Braking Distance and Reaction Time

Automotive Engineering · CHAPTER 3 / 5

Level: introductory undergraduate · Study session: 15–20 minutes including practice.

Learning outcome: Explain the different speed dependence of reaction and braking distance.

Before you begin: Complete earlier chapters in this discipline; revise algebra and units as needed.

1. Understand the idea

Before braking begins, the vehicle continues moving during perception and reaction time. With constant deceleration on a level road, braking distance grows with speed squared. Real stopping depends on tyres, road condition, brake response, gradient and driver behaviour.

2. Write the model

dreaction = vt; dbrake = v²/(2a)

Define the symbols from the worked case before substituting values. Keep units consistent, record the assumptions and round only after the calculation.

3. Follow a worked example

For v=20 m/s, reaction time 1 s and deceleration 5 m/s², reaction distance is 20 m, braking distance 40 m and total 60 m.

4. Explore the model

Figure: calculated teaching cases; straight segments connect sampled points, not experimental observations. Read exact values below.

Illustrative scenario sweep — synthetic educational data
Initial speed (m/s)	Reaction at 1 s (m)	Braking at 5 m/s² (m)
10	10	10
20	20	40
30	30	90

Double speed from 10 to 20 m/s and compare each distance term. Braking distance quadruples in this model.

5. Reproduce it yourself

Copy the input cases into Excel or a notebook and name each quantity and unit.
Recalculate each result from the stated equation or rule. For qualitative cases, explain the state or partition decision instead.
Change one input, predict the direction of change, then calculate again.
Write one sentence explaining where the model stops being valid.

6. Check your understanding

What is total distance at 30 m/s under these assumptions?

Reveal the answer and check your work

120 m.

Compare your result, units and reasoning. If they differ, return to the equation and test the worked case first.

7. Common mistake

These illustrative values are not a safe-following-distance rule.

Further study and academic note

Explore the discipline reference and further learning resources. This is an original IIMEEE introductory teaching note with synthetic examples; it is not a peer-reviewed research result or a professional design specification. Figures were constructed from the listed teaching cases. Resource links support further study and do not imply endorsement.