In case of a crash, higher speed results in greater impact, which increases the risk of serious injury [1] [2] [3] [4]. Moreover, at higher speeds, the braking distance is longer, there is less time to process and react to information [2] and, consequently, less opportunity to prevent a crash. Finally, at higher speeds, there is a large amount of information drivers have to process in a short time. If too much information is provided, they more or less automatically focus on information in their central field of vision. This focus is detrimental to observation and processing of information in their peripheral field of vision [5] [6]. Driving fast is sometimes supposed to benefit road safety, since it will make driving less boring and will (thus) increase alertness. However, there is no scientific proof for this supposition. At any rate, the net effect of higher speeds on road safety is negative (see the question Does driving speed affect alertness?).
In all, this implies that when the average speed on a road increases, crash risk also increases as does the risk of a serious outcome (see the question How does speed affect road safety?).
Absolute speed is not the only thing at issue here; speed differences also affect safety. Speed differences involve more encounters with other road users, more lane changes, and more overtaking manoeuvres (see the question How do speed differences affect road safety?).
