## 4 Classification and Core Components of DT Systems in Transportation Infrastructure
### Basic Framework of DTs of Urban Transportation Infrastructure
#### 4.3.1 Four Horizontal Layers
The core functions of the service layer include description, diagnosis, prediction, decision support, and control of the digital twin. In the context of full lifecycle management of infrastructure, the service layer is not just an analytical tool but a dynamic system that can directly determine and optimize real-world operations. By integrating data and predictive information from all phases of planning, design, construction, operation, and maintenance, the service layer is able to provide immediate decision support at all lifecycle stages, driving intelligent management of infrastructure. In the planning and design phases, the service layer provides constructive advice to decision-makers through diagnostics and predictive analysis; in the construction phase, it monitors construction progress in real time and proposes optimization solutions; in the operation phase, the service layer ensures the efficient execution of traffic flow, resource allocation, and maintenance operations through real-time data feedback and dynamic decision support; and in the maintenance phase, it analyzes the health status of infrastructures and optimizes resource allocation to achieve proactive maintenance. Through this continuous optimization and decision support, the service layer plays a bridging role in the whole lifecycle of transportation infrastructure, transforming the prediction and control capabilities of the model layer into actual operation and management results. 4. Application Layer  
The application layer is the final layer of the digital twin, with which the user interacts directly. It supports functions such as simulation, real-time monitoring, predictive optimization, active control, and virtual testing. In this layer, decision-makers and operators can intuitively apply the data and analytics provided by the service and model layers to optimize the performance and response of the transportation system. The application layer not only translates the digital twin into actionable practices but also ensures that the benefits associated with transportation management are effectively applied in the real world. By using this layer across the entire lifecycle of transportation infrastructure, decision-makers are able to make accurate decisions based on real-time data and predictive information during the planning, design, construction, operation, and maintenance phases. For example, in the planning phase, the application layer supports decision-makers in evaluating the effectiveness of different transportation scenarios; in the operation phase, the application layer helps to optimize traffic flow and resource allocation; and in the maintenance phase, it provides intelligent scheduling and proactive management recommendations to extend the service life of the infrastructure. By translating the predictive analytics and control functions of the modeling and service layers into real-world operations, the application layer facilitates the efficient management of transportation systems at different lifecycle stages and ensures the practical application and sustainability of digital twin technology in urban transportation management.