International comparisons are frequently used to assess the road safety performance of a country or to monitor its development over time, at a national or regional level. In most instances either accident or injury risks have to be considered, which results in the need for a quantitative estimation of the amount of travel, namely traffic volumes. Only in special cases may risks be indirectly compared using methods of induced exposure. The required data on traffic volumes is not always readily available. This is especially true if old data or data from several countries for a long period are to be used, as in comparative time series analysis. Several simplifying assumptions can be adopted to bypass this lack of data but often the estimates resulting from these simple assumptions are not entirely satisfactory. Between 1996 and 1998 the European Union (EU) sponsored, within the scope of the 4th Research Framework Programme, a COST Action (COST 329) to analyse and establish a common methodological framework for description and analysis of safety developments-past and future-that could also represent the effect of interventions. Researchers from 14 European countries participated in this action. One activity carried out was the development of a method for estimating the traffic volume of countries where such data is not available, using data on vehicle fleet and fuel sales in the studied country and mathematical models fitted to existing data (from other countries) on fuel consumption, vehicle fleet and traffic volume. In the European context, this method is especially useful for some Eastern and Southern European countries. In this paper the mentioned method is described, the results of its application to Portugal are presented and ways to use it in other countries to estimate their traffic volumes for the period 1980-2000 are described.