On fitting mode specific constants in the presence of new options in RP/SP models

We treat the problem of fitting alternative specific constants (ASC) in models estimated with a mixture of revealed preference (RP) and stated preference (SP) data to forecast the market shares of new alternatives. This important problem can have non-trivial solutions, particularly when some of the SP alternatives are completely revamped versions of existing ones (i.e., an advanced passenger train replacing a normal railway service). As there is no explicit treatment of this problem in the literature we examined it in depth and illustrated it empirically using data especially collected to analyse mode choice in a corridor to the West of Cagliari. We propose a hopefully useful guide to this art (as no practical recipes seem to serve all purposes). Careful specification of the systematic component of utility functions in RP and SP, including the ASC, serves to illuminate the true nature of the underlying error structure in the different data sets, yielding superior forecasting models.