It ' s to do with the amount of energy the car has . Kinetic energy i . e . the energy of something moving is given by the formulaKE 0 . 5 mass speed 2 Now , this speed 2 bit is what ' s important , it ' s speed speed . We can ignore the mass of the car , because that doesn ' t change so let ' s just look at what happens if we compare speeds . At 50 kmh , speed 2 is 2 , 500 . At 60 kmh , speed 2 is 3 , 600 , at 70 kmh it ' s 4 , 900 and so on . Now , you can see that speed 2 doesn ' t rise at steady rate , it keeps getting bigger and bigger and bigger , at a faster rate than the speed itself does . Because speed 2 is getting bigger and bigger , so does the energy that the car will hit the pedestrian with . More energy Worse injuries . Edit To put it into a bit more context , at 60 kmh , the car has nearly 1 . 5 times the energy that it does at 50 kmh at 70 kmh , it ' s nearly double .