Stated preference data are used extensively in transportation to measure relationships that are hard to measure based on actual choices. Data are generally analysed assuming some specification of a utility function that is taken to represent the preferences of the respondents also in real life.These preferences are assumed to be rather stable over time, such that they can be used as a basis for forecasting and policy evaluation. It is becoming increasingly clear that this view is too simple. Specifically, evidence is being collected that preferences, at least in SP situations, are reference-dependent. Loss aversion has been advanced as an explanation. A clear reference situation is defined in many SP exercises. Under loss aversion, characteristics of the choice alternatives are compared to the reference one by one, the disutility of a loss is larger than the utility of a same sized gain. This model has been shown to describe SP data extremely well. It explains the large gap between willingness to pay and willingness toaccept that has been found repeatedly. This study investigates which factors influence the degree of loss aversion. Using data from a Danish value of time study, the influence on loss aversion of socioeconomics, trip characteristics and strength of the reference, comparing experiments involvingchosen and alternative transport mode were investigated. The results haveimplications both for the understanding of the phenomenon of loss aversion as well as for the design of SP experiments. A general question that this research may help in answering is whether it is pertinent to seek to strengthen or weaken the reference when designing experiments. For the covering abstract see ITRD E145999
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