A higher speed increases the likelihood of an accident. Very strong relationship have been established between speed and accident risk: the general relationship holds for all speeds and all roads, but the rate of the increase of accident risk varies with initial speed level and road type. Large speed differences at a road also increase the likelihood of an accident. In the addition, drivers driving much faster than the average driver have higher accident risk; it is not yet evident slower driver.
That this is also the case of the high speed reduces the possibility to respond in time when necessary. People need time to process information to decide whether or not to react and finally to execute a reaction. At high speed the distance covered in this period is longer. At high the distance between starting to brake and a complete stand still is longer as well. The braking stand is proportional to the square of speed (v2). Therefore, the possibility to avoid a collision becomes smaller as speed increases.