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The increasing prominence of environmental sustainability as an objective of transport policy, and in people's thinking about their travel-related decisions, brings new challenges for data collection because, uniquely in the transport sector, it brings three issues to the fore: firstly, the difficulties associated with gathering data and opinions on a topic which is socially and morally charged; secondly, the fact that the resulting adjustments to behaviour may be in dimensions (such as choice of vehicle characteristics and mode choice for infrequent trips) which are not traditionally covered very fully in travel surveys; and thirdly, the likelihood that any change in behaviour may emerge only slowly over an extended period of time (as and when the opportunity arises for the individuals' behaviour to be adjusted to fit their aspirations).

It is argued that, because the topic is socially charged, data on sustainable travel behaviour is particularly prone to social desirability bias and other related biases. The nature and implications of these biases are addressed and it is concluded that individuals will tend to exaggerate the likelihood of behavioural change in response to sustainability concerns and policy initiatives. Methods, which might be used to study the emergence of sustainable patterns of behaviour in general, and responses to sustainability orientated policy initiatives in particular, are discussed and attention is given to ways of minimising bias in the data.

Purpose — The research was designed to explore people's willingness/ability to understand complex road user charges. However, the results raise issues about respondent engagement and ecological validity and so have important implications for questionnaire practice.

Methodology — Computer-based experiments administered in the United Kingdom and Germany gathered respondents' estimates of road user charges along with their response latencies, personal characteristics, acceptance of road charging, assessments of task complexity and attitudes to analytical tasks.

Findings — The results demonstrate questionnaire learning effects and show the effect of personal characteristics on the accuracy and speed of questionnaire completion. The tendency of males, younger people and students to complete the task more quickly is interesting as is the fact that fewer and smaller errors were made by participants who claimed to gain satisfaction from completing a task which has involved mental effort. Engagement was seen to vary with personal characteristics, attitudes to decision making, task complexity and acceptance of the policy being tested. A key finding is that disengagement was more evident among participants who were broadly supportive of road charging than among those who were not.

Implications — The findings have important implications for the design of data collection exercises and for the interpretation of resulting data. It is concluded that repeated choice experiments are an inappropriate source of data on responses to unfamiliar circumstances. The collection of data on response latencies and the inclusion of questions on respondents' attitudes to task completion is a strongly recommended addition to standard questionnaire practice. The extent to which disengagement in an experimental context is, or is not, indicative of real-world behaviour is an important and urgent subject for further research.