Household Travel Surveys (HTSs) have been conducted for more than 40 years throughout the world; however, their potential for analysing travel behaviour for different regions over time has not been fully realised. This paper develops a method for ex-post harmonising mixed-method, cross-sectional HTSs. The harmonisation includes the areas of survey coverage, survey definitions, and survey methods. The spatial harmonisation, based on the density of residents, and the temporal harmonisation, based on the definition of comparable survey periods, are introduced as further steps for harmonisation. The harmonisation method is developed and successfully validated based on HTSs from five European capital cities: Berlin, Copenhagen, London, Paris, and Vienna. Key travel estimates correspond between the cities, and, except for walking, the developments over time are consistent. The result of the data harmonisation is a pooled database that has enormous potential for analysing travel behaviour based on historical HTSs.