چالش ها در استفاده از مدل تعادل عمومی قابل محاسبه فضایی برای ارزیابی حمل و نقل

The use of spatial computable general equilibrium (SCGE) models for assessing the economic impacts of transport projects is one of the key items on the research agenda for project appraisal in the Netherlands. These models are particularly suitable for analysing indirect effects of transport projects through linkages between the transport sector and the wider economy. Potentially, according to the literature, indirect effects that are additional to first-order direct cost reductions can turn out to be up to almost 80% in magnitude of the direct impacts. Given the relevance of these models for policy appraisal, experiences with this new modelling approach are important to report. After two years of development and application of SCGE models for transport appraisal, we found that the translation of theory behind the spatial equilibrium models into practical model specifications and empirical applications is a challenging task, and may lead to problems in project appraisal in terms of inaccuracies in the assessment of impacts. This paper discusses some key challenges we encountered with the specification of the Dutch SCGE model RAEM. This chapter is especially useful for researchers developing SCGE applications for use in transport appraisal and those who want to get a better understanding of differences between theoretical and computable SCGE modelling.

In this paper we discuss a number of complexities in modelling changes in the economy of regions. We focus in particular on • changes that arise as a result of changes in the efficiency of transport processes • the modelling approach using transport and SCGE models. These problems in modelling have not yet received widespread attention, as the application of SCGE models for the appraisal of transport investments and policies is a new phenomenon. We describe 4 types of issues, explain the possible implications of neglecting these issues and, where relevant, propose approaches for their resolution. These issues concern 1) interfacing problems between SCGE and transport models, 2) the modelling of the influence of transport costs on sectoral production, 3) the intepretation of the conventional, micro-level specification of product variety in aggregate applications and 4) the problem of irrational agglomeration effects in economic activities. Our main conclusion concerning these points are as follows: 1. In order to have a consistent linkage between transport and SCGE models, the main variable that forms this linkage – transport costs- deserves special attention. We firstly observe that there is a severe lack of empirical data on the consumption of transport services by various sectors of industry. Secondly, we identify two cases of a possible mismatch in the definition of transport costs, as they are produced by transport models, and as they should enter SCGE models. Thirdly, we describe how the use of 4 step transport models may introduce inconsistencies in appraisal results. 2. The use of iceberg transport costs is theoretically convenient and empirically acceptable in the case of a one-sector economy. In a multi-sector economy, it may lead to strange results: an underestimation of the impacts in precisely those sectors that are most sensitive to the reduction of transport costs at hand. Moreover, estimations of factor costs involved in transport are based on the wrong production functions and therefore incorrect. 3. We find that the interpretation of the variable ni,s is less straightforward for our purpose of application than often presented in the literature. This variable seems to be merely a parameter in the regional demand function but of no importance in policy analysis. Moreover it was found that this variable is a theoretical abstract. The implication is that it cannot be used in the calibration, nor is the generally used condition of a large value for n or N still valid. 4. We propose a new method to take hysteresis and locational boundedness of production into account. This is necessary to have a more realistic policy analysis, particularly when it comes to predicting changes in spatial patterns of production and consumption. As a concluding remark, we feel that such a critical, yet constructive evaluation of the application of SCGE modelling for transport appraisal purposes is a necessary task for the research community. The advent of SCGE modelling, beside improving our insight in how regional economies interact, also includes a promise of improved quality of appraisal results for transport investments and policies. New research into the critical interface with transport modelling is needed, however, for this promise to materialize.