23 on a demand study. How to pick a method also depends on this, as much as on the data availability and the type of mode the regulators is focusing on.

Assuming that money is not an issue, in addition to the specific traffic type to be forecasted and the specific type of nfrastructure concerned, the ideal model will depend on a data availability, on how much detail is needed and on the specific goals of the forecast. Data provided by the historical operators of the service will help. For the most aggregated data this is generally possible but for more detailed data this can be a problem. Indeed, many operators will consider a lot of the relevant information to be confidential and would see as a threat to their commercial viability the obligation to disclose some data on their markets and clients. But on the other hand, precisely in the context of privatization, demand studies should be undertaken with the historical operator in preparation for the valuation of the assets to be "privatized"; this will also constitute a good initial data base for a regulator. In most countries, a useful compelemtn is data published by public (statistical offices) or private entities monitoring the sector. It is surprising to causal observers the number of think tanks or private survey firms collect and sell relevant information collected through regular surveys. Finally, all this can be compelemented by data produced through field surveys. The enthousiams for revealed and stated preferences is also generating vasts databases which often end up under a pile of dust after their initial use. A better organization of data bases should allow these surveys to become good sources of information to any regulator. Ultimately, the choice of methods will be driven by the data availability and for many of the most promising methods they new surveys are the way to go and do not require more than a couple of months to collect field data.

While data matters, the amount needed depends often on how much details are really needed from a regulatory viewpoint The devil can often be in the details but the cost of a survey is directly related to the level of detail that is needed. Of course, this depends on the questions at stake . If the concerns of the regulator is the potential payoff from the addition of service requirements such as a toilet or a business center in a train station, there are not too many alternatives to targetted field survey. But in many more general issues regarding prospects, fairly aggregate data can be sufficient as seen in the case of the port of Antwerp. The fact is that often the specific concerns are not well defined and this can results in wastes of time and resources. Targetting the issues also allow a better targetting of the data requirements and hence the design of demand studies and the choice of a methodolohy that meet their purpose in the most cost effective way.

# _6.2. After the study_

Once the study is done, the job is not over. First, it makes sense to ensure that the validity of the results of the study can be assessed. A first benchmark consist in a comparison with th results experienced in similar situations elsewhere. It is of course difficult to find exact comparability but the "distance" between the situations can be evaluated and provide an idea of the "distance" between the results. It is often useful to request from the consulting team doing the analysis to provide a survey of their own studies or similar studies. This information should include a review of the elasticities of traffic to prices and cross-price elasticities when substitutes or complements are involved. In this context, conventional wisdom can also help and surveys can be consulted to provide first order estimates.<sup>2 5</sup> Last but not least, the study should provide a point estimate of course, or at least a narrow range. But it should also indicate how is the traffic modified under different assumptions for the exogeneous parameters, such as value of time, 25 For instance Oum,Water and Yong (1992) and Small and Winston (1999)