# BIM-Based Description of Intelligent Transportation Systems for Roads
## 4 BIM-Based Description of ITS for Roads
### An IFC Schema Extension for ITS for Roads  
Figure 7 shows the proposed IFC schema extension. The entities, property sets, and an enumeration data type, which are newly proposed in this study (as described below), are shaded in gray. The IFC schema follows an object-oriented approach based on the EXPRESS data modeling language, which is standardized in [52]. The IFC schema contains a hierarchy of several layers, in which IFC classes, i.e., IFC entities, are structured from the most abstract entities in the core layer to extensive, domain-specific, and resource-specific entities in upper layers. As can be seen from Figure 7, the entity _IfcRoot_ of the core layer represents the most abstract class relevant to all entity definitions. The _IfcRoot_ entity provides capabilities for globally identifying entities, attributing name and description of entities, and defining ownership information of entities. The entities _IfcObjectDefinition_, _IfcPropertyDefinition_, and _IfcRelationship_ are subclasses of the _IfcRoot_ entity, representing type and occurrences of objects, generalized characteristics assigned to objects, and objectified relationships in the IFC schema, respectively [53]. Figure 7: Extract of the IFC schema extension showing entities relevant to ITS in gray color. The _IfcObject_ entity, a subclass of the _IfcObjectDefinition_ entity, defines semantic occurrences of any \"thing\" or process. Several subclasses of the _IfcObject_ entity exist, such as the _IfcProduct_ entity, which represents geometrical and spatial objects, and the _IfcGroup_ entity, defining a logical collection of objects. The _IfcElement_ and _IfcSpatialElement_ entities are two subclasses of the _IfcProduct_ entity. The _IfcElement_ entity defines physical components in an AEC product, whereas the _IfcSpatialElement_ entity defines a generalized representation of components that construct spatial elements for structures. The _IfcSpatialStructureElement_ represents spatial elements that construct a unit of project structure, such as construction sites, spaces, and facilities. The _IfcFacility_ entity further classifies instances of the _IfcSpatialStructureElement_ entity as built facilities, such as buildings (_IfcBuilding_), bridges (_IfcBridge_), and roads (_IfcRoad_). The _IfcRoad_ entity, representing land routes, is a built facility defined for highways, streets, cycle, and foot paths. It is worth noting that _IfcFacility_ and _IfcRoad_ entities are included in the latest draft of the IFC schema, but are not yet standardized [54]. For describing ITS infrastructure, the _IfcTTSStation_ entity is newly defined in this study. The _IfcTTSStation_ entity is introduced as a subclass to the _IfcDistributionElement_ entity, which represents elements participating in distribution systems. Additional to attributes inherited, the _IfcTTSStation_ entity is specified with attributes _StationID_ of type _IfcInteger_, _HasActuator_ of type _IfcBoolean_, and _StationType_ of the _IfcTTSStationTypeEnum_ enumeration type. ITS station types are listed in the enumeration type _IfcTTSStationTypeEnum_ as the following constants: _VEHICLEUNIT_, _PERSONALUNIT_, _ROADSIDEUNIT_, _CENTRALUNIT_, _USERDEFINED_, and _NOTDEFINED_. on-board resources of ITS stations are specified using the property set _Pset_ITSSResources_, as shown in Table 1.  
The entity _IfcSystem_ presents organized combinations of objects that provide specific services. As a subclass of the _IfcSystem_ entity, the _IfcDistributionSystem_ entity accounts for networks, through which a distribution medium is controlled or maintained. Therefore, intelligent transportation systems may be regarded as distribution systems maintaining ITS communications and data transmissions among ITS stations. Hence, the _IfcRoadITS_ entity is introduced as a subclass of the _IfcDistributionSystem_ entity for describing instances of intelligent transportation systems for roads.