# Leveraging Digital Twin Technology for Sustainable and Efficient Public Transportation
## 3 Challenges and Limitations of Digital Twins  
### Scalability  
Scalability is the ability of a particular system to function, remain responsive, and operate smoothly with an increase in the amount of work or data. The scalability of DTs is a primary concern when deploying technology, especially in bus transportation systems in urban areas. Many researchers have pointed out the lack of capacity to integrate more data from data sources than originally expected and handle increased computational demands for DT simulations to be performed in real case scenarios. Network architecture and application/technology stacking are some of the primary considerations when it comes to the scalability of DTs. A study discusses interoperability (ability of different systems to work together), security, and the correct determination of information sources for the long-term deployment of digital twins [25]. It is essential in bus transportation as multiple data streams from various sensors and systems must be integrated seamlessly in real time. Another study presented a methodology that prioritized digital twin use cases based on stakeholder satisfaction and infrastructure scalability, which is essential for effective implementation in transportation systems. The authors, however, mention a lack of standardized approach, which leads to inconsistencies and challenges in effectively managing and scaling DT applications across different industries and stakeholders [41]. DT technology also requires a systematic approach, which necessitates specific environmental needs. A study points out that there is a lack of a comprehensive methodology for developing scalable and reusable digital twin solutions tailored to manufacturing environments. This is concerning since the extrapolation to the public transportation needs to be smooth and flawless. The authors suggest the requirement of a broader framework that can readily adapt to unique challenges posed by bus transportation, such as real-time data-processing and dynamic routing [36]. Real-time data integration and processing, and bidirectional data interaction capabilities, are important for bus transportation. While the data integration for operational adjustments can be accurate, scalability is essential to improve efficiency and passenger services. The authors conclude that the management of challenges related to data management, model complexity, real-time integration, resource allocation, and adaptability is important to achieve scalability in energy consumption [57]. The concept of modularity (a system which can be broken down into separate parts that can also be combined to function as a whole unit) is also important to facilitate scalability. A study discusses a microservices-based architecture which focuses on reusability and modularity to enable dynamic responses to meet the growing demands in industrial settings. This approach can be beneficial in the context of bus transportation, where different operations require flexible and scalable solutions [42]. Furthermore, the integration of digital twins with emerging technologies, such as the Internet of Things (IoT), can enhance scalability. One study highlights the potential of the IoT and digital twins to create advanced features in smart grids, which can be analogous to the smart transportation systems that utilize digital twins for improved operational efficiency [97]. The synergy between these technologies can lead to more robust and scalable solutions in bus transportation. Additionally, combining DTs with the IoT can also be beneficial in increasing smart grid capabilities, which in turn can prove to be reliable for smart transportation systems. A multi-layered security strategy where traditional security techniques (such as encryption, firewalls, and access controls) are integrated with IoT-based security measures can prove to be beneficial for DT security and resiliency.