# Leveraging Digital Twin Technology for Sustainable and Efficient Public Transportation
## 1 Introduction  
### Background and Context  
Public transportation has been an integral part of urban mobility since the 19 th century. It has also gone through many technological advancements to meet the growing needs of cities. Forms of public transportation systems in the United States include buses, trains, ferries, and airlines, which provide accessible and publicly financed services to millions of people. According to the American Public Transportation Association (APTA), public transit ridership grew by roughly 21% between 1997 and 2019, even as private vehicle ownership increased, highlighting its critical role in the U.S. economy. As of April 2024, APTA reported that public transit ridership, which had dropped to 20% of pre-pandemic levels in April 2020, had rebounded to 79% of pre-pandemic levels [1,2]. In terms of the highest public transit usage, New York, followed by Boston, Chicago, and Washington, DC, has the highest public transit ridership. Figure 1 presents a bar chart showing the most recently published paid passenger ridership (in millions) data in the four cities.  
In a global context, the most recent comprehensive public transportation ridership data are unavailable due to variations in data collection methods and inconsistent reporting standards. However, Figure 2 provides the bus rapid transit ridership details from 2022 for cities with the highest utilization of bus rapid transit systems. The numbers highlight the critical role of daily bus transit in urban mobility worldwide [6].  
Urban transit agencies work continuously to improve operations, addressing challenges daily that include traffic congestion, unpredictable schedules, fluctuating passenger demand, and maintenance issues. These challenges grow with the rapid growth of urban cities and hence require more efficient, reliable, and sustainable solutions to keep up with the growth. Over the years, the traditional public transportation systems have operated on tight schedules and outdated infrastructure. As such, they lack the adaptability needed to respond swiftly and dynamically to unexpected disruptions in real time [7; 8].  
Sustainability is at the forefront of smart city initiatives. With rapid urbanization, the importance of efficient public transportation is further amplified as cities seek to reduce congestion, lower emissions, and enhance the quality of urban life. This can be achieved  
Figure 1: Transit ridership in US cities with the highest public transit usage [3; 4; 5].  
Figure 2: Bus ridership in cities with the highest bus rapid transit usage [6].  
through greener, more resilient transit solutions. Effective transit systems have the potential of reducing traffic congestion, lowering pollution levels, and enhancing overall quality of life. While advances in Intelligent Transportation Systems (ITSs) have improved urban traffic operations, many networks still rely on fixed schedules, unable to adjust to real-time events such as accidents, delays, or fluctuating demand on the road [9].  
In recent years, the emergence of digital twin (DT) technology has offered a novel solution to the time constraint issues in urban cities. A digital twin is a virtual replica of a physical system, which can integrate real-time data to simulate, predict, and optimize performance [10]. With the help of this tool, it is possible to model and simulate real-world systems in various sectors such as healthcare, manufacturing, and smart cities [10]. In the realm of public transportation, digital twins have the capability of modeling entire networks and optimizing routes, vehicle maintenance, and passenger flows based on live conditions. Such an approach and dynamic integration could dramatically improve efficiency, reliability, and the user experience in urban public transit.