Stopping Distance Calculator

About the Stopping Distance Calculator

Total stopping distance is the reaction distance (travelled while the driver reacts) plus the braking distance (travelled while decelerating to a stop).

Enter the initial speed (m/s), the driver reaction time (s, typically ~1.5 s) and the braking deceleration (m/s², ~7 on dry road). The tool returns reaction, braking and total distances.

How to use this calculator

1. Enter the vehicle speed, making sure the units match the calculator.
2. Enter the reaction time, which is the delay before braking starts.
3. Enter the deceleration, which is the braking strength while slowing down.
4. Read the stopping distance as reaction distance plus braking distance.

The formula explained

The formula computes total stopping distance, where the first part, \(v \cdot t_r\), is the reaction distance and the second part, \(\frac{v^2}{2a}\), is the braking distance.

• d = stopping distance
• v = vehicle speed
• \(t_r\) = reaction time
• a = deceleration

Step by step method

1. Find the reaction distance by multiplying speed \(v\) by reaction time \(t_r\).
2. Find the braking distance by squaring the speed \(v\), then dividing by \(2a\).
3. Add the two distances to get the total stopping distance.

Worked example

Problem. A car is moving at \(20\,\text{m/s}\), the driver's reaction time is \(1.5\,\text{s}\), and the deceleration is \(5\,\text{m/s}^2\). Find the stopping distance.

1. Reaction distance: \(20 \cdot 1.5 = 30\,\text{m}\).
2. Braking distance: \(\frac{20^2}{2 \cdot 5} = \frac{400}{10} = 40\,\text{m}\).
3. Total stopping distance: \(30 + 40 = 70\,\text{m}\).

Tips and common mistakes

• Make sure the speed, reaction time, and deceleration use compatible units, such as \(\text{m/s}\), \(\text{s}\), and \(\text{m/s}^2\).
• A larger reaction time or a higher speed increases stopping distance quickly, because speed is squared in the braking term.

Frequently asked questions