TRL has undertaken a major programme of research for the Department of the Environment, Transport and the Regions (DETR) to investigate the impact of traffic speed on the frequency of road accidents. This has built on results from a comprehensive review of relevant research, published by TRL in 1994. The evidence then available indicated a 5% increase in injury accidents per one mile/h increase in average speed. Extensive road-based and driver-based studies have been used to address the complex task of understanding the speed-accident relationship more fully. Statistical modelling has been used to develop relationships between: the accident frequency on urban and rural roads, and how it depends on the speed of traffic, the volume of traffic movement and characteristics of the road layout; the speeds at which individuals choose to drive and how often they have accidents. The report describes the collection and analysis of data and the models developed. These models allow accident changes to be predicted from the speed changes that might result, for example, from the introduction of speed management measures. The report illustrates the effect of the speed (and other) variables that determine accident frequency. The application of the results to the identification of priorities for speed management is then discussed. The overall potential for accident reduction from measures to restrain speed is large.