Speed is a divisive issue not just for the public but for the traffic engineering community as well. On the one hand faster speeds are viewed as a positive attribute of the road network, suggesting that mobility is optimized and efficiencies are at their peak. On the other hand, faster speeds are seen as detrimental to road safety, and an attribute of the system that creates a barrier to comfortable and safe movement for pedestrians and cyclists. The conventional wisdom concerning the relationship between speed and crash risk has been somewhat fractured, and this in part has fuelled the burning debate concerning speed, and establishing speed limits and target operating speeds. While the traffic engineering community has consistently concurred that crash severity increases as operating speeds increase, the community is less agreeable concerning the effect of operating speed on crash probability/occurrence. For a long time, traffic professionals have advocated that the probability of a crash follows a U-shaped curve, where excessively low and exceedingly high speeds have higher crash probabilities than moderate operating speeds. The most recent, and statistically robust, research has indicated that this long held belief concerning speed and crash risk is not valid, and that crash probability also increases with operating speed. The purpose of this paper is to articulate the speed and crash relationship by presenting the statistical evidence from the available research. In addition to the direct examination of correlation between speed and crash risk, the paper will also examine some of the impacts that operating speed has on road user behaviour, and how these impacts help establish the causal relationship between speed and safety.
