## 4 Model Considerations
### Spatial Transferability  
Currently, the FEATHERS framework is operational at the so-called Subzone zoning system for Flanders (see Figure 2), which is defined in the form of a spatial hierarchy composed of three layers. In order of increasing detail, they are: Superzone (compatible with 327 municipalities of Flanders), Zone (consisting of 1145 administrative units at one level lower than municipality), and Subzone (consisting of 2386 virtual areas according to homogeneous characteristics with an average area of 5.8 km\({}^{2}\)). Hence, using the current FEATHERS framework, the most detailed travel demand data can be obtained at the Subzone level. Although a large amount of useful information can be deduced at this layer, the size of each Subzone--an average area of around 6 km\({}^{2}\)--is still too large to avoid trip generation and attraction focusing too much on one point (e.g., TAZ centroid), which might result in unreasonable traffic allocation with some congested links and some empty links. Therefore, a zoning system with higher resolution is desired to implement the FEATHERS framework on. To this end, a Building Block (BB) layer is introduced. It is the most disaggregated geographical level currently applicable in Belgium, which consists of 10,521 units with an average area of 1.3 km\({}^{2}\). Taking the city of Leuven as an example, it is one of the Superzones in the geographical structure, which consists of 24 Subzones or 129 BBs (see Figure 3). To apply the FEATHERS framework in the BB zoning system, which contains the geographical layers of Zone, Subzone, and BB, some input data currently derived from the Subzone zoning system have to be updated accordingly. The information on the fundamental transportation system is one of the most important aspects, as it monitors the availability and performance of the transportation system between every two zones in the study area, such as the travel distanc  
Figure 1: The proposed regional light rail network surrounding Leuven. respect to different transport modes. All these data are represented by a set of Level of Service (LOS) matrices in the FEATHERS database. However, these data are not directly available for the BB level. Taking the car mode as an example, due to the fact that travel demand is highly dependent on the time of the day, the free-flow and the congested (morning and evening peak) travel time between every two building blocks have to be obtained, as they are not directly available in the current database. To obtain this information, we apply the network skimming tool in TransCAD (Version 5.0 R4 Build 2035, Caliper Corporation, Newton, MA, USA, 2015) to compute the travel distance and travel time for every two BBs based on the Flemish road network. More specifically, the Flemish road network provides all the road links with attribute values regarding travel distance and travel time. Thus, by identifying the travel path connecting every two BB centroids (based on the shortest travel time, for example), the total travel distance and free-flow travel time between every two BBs can be calculated. Apart from this, the activity-based schedule diary data, the synthetic population data, and some other environmental data (such as the land use system) have to be updated as well. For instance,  
Figure 3: Classification of Subzones and BBs in Leuven. Figure 2: Geographical structures of Subzone and BB zoning systems. the schedule diary data should cover the demographic (e.g., age, gender), socioeconomic (e.g., income, work status), activity (e.g., beginning time, duration and the location of the activity), household (e.g., number of cars, age of children), and travel information (e.g., transport mode, origin and destination of the journey), as well as the interrelationship between a multitude of facets of activity profiles on an individual level.