Speed and Crash Severity
Small changes in speed produce large changes in outcome.
Synopsis
Speed affects both crash occurrence and injury severity. Managing speed remains one of the most effective road safety interventions.
Why this matters
Impact energy rises with the square of speed. A small speed reduction produces a disproportionately large safety benefit.
Expected outcome
You will understand how speed drives stopping distance and impact energy, and why safe-speed choices save lives.
Learning objectives
After completing this lesson learners should be able to:
- Understand stopping distances
- Explain impact energy
- Understand speed management
- Recognise survivability thresholds
Why Speed Matters
Higher speeds reduce available reaction time, increase stopping distance and increase impact energy. Speed is the single variable that touches every phase of a crash — before it, during it and after it.
Stopping Distance
Total stopping distance is reaction distance plus braking distance. Reaction distance grows in a straight line with speed; braking distance grows roughly with the square of speed. That is why a modest increase in speed produces a much larger increase in the distance needed to stop.
Impact Energy
Kinetic energy scales with the square of speed. Going from 40 km/h to 60 km/h raises impact energy by more than 2×, not 1.5×. Small increases in speed translate into disproportionately large increases in injury severity.
Pedestrian survivability
A pedestrian struck at 40 km/h has roughly a 90% chance of survival. At 60 km/h, that survival probability drops to around 20%. Same pedestrian, same driver — different speed, very different outcome.
Real-world scenarios
Traffic ahead slows unexpectedly
You are driving on a busy road when traffic ahead slows unexpectedly.
→ What is the safest response?
Show suggested response
Ease off, hold your lane and increase following distance. Extra separation preserves reaction time — the single biggest defence against a rear-end collision when the car in front brakes hard.
Key takeaways
- Speed influences survivability.
- Speed management saves lives.
- Safe speeds depend on conditions — not just posted limits.
- Small speed reductions produce large safety gains.
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Lesson 5 of 83 available · 20 min · India-specific