Travel Survey Design and Description of Multimodal Trips

A majority of the trips are made by just one transport mode that is mostly the car, walking, or –in some countries– the bicycle. For a small proportion of the trips, generally a few percent, two or more modes are used. The latter include particularly trips where a collective mode (e.g. bus, train, airplane) is the main mode and the trip distance is rather long. In travel segments with high shares of collective public transport and rather long trip distances, the proportion of multimodal trips can exceed 50%. Knowledge on volume and characteristics of multimodal trips is important, among others, for policymakers that aim at promoting public transport, for those who have to plan and design facilities at train stations and other nodes where people make interchanges, and for those who want to simulate multimodal travelling in transport models.
Information on the use of transport modes, including multimodal use, is generally asked for in the national and regional travel surveys. Because of the relative complexity of multimodal trips, reporting on such trips is more troublesome and may be less accurate than reporting on unimodal trips. As a consequence, multimodal trips demand for more effort to achieve accurate information. Consecutively, the quality of the registration of multimodal trips may depend relatively strong on the set-up of the survey.
The paper explores the impact of travel survey design on the quality of the registration of multimodal trips using survey data from the Netherlands. This country has a long tradition in surveying mobility behaviour; since the start in 1978 every year a large survey has been conducted. In the long period after 1978, the survey design has a few times been strongly adapted. The adaptations caused several trend breaks in registered travel behaviour, including huge changes in the performance of multimodal travelling. The paper analyses the relation between the survey design and the quality of the description of multimodal trips and gives some recommendations about how an accurate registration of multimodal trips can be achieved. It also shows which aspects of multimodality are rather robust with respect to survey design changes, and which aspects are highly volatile. The most prominent example of the latter is the registration of interchanges between vehicles of the same mode (e.g. the train); an accurate registration of such interchanges makes high demands on the set-up of the survey.