Excessive or inappropriate speed is the leading cause of fatal accidents. Law enforcement officials report this factor in one in three fatal accidents. In fatal accidents, this speed factor occurs more often than the average on roads limited to 70 km/h, which probably reflects the difficulty for drivers to perceive the specificity of these sections and to become aware of the associated dangers.
It's less present on motorways limited to 130 km/h. Speed affects the driver's ability to adapt to encountered situations and unexpected events (distance travelled during the reaction time, possible or not, then braking distance), and his injury severity (energy dissipated in the impact).
Despite technical progress, vehicles aren't designed to withstand impacts at high speeds (above 55 km/h at impact). Equipment, belts or airbags, and the absorption of energy by deformation, are not sufficient to protect the internal organs of human body. However, most fatal accidents for passenger vehicle occupants occur at residual speeds (after braking) between 40 and 80 km/h.