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Speed and hitting pedestrians

When a car hits a pedestrian, speed decides how much energy is transferred. Kinetic energy rises with the square of the speed. That means a little more speed gives much worse injuries. That is why the limit is often 30 km/h where there are many pedestrians, and why you should slow down at crossings, schools and residential areas.

E = \tfrac{1}{2} m v^2

the kinetic energy

\dfrac{E_2}{E_1} = \left(\dfrac{v_2}{v_1}\right)^2

how many times larger the energy becomes

Symbols

$m$	mass of the car	kg
$v$	speed	m/s

Example

Compare 50 km/h with 30 km/h.

$\left(\dfrac{50}{30}\right)^2 \approx 2.8$ . At 50 km/h the car has almost three times as much energy as at 30 km/h.

Lower speed also makes the stopping distance much shorter, so you may not hit anyone at all.

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Part of Car licence (class B): Vulnerable road users and interaction.