Like any city, Boston is constantly changing: old buildings are demolished, new ones rise, public transit winds through, and schools, grocery stores, and even street trees dot the landscape. For years, Boston wanted to integrate these elements into a three-dimensional digital model to simulate planning decisions before implementing them in the real world. The model was built in 2015 and opened to the public three years later.

The digital twin covers the landscape of current buildings, transportation, trees, sunlight and shadows, and points of interest, while also including proposed and under-construction buildings, and serves as an archive of Boston's appearance over the years. Carolyn Bennett, deputy director of geographic information systems (GIS) at the Boston Planning & Development Agency, said: "It's a living, dynamic thing, and it includes historical archives, current conditions, and the future—almost the full spectrum, and I think that's very helpful for the agency and the city of Boston as a whole."

A digital twin is a three-dimensional virtual replica of a specific system, place, or thing, allowing cities and property owners to test changes before implementing them in the real world. Cities from Los Angeles to coastal Texas are increasingly using this technology to study the impacts of development, transportation, climate change, and many other scenarios facing urban areas.

Ankit Srivastava, associate professor of mechanical and aerospace engineering at the Illinois Institute of Technology, noted: "You can simulate the effects of decisions before making actual changes to the physical object—here, a city or campus. This makes decisions more data-driven and less costly because you can try many possibilities." However, ensuring that digital twins stay up to date and help cities grow requires dedicated time and effort.

Capturing the full scope of a city in the digital realm

Bennett explained that since 2005, Boston has been exploring how to integrate the city's vast data into a three-dimensional model, using available data such as water and sewer, transportation, and tax parcel geospatial data. Today, Boston's digital twin includes these data repositories. The city can use it to assess the impact of development proposals on community housing, zoning, and parking. For example, planners can overlay urban heat island data from other sources onto the twin model to visualize the relationship between temperature and buildings, impervious surfaces, and tree canopy.

Bennett said: "It allows for better visualization of our work as planners. We plan in three dimensions; we're not a two-dimensional agency. We look at the world as it exists, so we have to look at it in three dimensions." The city of Chattanooga, Tennessee, has also found digital twins to be an effective tool for integrating the data it collects.

Jibo Sanyal, leader of the Computational Urban Sciences group at Oak Ridge National Laboratory, posed the question: "Can we bring all real-time data into a common ecosystem to clearly understand what's happening?" Researchers from Oak Ridge and the National Renewable Energy Laboratory partnered with Chattanooga to build a digital twin that helps predict and mitigate traffic congestion, with Sanyal serving as the project's technical and strategic lead.

Information from 500 different sources—such as traffic cameras, 911 data, radar detectors, and weather stations—flows into Chattanooga's digital twin. Sanyal said that traffic congestion experiments in this virtual environment showed traffic flow improvements of up to 30%, thereby increasing energy efficiency. Researchers used findings from these digital twin experiments to implement changes in the real world. For example, traffic planners typically collect data during morning and evening peak hours, but when researchers conducted digital twin experiments on Shallowford Road in Chattanooga, they found that during the midday peak, more than 90% of cars were stopped by red lights.

In reality, researchers used experimental insights to recalculate the timing of traffic signals along Shallowford Road to reduce congestion. Sanyal said they can now change signal patterns every four minutes based on traffic conditions. Chattanooga has one of the busiest freight corridors in the nation, so the digital twin also supports long-term decisions. For instance, the city wanted to know if there were specific times of day when it would be appropriate to let drivers use the shoulder as a travel lane. Sanyal said using the digital twin for such work reduces evaluation costs.

He said: "A lot of research stops after publishing a nice paper—often a very compelling paper—but the opportunity for real-world application often doesn't exist. Chattanooga is great; they brought us in and collaborated to adjust traffic light settings, fine-tune them when necessary, and conduct experiments in the real world. Then we look at before and after to see what improvements we observed." Although researchers currently control signals on only one corridor, Sanyal expects that within a year they will control about 100 signals, covering downtown Chattanooga and other high-traffic areas.

The buildings that make up a city

Digital twins can also help cities and property owners reduce energy consumption. Cities account for about 75% of global greenhouse gas emissions. To reduce pollution, cities such as New York, Boston, and Washington, D.C., have set goals for future buildings to achieve net-zero carbon emissions. But decarbonizing buildings requires analyzing and implementing energy management systems and renewable energy strategies, while also purchasing carbon offsets. Cityzenith is one company focused on developing digital twins of cities to aid decarbonization.

The company's CEO, Michael Jansen, said the model can start with as few as two to five buildings. Property owners use templates to input data, and the resulting digital twin helps them run and optimize simulations based on factors such as building age, condition, and use, with the goal of achieving more energy-efficient buildings and ultimately cities. Las Vegas and New York City have adopted the technology, and Phoenix and other U.S. and European cities are expected to follow. Jansen said Cityzenith plans to donate the technology to a total of 10 cities by Christmas this year and to a total of 100 cities by 2024.

Barriers to digital twins

Jansen noted that one of the main barriers to implementing digital twins is ensuring all necessary data is in place to build the model. "A digital twin is not bought," he said. "It's built." Mohammad Heidarinejad, assistant professor of architectural engineering at the Illinois Institute of Technology, said that since large cities already have vast amounts of data, more cities are expected to use digital twins. He added that within five to ten years, these models may also enter more smaller cities.

But cities and property owners will face obstacles. Heidarinejad said capturing reality with drones or scanning requires significant storage space, and digital twins must evolve as the real world changes (Boston updates its model twice a year). Srivastava of the Illinois Institute of Technology said one way to update digital twins is through crowdsourced data, including photos people take of the city and agree to contribute to the project. He suggested that perhaps a central system could analyze these photos and their GPS coordinates and automatically update the digital twin.

The data contained in digital twins can raise privacy concerns. If people can access the information behind the twin, so can hackers. Srivastava said cities must decide how much detail to capture in a digital twin and which elements to make public, especially as the amount of data in the world continues to grow. He said: "There will be more and more virtualization of the real world, simply because data becomes much easier to obtain. I think future cities will take advantage of it."

This may require investment from cities. Bennett said Boston has a three-dimensional data analyst whose primary responsibility is maintaining the city's digital twin. She said: "Cities have to make a commitment to it because it's dynamic content and data that needs continuous updating and maintenance. It's challenging because if the model becomes too outdated, people won't want to use it." Sanyal of Oak Ridge National Laboratory said personnel and IT overhead can be difficult for cities to bear. After all, cities want to be good stewards of taxpayer money, so they tend to prefer proven tools.

The digital twin work conducted by Oak Ridge and Chattanooga was designed with this in mind. Oak Ridge focuses on solutions that can be scaled to other cities, so Sanyal hopes companies will license the national laboratory's technology. Nevertheless, digital twins are just one tool for cities and properties to plan for the future, and their value depends largely on the data available. Sanyal said: "Because it's highly data-dependent, it's crucial to carefully choose the data used for decision-making. Ultimately, a digital twin is a tool, and large-scale planning decisions are usually made by humans."