distance

It is important to understand this relationship in order to adjust your speed and maintain a safe distance.

Why is it important to understand reaction distance?

Being aware of reaction distance makes you a safer driver. It helps you:

• Adjust your speed: Higher speeds give you longer reaction distance.
• Keep your distance: Understanding reaction distance allows you to maintain a safe distance from the vehicle in front.
• Avoid accidents: Being aware of your reaction time allows you to act faster in critical situations.