This three part monograph aims to enable road safety researchers and professionals to interpret correctly the results of one of the main sources of knowledge about the effect of road safety engineering measures, the "observational Before-After study". Part I, Essentials - contains information the author regards as essential for forming an opinion of results obtained by others, and for planning and analysing such a study. This is written to be accessible to all. Part II, Adaptations of conventional approaches - explains how to avoid the errors and improve the results obtained from the predominant methods currently used. This Part employs algebra and statistical analysis. Part III, Elements of a new approach - presents new approaches to improve future methods of observation and analysis.