# BIM-Based Description of Intelligent Transportation Systems for Roads
## 3 A Semantic Model for Intelligent Transportation Systems
### Semantic Content and Structure  
The semantic model, representing a metamodel for conceptually modeling ITS for roads, is developed based on the knowledge sources analyzed in the previous section. Figure 4 shows the ITS semantic model in terms of a class diagram. The main elements of the semantic model are described in the following paragraphs. As depicted in Figure 4, the _DigitalRoad_ class represents \"non-conventional\" roads that are furnished with ITS intelligent infrastructure. The _DigitalRoad_ class is thus composed of the _RoadStructure_ and _ITSStation_ classes. The _RoadStructure_ class displays the underlying physical body of a road, including spatial structural elements (e.g., bridges, tunnels, roads, ramps, and resting areas), as well as detailing of structural elements, such as cross section geometry, location, and material properties. Attributes defined for the _RoadStructure_ class may specify standalone structural elements (e.g., a bridge slab) or grouped structural elements (e.g., a bridge) using _id_, _geometry_, _location_, and _type_. The abstract class _ITSStation_ represents core elements of the ITS architecture. Every ITS station is uniquely defined by an identification number, denoted by the _id_ attribute, and on-board resources, expressed by the _resourceInit_ attribute. As mentioned earlier, ITS stations may be mobile or fixed. Thus, the _ITSStation_ abstract class is modeled as a superclass of the _Mobile_ and _Fixed_ abstract subclasses. In the abstract class _ITSStation_, the attribute _isFixed_, with an initial value of false, represents mobile ITS stations. The _Mobile_ abstract subclass is further categorized into _Personal_ and _Vehicle_ subclasses, which represent personal smart devices and vehicles of any type, respectively. The _Fixed_ abstract subclass is depicted as a parent class to _Central_ and _Roadside_ subclasses to account for  
Figure 4: The semantic model for ITS for roads. ITS control centers and roadside intelligent infrastructure, respectively. Furthermore, as a specific type of network nodes (Interface _NetworkNode_), ITS stations may perform a variety of operations, such as data processing, signaling, recording, data packet routing, and resource renting. Moreover, the abstract class _ITSStation_ can perform operations, i.e., _measureSpeed()_, _sendMessage()_, _receiveMessage()_, _rentResource()_, and _checkStatus()_. The _SensingUnit_, _ComputingUnit_, _PowerUnit_, and _CommunicationUnit_ classes, conforming to the assumptions made in Section 2.3, represent the four main on-board units of all ITS stations in the ITS architecture. Therefore, the _ITSStation_ abstract class is connected to _SensingUnit_, _ComputingUnit_, _PowerUnit_, and _CommunicationUnit_ classes using composition associations. However, some ITS stations may also have actuating capabilities, such as roadwork warning devices or traffic signal controllers. The actuating capability is represented via the _ActuatingUnit_ class, which is connected to the ITSStation abstract class with an aggregation relationship. The abstract class _ITSCCommunication_ portrays communication in the ITS architecture. The attribute _domain_ represents communications in the generic domain or in the ITS domain, as described above. The _communicationType_ and _networkType_ attributes further characterize the instances of _ITSCCommunication_ abstract class with respect to communication type (internal and external) and network type (ad-hoc, ITS access, public, or private). The _ExternalNetwork_ class depicts communications between ITS stations, whereas the _InternalNetwork_ class presents communications within an ITS station. As introduced earlier, internal networks are dependent on a reference architecture, which is reflected in the semantic model as an interface that is realized by the _InternalNetwork_ class. on-board units and all equipment and assets attached to each ITS station, whether connected wirelessly or with wires, are denoted by the _ProprietaryNetwork_ class.