A Methodology for Using Dynamic Visualizations to Enhance Citizens Engagement in Mobility Planning in Thessaloniki

Abstract

Many cities are developing semantic 3D models as digital representation of their environment to assess the outcomes of the implementation of new mobility services before their real-life piloting. So far, digital twins have reinforced evidence-based approaches for policy-making. The purpose of this paper is to present how digital twins can be used as a tool for vulnerable citizen engagement, improving efficiency of co-creation in mobility planning. An innovative methodological approach of integrating digital twins in engagement processes, by transforming them from real-time data-driven replicas, targeted to the facilitation of the decision-making process for the public authorities to a tool for enhancing citizen engagement in the mobility context has been developed and tested in Thessaloniki Smart Mobility Living Lab. Through a 3-steps workshops’ approach, the mobility needs set by vulnerable users are collected, prioritized and matched with policy plans. By combining citizens feedback with real-time data incorporated in digital twins, citizens receive back visualized scenarios of the impact of specific mobility measures on their neighborhoods. By lifting the citizen engagement in mobility planning to the next participatory level, the results of the study create a bridge between participatory processes and digital twins.

1. Introduction

Citizens’ engagement is fundamental in planning processes of forming an urban mobility strategy. End-users’ feedback is an important feature for policy-makers to understand mobility services’ acceptance and improve the provided transport system. Traditional ways of receiving feedback include actual public consultation in different forms. These methods of identifying the impact of a service are time-consuming in terms of data collection and interpretation and most of the time are not citizen-friendly or even accessible to all citizen groups. This leads to the exclusion of certain groups, such as elderly individuals, people with disabilities, and low-income communities [1], from the mobility planning process and, consequently, the non-responsiveness of the provided mobility services to their specific needs. Moreover, conventional methods are quite generic in terms of geographical focus as the feedback received by the citizens is interpreted as a call for implementing a specific service in the whole city, and rarely is a spatial correlation to target neighborhood-specific requirements performed [2]. Since the advances in technology have resulted in a new efficient representation of the transportation systems in the real world, new digital tools such as digital twins are available, creating a digital environment in which citizens and policy-makers can collaborate to provide better solutions [3].

Digital twins first appeared in the product and manufacturing design industries and gradually have been integrated in industries such as aerospace [4], automation [5], shipbuilding [6], healthcare [7] and energy [8,9]. A broader application is now found at the city level. Cities are dynamic complex ecosystems with different services, operating simultaneously. Urban mobility is one of the main activities in urban environments. A vital component to ensure coordinated actions between services in this complicated environment is the representation of these activities in a holistic ecosystem to coordinate further actions to improve its performance [10]. Advances in technologies have resulted in a new representation of the systems in the real world. Big data and Internet of Things (IoT) [3], cloud computing [11] and artificial intelligence (AI) [12] collecting and processing data from different sources create a digital environment in which different groups of stakeholders, policy-makers and citizens can collaborate to select the right mix of new mobility solutions to be implemented.

Digital twins align directly with the objectives of smart cities. A smart city is defined as “a place where traditional networks and services are made more efficient with the use of digital solutions for the benefit of its inhabitants and business” [13]. It collects data through both information and communication technologies (ICT), as well as physical devices connected to the IoT network, enabling the collection, processing, storage, and transmission of data to facilitate communication and decision-making. All the collected data are used to optimize the efficiency of the city operations and services. A digital twin is a virtual model that replicates a real-world urban area/city, allowing the creator to determine the performance of its different systems [10]. Therefore, digital twins act as virtual representations of a physical environment that support smart city operations [14].

Digital twins’ applications have demonstrated significant societal and economic value, benefiting municipalities, businesses and communities [8]. By accelerating communication between decision-makers and citizens, digital twins enable more inclusive planning processes. Their interactive features facilitate citizen engagement and empower the collaborative relationships with decision-makers to co-create a resilient and eco-friendly society.

A digital smart-city twin can be used as a tool to represent and engage all stakeholders in a project. It can also provide tools to incorporate feedback through comment sections and polling. Using digital twins, people do not have to be physically present and can access the city’s services at their own convenience. Furthermore, a digital twin replica can be updated at any given moment, so the latest changes can always be displayed. Communication between the interested parties can be enhanced since the flow of information is facilitated through new channels [15].

This paper aims at presenting extended opportunities of the use of digital twins as tools in engagement processes. Through the implementation of a three-step methodology, a twofold goal will be achieved: (1) a higher level of engagement of citizens that are typically under-represented in participatory processes by the use of digital twin models, and (2) the validation of citizens’ input through visualizations in digital twins, contributing to a more effective and accurate incorporation of citizens’ feedback into policy plans. After introducing the benefits of digital twins in general and specifically in citizen engagement in Section 1.1 and Section 1.2, respectively, Section 2 presents the methodological approach followed, including the target group and the area of study. Last, the results and their interpretation are described in Section 3 and Section 4, respectively.

1.1. Benefits of Digital Twins

Large cities are usually characterized by a dynamism that conceals highly complex social structures and services. Thus, changes to some services affect the operational elements of others, increasing the level of uncertainty for proposed actions during planning processes. Digital twins allow for the simulation of plans before implementing them, exposing problems before they become a reality [16]. From land-use optimization to urban planning, policy-makers will have the power to govern an ecosystem in an effective way [17]. It is now possible to transfer the complicated system of a city to the digital world, enabling town planners to test innovative mobility schemes without taking too many risks, such as low operational performance. This capability of conducting physical experiments based on services while optimizing their efficiency under real-world complex conditions provide policy-makers with a valuable benefit: prior implementation risk management, reducing the uncertainty of the investments [14]. The element that distinguishes a digital twin from any other digital model is its connection to the process. Based on data from the physical system, a digital twin unlocks value by supporting improved decision-making, which creates the opportunity for positive feedback into the physical twin. Digital twins can assist decision-makers in evaluation for a variety of purposes, such as existing situations and future scenarios [18].

Digital twins produce better outcomes for the end users of the services, including increased satisfaction and improved mobility experience, through higher-performing services. Data computational models perform analytics and processing for the city’s digital model, providing comprehensive results to policy-makers. Furthermore, digital twins contribute to financial sustainability by reducing infrastructure maintenance costs through predictive analytics, optimizing public transport operations for cost efficiency, and enabling new revenue streams such as monetized mobility data [19]. Digital twins aim to fundamentally transform the decision-making process; the innovative evaluation of a holistic system through a digital replica will be an accelerator key for decision-makers to adopt targeted solutions for a resilient urban environment [10].

Reduced implementation costs could be provided in a digital twin model as the system can be monitored and evaluated using real-time data. Based on information from the cities, industry can evaluate and place the infrastructure needed in specific locations, fueling financial revenue for business. New market corridors and investments will be established as new services are implemented in cities, creating job opportunities for operating the digital twin model and supporting all aspects of the services in the urban environment [10].

Built environments of future cities will need to cope with climate change and align with more intricate energy needs. Testing in a digital environment creates desire features and benefits externally. The reduction of physical experimentation concerning services in the real world will lead to minimized disruptions in the everyday life of citizens in the real world, as well as create the pillars of a more resilient urban environment [19]. Air pollution, congestion, delays and disruptions in the road network are features that decrease the level of resilience of a city. BING and SEA models primarily focus on spatial analysis and strategic environmental assessments, but they do not offer real-time replication, continuous feedback loops, or predictive analysis at the same scale as a digital twin [20]. Also, GIS-based transport planning tools are excellent for visualization and analysis, but they lack the real-time interaction that a digital twin provides. So, in contrast to the GIS-based planning tools, BING and SEA models, in digital twins, the real-time replication and evaluation of sustainable mobility solutions can be achieved, mitigating the levels of disruption occurring in cities. The operation of the proposed mobility interventions can be evaluated alongside other transportation modes in the city using real-time performance data. Parameters and features of the services can be examined, without additional implementation costs [15]. Historical data stored in the smart cities can be used to evaluate the current situation in an urban environment, such as congestion and quality of air, and evaluate future scenarios from the decision-making perspective.

Therefore, digital twins revolutionize traffic management and transport planning by integrating real-time data from IoT sensors, AI-driven decision-making and predictive modelling, unlike traditional tools that rely on historical data and static models, have mainly a reactive nature. By combining multiple data sources, digital twins enable dynamic simulations, scenario-based planning, and automated incident detection. This enhances traffic efficiency, safety, and environmental sustainability by optimizing signal timing, reducing congestion, and minimizing fuel consumption. Moreover, digital twins seamlessly integrate with smart city infrastructure, supporting connected vehicles and next-generation mobility solutions. Their scalability allows for applications in large cities’ traffic control, autonomous vehicle coordination, and emergency response optimization, making them a cost-effective and future-ready solution for urban planning and transportation management.

The effectiveness of digital twins in improving traffic management are also recorded in a literature review: a digital twin technology based on Adaptive Traffic Signal Control (ATSC) showed significant reductions in control delays, ranging from 1% to 52% for low traffic demands, from 3% to 19% for moderate demands, and up to 45% for high demands, compared to traditional methods [21]. Research on digital twin-assisted cooperative driving systems at non-signalized intersections demonstrated a 20% reduction in travel time and a 23.7% decrease in energy consumption compared to traditional signalized intersections [22].

1.2. Digital Twins in Citizens’ Engagement

The citizen engagement processes rely on three main pillars: connect, exchange, and empower. The connection between citizens and policy-makers is one of the major pillars of citizens’ engagement processes. The main goal is to renew the relationship between policy-makers and citizens, to form an integrated transport network in a city. Raising public awareness about the new digital tools through campaigns allows for a more active form of participation from the citizens’ perspective. The users of the services understand the importance of co-creation and engagement in the implementation process of mobility services. The policy-makers regain the trust of the citizens, as the new tools will allow the integration of the information received in the real world in a quicker way [2].

Since technological advances have resulted in a new efficient representation of the transportation systems in the real world that citizens have access to, they can collaborate with policy-makers to co-design better solutions [3]. Collaboration with the end users of a mobility service could reveal a handy visualization of a subjective point of view of the services. The extraction and creation of knowledge resulting from the fusion and processing of the information received from the end users can also feed dialogues and debates with citizens and create common understanding on mobility issues and the real needs of the society/neighborhood. Thus, the digital twins’ function as an accelerator tool of the traditional time-consuming assessment activities of mobility services [3].

Currently, many cities are integrating digital twin technology into their daily operations for simulating potential scenarios and being able to address urban transportation issues proactively by following efficient decision-making procedures. New York City has adopted digital twin technology to optimize traffic flows and transportation networks [23]. In Helsinki, digital twins are used for public engagement, allowing residents to visualize and provide input into urban development projects before they begin. For example, the Kalasatama Digital Twin project in Helsinki in 2019 produced a high-quality digital twin city model of the Kalasatama area that was shared publicly, serving as a platform for designing, testing, applying and servicing the whole lifecycle of the built environment [24].

A public and open digital twin of the Docklands area in Dublin has been developed for the urban planning of skylines and green spaces. The digital twin serves as a virtual feedback loop where citizens can interact by reporting problems in specific areas and provide feedback on planned changes. It also serves as a co-creation space where citizens can also suggest their own ideas for green space planning. The goal is to engage citizens through digital twins and use the collected valuable feedback for key urban planning and policy decisions. However, the current digital twin does not include simulations related to mobility planning issues, so the evaluation of the impact that a potential urban mobility planning decision may have is not feasible at this stage [25]. Also, the prototype of an urban digital twin for Herrenberg in Germany was demonstrated to the public through a visualization platform for virtual reality and was positively assessed as a technology that could significantly contribute participatory processes [26].

Empowering citizens in decision-making addresses a high level of citizen engagement, where citizens advise and ideas are included in the decision-making and implemented to the maximum extent possible [2]. Digital twins can be directly linked to digital platforms dedicated to collect citizens’ feedback, such as the vCity project in Barcelona, which integrates feedback from Decidim (a digital platform already used by over 200 cities for citizen participation) into the digital twin platform of the city, showcasing a structured and direct method for ensuring that urban planning is informed by the perspectives of the people it serves [27]. Citizens can share “objective” data-driven information through their first-hand experience with the implemented services.

Unlike static or simulation models, digital twins are continuously fed real-time data, enabling more informed and participatory decision-making processes, as stakeholders can see the direct impact of changes in real-time. Additionally, digital twins can predict future outcomes by running “what-if” scenarios. This is particularly valuable for participatory decision-making in mobility planning. Also, digital twins often incorporate 3D visualizations and AR/VR capabilities, presenting the city environment in an easily understandable way and thus making it easier for non-experts to contribute to discussions. The interactive environment of digital twins makes them more engaging than traditional reports or workshops.

However, while digital twins offer powerful tools for urban planning and efficiency, their success depends on their alignment with social values, ethical principles, and inclusive policies. A holistic approach that integrates technology with social, political, and cultural considerations will help the integration of social dimensions into urban digital twins. Ensuring equitable access to smart city benefits requires policies that integrate diverse community needs, including elderly individuals, people with disabilities, and low-income communities [1]. For example, real-time traffic monitoring should support accessible transportation options, ensuring that pedestrian pathways and public transport systems are inclusive. Moreover, digital literacy initiatives should accompany the implementation of digital twin systems so that all citizens, regardless of socioeconomic background, can engage with the technology.

The literature on digital twins that integrate social aspects identifies three focus areas, covering both their conceptual development and their practical application: (1) conceptualization and modelling, which aims at replicating human behavior in digital environment; (2) human participation and governance that promotes citizen involvement in the governance of digital twin for enhanced decision-making; and (3) social applications, where digital twins operate as tools to address social challenges [1]. For example, in the city of Bologna, the concept of a Civic Digital Twin has been introduced to support a citizen-centric approach to urban planning and the active engagement of city neighborhoods [28]. Additionally, it is noteworthy that advancements in urban digital twins and citizen engagement is a topic that gathers high interest in the internationally community and lot of workshops are organized bringing together researchers from various countries, including Greece (e.g., the international Digital Twin Cities Centre (DTCC) workshop on urban digital twins where researchers from the Aristotle University of Thessaloniki actively participated) [29].

Although in many cities digital twin users have the ability not only to be informed but also easily exchange information with decision-makers, there is no extended research of digital twins that are used as part of the ideation process and for jointly co-creating and testing suitable mobility solutions with the decision-makers. So, this paper aims to answer the following research questions: Can digital twins serve as a co-creation environment in mobility planning that enables policy-makers to implement improvements in transport systems? Can digital twins enhance citizen engagement and strengthen the communication between citizens and policymakers, leading to more responsive urban mobility strategies?

2. Methods

A Thessaloniki pilot project was implemented in the framework of the DVECE project in 2022 within Thessaloniki Smart Mobility Living Lab—a user-driven innovation environment where users and producers co-create innovation in a trusted, open ecosystem that enables business and societal innovation. The pilot was focused on a selected vulnerable population group in a specific neighborhood, thus representing a variety of mobility challenges and a geographical correlation with the citizens’ needs. Through a series of ideation, co-creation and validation/evaluation workshops, the mobility needs set by the vulnerable users were collected, prioritized and matched with policy plans and real-time data incorporated in Thessaloniki’s Digi Twin.

This approach allows for the possibility to feed the digital twin with “bottom up” data coming from the citizens and merge policy-driven mobility plans with actual perception and views of the neighborhood inhabitants, testing how the inclusiveness of the developed mobility solutions is improved. The ideation–co-creation–validation/evaluation workshops with the focus group are the core of the methodological approach.

2.1. Target Group

The Thessaloniki pilot focuses on pedestrians in the specific area of the Egnatia corridor. In the ideation and co-creation workshops, a group of people from the Pedestrian Association of Thessaloniki participated, discussing the specific characteristics of Egnatia road, the mobility problems in the area and specific suggested measures to improve mobility conditions for pedestrians moving in the examined area.

In the validation and evaluation workshop, the target group of pedestrians, policy-makers and other stakeholders participated to evaluate the results from the two previous workshops. The Thessaloniki Smart Mobility Living Lab lies at the center of an ecosystem of mobility-related stakeholders in the city of Thessaloniki, which consists of public administrations such as the Municipality of Thessaloniki, public and private transport operators, technology providers, research and academia. The long-term relationships of collaboration and trust between Thessaloniki Smart Mobility Living Lab and the mobility ecosystem enabled the engagement of a significant number of them for the workshops’ purposes.

2.2. Selected Neighborhood

Thessaloniki’s pilot focuses on Egnatia road—one of the main roads of the city center. Due to high levels of congestion, the lack of parking slots and insufficient infrastructure (e.g., inconsistencies in ramp corridors) it is one of the most challenging areas for pedestrians. The study area was Egnatia road’s full length, but greater emphasis was given on the section of the road from Ethnikis Aminis to El. Venizelos Street (Figure 1), where most of the problems are appeared and the largest flows of the pedestrians are observed.

Also, Egnatia road is one of the focus areas of Thessaloniki’s Sustainable Urban Mobility Plan (SUMP). Thessaloniki’s SUMP suggests emblematic interventions in Egnatia road, aiming to reduce motorized traffic. Through interventions focused on the redistribution of the public space—such as the extension of the bicycle network and the available space for pedestrians, the reduction of traffic lanes, and the separation of the two traffic streams with a planted divided median strip—SUMP set as one of its main priorities an increase in the safety and accessibility of the pedestrians on Egnatia road [30].

2.3. Thessaloniki’s Digital Twin

Thessaloniki’s digital twin (Figure 2), developed by CERTH/HIT, is a cyber replica of the physical transportation system of the metropolitan area of Thessaloniki, allowing for the evaluation of the performance of various mobility solutions of the city’s transport system. It consists of static data such as transportation infrastructure and dynamic data from heterogeneous sources [31], with the functionality to formulate predictions throughout the network. It leverages real-time data for predictive traffic management, achieved by the combination of multi-resolution simulations and artificial intelligence algorithms for solving specific transportation problems, such as traffic flow forecasting, managing fleets and providing routing services, ensuring that the transportation network of Thessaloniki operates smoothly under various conditions.

Through the virtual representation, a new technology or/and solution can be examined, and possible behaviors can be predicted to optimize the results and the performance and better respond to citizen’s needs. Applied to a system or process, Thessaloniki’s digital twin aims to eliminate the need for extended physical experimentation while optimizing performance under different conditions. The employment of Thessaloniki’s digital twin allows for the exact simulation of the real transportation system and road network, as well as the implementation of cutting-edge tools and methodologies based on predictive analytics and artificial intelligence for making decisions on the development of the transport network, such as performing calculations to estimate its congestion [32].

The cyber replica of the road network of Thessaloniki Is utilized for combining real data obtained by heterogeneous sensors with simulated data generated through the digital twin for training artificial intelligence algorithms capable of capturing complex spatiotemporal patterns of transportation data [33]. Thessaloniki’s digital twin is also harnessed for training agents on challenging tasks and testing their performance in real conditions. Thessaloniki’s digital twin can precisely forecast the future status of the transport network, providing management strategies by combining real transport data feeds, artificial intelligence (AI) and machine learning (ML) techniques, as well as high-speed simulations with the emulation of congestion/incident-mitigation measures. It allows us to study fictional scenarios and predict how external actions affect the system while offering added value to policy-makers, since they gain basic insights into aggregated behavior to prevent losing control and understanding of the system. To that end, it can be utilized as a test bed for assessing cutting-edge tools and technologies and examining the effects of potential soft or hard interventions in the transportation system.

The capacity of Thessaloniki’s digital twin to translate analytical and simulated outputs into distinctive performance metrics is essential for facilitating data-driven decision-making. Having access to delays, travel times, pollution emissions, speed and routes, performance metrics can be developed, enabling the development of evidence-based transportation management strategies. For performance indicators that cannot be directly measured, such as emissions, Thessaloniki’s digital twin can formulate predictions throughout the network, even where detectors are not present [34].

Different types of data are integrated in Thessaloniki’s digital twin: digitized road networks, floating car data, Bluetooth sensor data, conventional traffic counter data, public transport schedules and timetables, traffic control systems and shared mobility data. These datasets are used for developing different traffic simulation scenarios.

Thessaloniki’s digital twin is mainly used to address issues related to traffic management and routing. Since it simulates traffic patterns in real time, it enables city planners and traffic management centers to implement adaptive signal controls and dynamic routing strategies. It is also used for managing public transportation systems, sustainable urban logistics and emergency response.

The use of Thessaloniki’s digital twin as a citizen engagement tool required the integration of new types of data, such as crowdsourced citizen data collected during the ideation and co-creation workshops (feedback on the current mobility situation and suggested improvement measures) and data from field measurements and observations (including measurements of vehicle/bus/motorcycle/bicycle/scooter flows, pedestrian flows on crossings, pedestrian origins-destinations (Ods) and observations of traffic lights intervals). The field measurements of vehicle flows were additionally integrated into Thessaloniki’s digital twin for further validating and calibrating the traffic simulation scenarios. The series of simulated observations of the digital twin were statistically compared with the real measurements of the actual system to determine whether reproducing the system behavior is of desirable accuracy and reliability.

Additionally, simulation outputs of the area of interest related to pedestrian flows, numbers of stops for pedestrians, pedestrian speed, pedestrian stop time, pedestrian travel time, and pedestrian walking time were extracted (Figure 3).

The integration of crowdsourced data into Thessaloniki’s digital twin was performed following three phases (Figure 4).

2.4. Ideation, Co-Creation and Validation-Evaluation Workshops

The ideation, co-creation and validation/evaluation workshops were performed in person and were at the core of the methodological approach.

The main objectives of the three workshops were as follows:

• To collect the mobility needs and requirements of the citizens within the ideation workshop;
• To match these needs and requirements with policy plans and real-time data incorporated into the digital twin;
• To co-create neighborhood scenarios within the co-creation workshop;
• To bring designs from the mobility plans and citizens’ opinions/feedback into the digital twin;
• To discuss and validate with the policy-makers the results from the visualizations of the proposed measures in the digital twin.

The ideation workshop took place in the presence of individuals from the Pedestrian Association of Thessaloniki. The main objective of the ideation workshop was to engage the participants in a discussion about the specific characteristics of Egnatia road, the definition of their specific mobility needs and the mobility related problems that they face as pedestrians on this street. A detailed report including the main problems that were identified in the field “scanning” was prepared by CERTH/HIT team before the workshop. These problems included bad pavement conditions at several points in the examined area, which makes it difficult for pedestrians to walk safely on the pavements, and short intervals of green traffic lights at specific intersections. More specifically, the green interval of the traffic light for pedestrians at the Kamara crossing was 10 s, which is not sufficient considering the pedestrian load that passes through this specific crossing. The moderators presented these problems along with the related photos and used them as the basis for the discussion with the workshop participants. The participants were encouraged to write down additional problems on Post It notes and put them on the specific location on printed maps to facilitate the next phase of the co-creation workshop.

The co-creation workshop took place with the same group of participants from the Pedestrian Association. The objective of this workshop was to confer with the participants on a list of prioritized mobility-related measures and discuss the impact they could have on the mobility conditions of the examined area. Considering the suggested interventions of the Sustainable Urban Mobility Plan in Thessaloniki and building on the problems discussed during the ideation workshop, participants were asked to propose specific measures to address the main problems of the Egnatia corridor, to improve the safety and the accessibility of the pedestrian network.

The validation/evaluation workshop took place with a wider group of participants, including representatives from the Municipality of Thessaloniki, the Major Development Agency of Thessaloniki, the Pedestrian Association, transport operators (shared bike operators and taxi association) and Thessaloniki Transport Authority.

The main purpose of the third workshop was the validation and evaluation of the measures visualized by the participants. The measures co-created by the participants were visualized in the digital twin, and different impact scenarios were illustrated in charts and discussed with the participants. This gave participants the opportunity to develop a common understanding on the main problems of Egnatia road and the measures that could be implemented to address these problems. In parallel, they were able to gain quantitative insights into the impact (e.g., decrease in waiting time and near-miss incidents) that the suggested measures would have in terms of improving moving conditions for the pedestrians in the examined area. The alignment of the suggested measures that were visualized through the digital twin with the priorities and interventions of Thessaloniki’s SUMP was validated by the policy-makers, who agreed to consider them in the revised version of Thessaloniki’s SUMP.

3. Results

The key output of the ideation workshop was a list of problems in the examined area, which created a clear image of where the suggested measures of the second co-creation workshop should be focused. The main problems as summarized through the discussion were clustered in five groups:

• Pedestrian crossings: Absence of a fourth pedestrian crossing at some intersections and a long distance between successive pedestrian crossings;
• Pavement condition and geometric characteristics: Fluctuations in pavements’ width and damaged surfaces;
• Obstructions and illegal parking: Occupation of the pavements by goods and illegally parked vehicles, and existence of barriers such as signs;
• Infrastructure for disabled people: Absence of tactile paving and acoustic traffic signal for visually impaired people, and the ramps for impaired people do not comply with the requirements;
• Signing: Insufficient green intervals for pedestrian crossings and existence of useless signs at specific locations that cause confusion for pedestrians.

During the co-creation workshop, specific measures of high importance were suggested to improve the safety and accessibility of the pedestrian network of Egnatia road; they were clustered as follows:

• Measures related to intersections: Addition of a fourth crossing (where missing) at some intersections and longer green intervals of traffic lights for pedestrians;
• Policy measures: Reduction of the speed limit for vehicles from 50 km/h to 30 km/h and creation of low-speed/-emission zone;
• Infrastructure-related measures: Separation of the two traffic streams with a planted divided median strip;
• Multimodality measures: Restructuring of the public transport network (bus lanes) due to the operation of the metro, creation of a bicycle lanes in the examined area and connection with the existing bicycle network, aligning with the promotion of micro-mobility and active mobility modes.

Additionally, a list of more generic measures was developed, including interventions related to the regular maintenance of sidewalks and removal of the obstacles; the existence of tactile paving and acoustic traffic signal for visually impaired people along the entire length of the sidewalks; the placement of protective (trees, etc.) parallel to the sidewalks; yellow grid lines where needed in roads; the creation of parking spaces for cars and motorcycles; widening sidewalks; and reconsidering the location of traffic lights and their placement before pedestrian crossings.

During the validation/evaluation workshop, the priorities, needs and suggested measures for Egnatia (as they derived from the ideation and co-creation workshops) were discussed and validated with the policy-makers collecting feedback on the technical and economic feasibility of the measures’ implementation.

Some of the suggested measures were visualized in Thessaloniki’s digital twin, and their impacts were captured in related charts. Two of the main interventions simulated were (a) increasing the green interval of the traffic lights for pedestrians at the Kamara crossing (as shown in Figure 5) that has important impact on pedestrian waiting time and on near-misses (as shown in Figure 6) and (b) adding a fourth crossing at the Iasonidou and Egnatia intersections (as shown in Figure 7) that decreases the pedestrian waiting time and the near-miss incidents (as shown in Figure 8).

The measure of adding a fourth crossing was considered particularly important by the participants. However, it was mentioned that the fourth crossing would in some cases create a problem for vehicles turning left and right, as it would significantly limit the time available to drivers for these movements. The increase in green interval time for pedestrians, observing the charts presented, was considered a measure that must be implemented, considering the assumption that such a measure would cause an increase in vehicle traffic congestion. Regarding the illegally parked vehicles on the sidewalks, the responsibility of the Municipality in their removal (especially motorcycles parked illegally on the sidewalks) was emphasized.

4. Discussion

The current paper presents an innovative methodological approach of using digital twins by transforming them from real-time data-driven replicas targeted to the facilitation of the decision-making process for the public authorities to a tool for enhancing citizen engagement in a mobility context. Collaboration with the end users of a mobility service could reveal a handy visualization of a subjective point of view of the services. Thus, digital twins, when combined with structured participatory processes, can function as an accelerator tool for the traditional time-consuming assessment activities of mobility services.

By using Thessaloniki’s digital twin and a participatory three-step workshop methodology (ideation, co-creation, and validation), citizens were able to articulate their needs, evaluate mobility solutions, and visualize the potential impact of proposed interventions. The use of digital twins in co-design processes was positively assessed by all the participants in the workshops, and the knowledge extracted from the methodological approach on how to update and bring designs from the mobility plans and citizens’ opinions into the digital twins was considered added value.

The findings underscore the ability of digital twins to visualize the direct impact of proposed measures, thereby fostering transparency, trust, and more informed decision-making among all stakeholders. The suggested measures will be taken into account for the revision of Thessaloniki’s SUMP.

An important conclusion extracted from the three workshops was the need of the different stakeholders to communicate regularly—not only within specific projects but also in broader conversations about the mobility conditions in the city of Thessaloniki and what is really needed for improving them, considering different groups of end users, including vulnerable users. The need for scheduling regular meetings between all stakeholders (citizens and policy-makers) was highlighted as a critical point to achieve ecosystem cooperation, moving towards the achievement of better transport conditions for all citizens, including vulnerable groups. Key findings of the applied methodology indicate that digital twins not only improve the efficiency of participatory planning but also foster stronger communication between citizens and policymakers, leading to more responsive urban mobility strategies.

The extraction and creation of knowledge resulted from the fusion and processing of the information received from the participants of the workshops within Thessaloniki Smart Mobility Living Lab fed dialogues and debates between the participants. The use of a digital twin created a common understanding about the real needs of the pedestrians on Egnatia road, the mobility issues and the measures that need to be implemented to facilitate the communication between the interested parties and keep them engaged during the whole process.

The findings of the present study corroborate and go beyond the existing literature that proposes digital twins as promising tools for participatory urban planning. In contrast with the Kalasatama digital twin project in Helsinki [24] and the Dublin Docklands project [25] that focused mainly on infrastructure-change visualization and feedback collection, the present study shows how digital twins can also support the co-creation of mobility measures and validate citizens’ input through simulation. The integration of citizens’ needs into the digital twin is aligned with civic digital twin research, such as that in Bologna [28], but goes further by applying the digital twin for the quantitative evaluation of specific pedestrian-oriented interventions (e.g., pedestrian green light timing).

Moreover, this study provides results on digital twins’ ability to improve mobility conditions (e.g., travel time and safety), confirming previous findings from Wang et al. [22] and Dasgupta et al. [21]. The outcomes contribute to this part of the literature by demonstrating how even small-scale pedestrian-focused measures (e.g., green interval adjustments) can be evaluated through digital twins using co-produced data.

The application of the methodological approach in Thessaloniki Smart Mobility Living Lab has two main policy implications. This study demonstrates how digital twins can examine the impact of specific mobility-related interventions (e.g., improvement in pedestrian waiting times and decrease of near-miss risks) and directly inform and revise policy strategies and plans such as SUMPs. It also highlights the need for institutional coordination among municipalities, researchers, mobility operators and civil society. This calls for governance frameworks that support regular data-sharing and joint decision-making. Enabling real-time participatory feedback loops through the integration of participatory platforms such as Decidim in Barcelona [27] will ensure the replicability of the suggested methodology in other cities.

Based on these implications, five main policy recommendations were also extracted:

• Incentivize citizens to participate in planning processes.

Citizen engagement is a critical point for policy-makers to be able to address mobility challenges at local scale. Participation in workshops does not mean active engagement in planning processes. A certain level of frustration was expressed many times by the citizens, as they are regularly invited to workshops organized by the cities, which requires investing part of their personal time, but their expectations of what should be done with the results of the workshops are not met at the end. Citizens should participate in all the steps of the planning processes—from the design to the implementation and evaluation. Giving them incentives to participate (e.g., by using gamification and/or rewarding schemes) and offering feedback on the procedure results after the end of the project/initiative will keep them engaged long term.

2.

Cooperation of all mobility-related stakeholders.

The implementation of radical measures that improve mobility conditions in local areas for vulnerable users requires the cooperation of all related stakeholders and citizens. It is important for all the key stakeholders involved to have continuous contact and exchange views—not only within specific projects but also in broader conversations about mobility conditions in the city, to identify common goals and action points. The establishment of a regular dialogue and communication ensures a better understanding of what is really needed for improving mobility for different groups of end users, including vulnerable users, as well as the potential and limitations in the adoption of new engagement tools.

3.

Adoption of the digital twin for citizen engagement with regards to accessibility and/or participation in the city planning processes.

It is recommended to have a proper introduction of what a digital twin is and the purposes it serves, especially for people who hardly or never work with a data-driven approach. Depending on the policy life cycle, digital twins can be used differently and have different functionality needs, e.g., in the planning phase, more focus might be necessary on numbers and facts, while the engagement phase might benefit from better visualization and a gaming component. Digital twins also enable the visualization of planning processes in 4D, with time being the fourth dimension alongside 3D. A digital twin is a support tool for a policy-making or engagement process and it is not the complete answer to a discussion. Also important is finding key stakeholders, securing resources, commitment to long-term adoption, maintenance, moderation and marketing the citizen engagement tool.

4.

Inclusion of relevant stakeholders and citizen groups in the development of new engagement tools.

All citizens should be informed about the existence of digital twins or any available dynamic visualization tool and the opportunity to visualize the impact of their decisions through it, actively participating in decision-making. It is important to include various user groups in the design and development process of new tools in the early phase. With regards to accessibility, persons with first-hand experience of accessible routes and services should be included in the ideation process of tools for engagement. Workshops and webinars/seminars should be conducted with different user groups and stakeholders, to collect and exchange ideas and information and to raise citizens’ awareness about existing/new digital tools. The commitment of local authorities, interest organizations and national authorities to secure adoption, development and moderation of new engagement tools is imperative.

5.

Use of the digital twin as an engagement tool accessible to all.

The digital twin as an engagement tool does not offer fully equal and accessible possibilities for participation if considering all possible vulnerable user groups, such as visually impaired persons or persons with cognitive disabilities. It is a valuable addition to the current set of tools available, but in its current form, it is not able to replace more accessible tools, such as 2D maps. Further developments are required for the digital twin to become an engagement tool accessible to all.

By adjusting their recommendations regarding their needs and integrating them into their local strategies, cities can benefit by building on the potential of digital twins as an engagement tool in participatory processes in the mobility field.

However, the present study has some limitations mainly related to the local scope and the automated integration of the citizens’ feedback into the Thessaloniki’s digital twin. The study focused only on a single urban corridor in Thessaloniki. While this allowed for detailed analysis, it limits the generalization of the study’s insights. Also, at this stage, the direct link of Thessaloniki’s digital twin with a participatory platform that would enable the automated integration of citizens’ feedback into the digital twin was not feasible.

Therefore, to further advance this research, investigating the scalability of this methodology in different urban contexts and urban typologies (e.g., in suburban areas) would provide valuable insights for its replicability across cities with different infrastructure and policy frameworks. Future research could also focus on leveraging AI and ML to automate the analysis of citizen feedback, enhancing the responsiveness of planning processes. Additionally, the long-term impact of digital twin-driven engagement on urban policy development should be assessed.

From a technological perspective, the integration of standardized digital twin platforms across municipalities could facilitate more consistent and data-driven policy decisions. Policy-makers should prioritize the development of user-friendly interfaces that allow seamless citizen participation, ensuring accessibility for diverse social groups, including individuals with disabilities.

Strengthening the alignment between technology and societal needs will ensure that transport planning in smart cities is both innovative and inclusive, ultimately improving life quality for citizens and all urban residents.

5. Conclusions

This study develops and validates a methodological framework that suggests a digital twin as a participatory planning tool, moving beyond its traditional role in infrastructure monitoring and simulation. This approach was applied in Thessaloniki’s Smart Mobility Living Lab, focusing on enhancing citizen involvement through a structured three-step process involving ideation, co-creation, and validation workshops.

The application of this methodology shows that digital twins can serve as collaborative environments where citizens and decision-makers jointly identify urban mobility challenges, suggest solutions, and visualize their potential impacts. The integration of crowdsourced qualitative data with real-time data enabled the evaluation of specific pedestrian-oriented interventions that, when simulated within the digital twin, were shown to significantly reduce pedestrian waiting times and near-miss incidents, offering quantitative evidence-based insights to support their implementation.

A major contribution of this study lies in demonstrating the role of digital twins in bridging the gap between top-down policy planning and bottom-up citizen feedback, as the inclusion of citizen-generated insights into Thessaloniki’s SUMP validates the practical utility of the suggested methodological approach. Although the methodology developed in the context of Thessaloniki, it can be applied also in other cities, particularly those with existing digital twins. Cities such as Helsinki, Dublin, and Barcelona that are already exploring digital tools for public engagement could benefit from adopting this participatory digital twin approach to enhance citizen involvement in mobility and urban planning processes. Future research should build on these findings by exploring long-term impacts of digital twin-driven engagement on urban policy development.