If the average speed on a road increases, crash risk also increases, as does the risk of a serious outcome. This is true in general terms, but more so when motorised vehicles crash with unprotected road users, such as pedestrians, cyclists and (light) moped riders. Furthermore, speed differences between vehicles at any one time or place are related to a higher crash risk. Drivers that maintain a speed that is higher than the average speed on that road run a higher crash risk; drivers that maintain a speed that is lower than average do not. Drivers often exceed speed limits, for example because they are in hurry, or they enjoy speeding, or go with the flow, or because they do not notice their speed. In general, we assume that almost one third of fatal crashes involves speeding or driving at a speed that is not appropriate to the circumstances.
Speed management consists of several steps, of which determining a safe limit is the first step. Which limit is safe, depends on the road function, traffic mix, and road layout. Always and everywhere, the limit should be credible (logical) and apparent. It can be supported by physical speed reduction measures, such as speed humps, and by traffic enforcement. Physical speed reduction measures greatly affect speed at location level. Intelligent Speed Assistance (ISA) greatly affects driving speed and therefore road safety, an intervening ISA system (which prevents the vehicle from exceeding the speed limit) being more effective than an advisory ISA system. On the main road network, Variable Message signs or Dynamic Lane Control Signs with dynamic speed limits contribute to more homogeneous speeds, fewer abrupt braking manoeuvres, and fewer crashes or near miss crashes. The effects of less stringent measures, such as educational measures or rehabilitation courses, publicity campaigns, community projects and nudging are often minor and short-term.