In the case of international research, it is essential that data are comparable or are made comparable. This leads to the conclusion that it is advisable to pursue harmonization of road safety data. This starts with definitions to be used (e.g. the 30-day definition of a road death) and also involves the harmonisation of methodologies for data collection (e.g. when collecting data about Safety Performance Indicators). In every case of international comparisons, comparability of data has to be ascertained.