Lausanne-Morges agglomeration will experiment strong increases in the number of inhabitants and jobs until 2020. The additional trips generated by these increases will have to be carried out in a larger proportion by soft mobility and transit. There are many projects aiming at facilitating this orientation, in particular projects of important transit lines. As these projects are very expensive, it is important to evaluate their impact before making a choice. This must not be delayed, in order to get Federal funds. However the existing data concerning the demand are limited. For the past, one has data of the federal census of 2000 on commuters trips, as well as cars and transit passengers counts for 2005. For the future, there are estimates of the forecasted increases for population and employment by zone. A modal shift model was established and calibrated for 2005, in order to have a better idea of transit and cars trips in 2020. The main feature of the approach is related to the lack of behavioural data, and the mere availability of a estricted number of data. For example, the parking data, very important to explain the modal split, are not available. Nevertheless it was possible to identify a model exploiting the few data available. We will discuss its limitations and present some hints in order to improve it.