# BIM-Based Description of Intelligent Transportation Systems for Roads
## 3 A Semantic Model for Intelligent Transportation Systems
### Validation of the Semantic Model  
To illustrate the relationships between the _ITSCCommunication_, _ITSStation_, and _RoadStructure_ classes, and for validating the semantic model proposed in the previous subsection, a benchmark ITS scenario, shown in Figure 5, is semantically modeled. In other words, an instantiation of the semantic model as a _metamodel_ is made to describe the ITS scenario, which is instantiated as a _model_. In the ITS scenario, it is assumed that an accident, e.g., a rear-end collision, occurs in a section of a highway and external networks of ITS stations form a vehicular cloud to disseminate safety-related messages. Therefore, to avoid chain accidents and traffic congestion, and to divert the traffic, ITS stations receive and send data packets by geographically addressing other nodes with potential interest in the content of data packets, or forward data packets to destination nodes.  
In Figure 5, a highway section of the German Autobahn A9, labeled _HA9_, is shown, where the vehicle ITS station _V1_ broadcasts collision risk messages using the GeoNetworking protocol (\"_GeoBroadcast_\") to ITS stations in the vicinity with potential interest, e.g., vehicles approaching towards the accident. The vehicle ITS station _V1_ uses short-range wireless communication to send a message (_M1_) to the vehicle ITS station _V2_. For sending and receiving the messages, the vehicle ITS stations _V1_ and _V2_ employ the ITS-G5 protocol.  
\(V1\) also sends a media footage it has recorded from the accident to the nearest infrastructure node, here roadside ITS station \(RSU1\), using short-range wireless communications (\(M2\)). In turn, vehicle ITS station \(V2\) utilizes the point-to-multipoint networking protocol to ask the nearest infrastructure node (roadside ITS station \(RSU2\)) for an alternative route (\(M3\)). Using long-range wireless communication based on 5G cellular networks and the GeoNetworking protocol (\"_GeoLnicast_\"), \(RSU1\) requests traffic-related data from the next fixed ITS station, here \(RSU2\), and sends a detouring alert as well as traffic signals to \(RSU2\) (\(M4\)).  
The scenario shown in Figure 5 is semantically illustrated in Figure 6 in terms of an object diagram, i.e., an instance of the proposed semantic model. The object diagram comprises the \(HA9\) object of type \(RoadStructure\), four objects of type \(Vehicle\) and \(Roadside\) (\(V1\), \(V2\), \(RSU1\), and \(RSU2\)), objects of type \(ITSCommunication\) characterized by the messages shared among the ITS stations (\(M1\ldots M4\)), and \(Wireless\)-type objects displaying the wireless communication standards employed for each communication (\(5G\) and \(ITS\)-\(G5\)).  
The scenario illustrated above may be interpreted from the object diagram depicted in Figure 6, corroborating that the proposed semantic model may define any scenario of vehicular cloud formations and ITS for roads, respectively. In the following section, a BIM-based description of ITS for roads in terms of an IFC schema extension is presented.  
Figure 5: Cloud formation scenario for validating the proposed semantic model.  
Figure 6: Object diagram corresponding to the scenario shown in Figure 5.