To put to bed the argument about the effects of an impact at 30mph compared to 20mph we just need to go back to the basic physics. An object moving at a given velocity (speed) will have a kinetic energy (energy due to its mass and motion) as shown below:


Kinetic energy (in joules) = ˝ MV2 - where M = mass (kg) and V = velocity (m/s)

30 mph = 13.4 m/s (meters per second) - 20 mph = 8.9 m/s

Weight of average sized car = 1900kg


At 30 mph the kinetic energy of a car is:

0.5 x 1900 x (13.4 x 13.4) = 170,582 J


At 20mph the kinetic energy of a car is

0.5 x 1900 x (8.9 x 8.9) = 72,250 J


That is a percentage difference of 57.6%. Therefore a car striking a pedestrian at 30mph will have more than double the energy of a car travelling at 20mph. This is because the energy is dependant on the square of the speed.