I was involved in a technical review of adjusted projections for a region. To my surprise, some of the technical users were expecting higher numbers for some of the assigned functional classes. One of the common suggestions tend to dis like differential projections stating the projections from the calibrated models should be higher. Well, there are many questions that come to mind when we look at this assumption. Some of them include:
1) What is the average annual growth rate of demographics especially related to the model inputs for the region where lower forecasts are observed?
2) How about some of the newer infrastructure development projects added to the existing landuse; some of those projects if large enough may change the framework of the trip generation process.
3) How about the synchronic inertia that exists between the mutual interaction of these new projects, diverse route choice, and socio-economic characteristics.
In any case, throwing the blames on mathematical models always sound easy; in reality the picture may show something different.
It is therefore important to have validated input data; especially the demographics should be validated thorougly. The complex models though calibrated and validated, are heavily dependent on input data.....
I hope we will be able to explain this to the people in professional practice one day where they will not expect model to be the "solution generator", in stead they would take it as one of the many available tools used in the infrastructure planning process.
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