The estimates of the incidence of deaths and non-fatal injuries from road traffic crashes are essential inputs for prioritising national health and transport policies. This article sketches a methodology for assembling such estimates at the country level by piecing together data from a wide array of sources that include death registers, hospital records, funeral records, health surveys and police reports. Using examples of the types of data sources available in four developing countries (Iran, Mexico, Ghana and India), methods are proposed for making these consistent and extrapolating to estimates of injury incidence at the country level. This requires filling information gaps, mapping from varying case definitions, deriving population-based incidence estimates from sources that may not track denominator populations, and appropriately reapportioning cases assigned to poorly specified causes. The principles proposed here will form the methodological basis for a series of country reports to be published in the future.