Improving comparability of survey results through ex-post harmonisation - A case study with twelve European national travel surveys

An essential prerequisite for research, decision making and effective policies in the field of sustainable transport are reliable data on travel behaviour. In particular comparative analyses over space or time allow for a better understanding of transport systems and their impact on travel behaviour. Further, there is an increasing need for comparable transport indicators at the international level. All along, National Travel Surveys (NTS) have been used to perform such analyses and to generate respective indicators. Despite their similar intention to elicit basic information on travel behaviour, there are considerable differences in terms of the overall survey design across countries. While this variety clearly reflects requirements of the stakeholders and national particularities, it is unclear to which degree the results are comparable. In order to obtain comparable indicators that reflect behavioural differences rather than methodological ones, in the context of the COST Action SHANTI (Survey Harmonisation with New Technologies Improvement, TUD0804) an ex-post harmonisation approach was developed using microdata from twelve European NTS. The paper presents both concept and basic principles of ex-post harmonisation, beginning with some general reflections on the potential impact of survey characteristics such as methodology, coverage and definitions on results. The process of ex-post harmonisation as performed by SHANTI is briefly outlined, followed by a discussion of benefits as well as challenges and current limitations. As long as international standardisation in terms of ex-ante harmonisation of survey methods is not taking place, ex-post harmonisation is considered as pragmatic and important approach to utilise data previously incomparable