Speed is the single biggest factor in both the likelihood and severity of collisions. Understanding the physics makes clear why small speed reductions have dramatic effects on safety.
Kinetic Energy
A vehicle’s kinetic energy increases with the square of its speed. Double your speed and you have four times the energy to be absorbed in a crash. At 50 km/h a collision has roughly four times the destructive force of the same collision at 25 km/h. This energy must go somewhere — into deformation of metal and, ultimately, human bodies.
Stopping Distance
Stopping distance also increases with the square of speed. From 50 km/h, total stopping distance is approximately 25 metres (in dry conditions). From 100 km/h it is approximately 100 metres — four times as far, for twice the speed.
Pedestrian Survival Rates
When a vehicle hits a pedestrian at 30 km/h, survival is likely (around 90% chance). At 50 km/h, survival drops to about 50%. At 70 km/h, survival is unlikely. This is why 30 km/h zones exist in pedestrian areas and near schools.
Exam tip: The relationship between speed and stopping distance is quadratic, not linear. This appears in exam calculations — if speed doubles, stopping distance quadruples.