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.
Knowledge check
Reinforcement only — not scored. Reveal the answer to check your understanding.
Q1. As speed increases, braking distance:
- Increases roughly with the square of speed ✓
- Stays about the same
- Falls because momentum carries the vehicle
- Depends only on the driver's height
Braking distance scales with the square of speed. Doubling speed roughly quadruples the distance needed to stop.
Q2. A pedestrian struck at 60 km/h compared with 40 km/h is:
- About equally likely to survive
- Far more likely to be killed ✓
- Slightly less likely to be injured
- Only affected if not wearing a helmet
Survival probability drops from around 90% at 40 km/h to around 20% at 60 km/h — impact energy rises with the square of speed.
Q3. The safest speed to travel at is:
- Always the posted speed limit
- The speed of the fastest surrounding vehicle
- The speed that suits the current road, traffic and weather conditions ✓
- The maximum speed your vehicle can safely reach
Posted limits are upper bounds for good conditions. Rain, fog, darkness or heavy traffic all call for a lower safe speed.
Complete this lesson
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Lesson 5 of 16 available · 20 min · India-specific