Kinetic energy is a term that describes the energy that a car has due to its mass and velocity. Although the formula is simple, it say very much.
Ec = ½ m x v2
Therefore, the kinetic energy of the car will increase with the square of velocity (mass remains, of course, constant). This means that if the speed doubles, kinetic energy will increase four times. This increase in energy does not create any problem until is will come a necessary dissipation (braking / stopping) or fast forwarding (change of direction).
One way in which kinetic energy can be dissipated quickly is the impact with a solid object. In this case, when the speed is doubled, there will be four times more kinetic energy to destroy the car and injured its passengers. Kinetic energy dissipation can be done either by impact, as well as using the brakes. Braking distance is related to the square of the speed; therefore, a car moving at a speed of 50km/h will require a braking space four times greater than if the same car would move at a speed of 25km/h.