## 1 Introduction
### Challenges of Stakeholder Engagement in Urban Planning and Design
#### 1.1.2 Facilitating Remote Two-Way Communication  
Mixed-reality visualization technologies for stakeholder communication have been studied and, to some extent, been used in the disciplines of architecture and urban planning and design [20; 40]. Previous research indicates that the use of AR in conjunction with the use of mobile phones for citizen evaluation of built environment proposals bring enhanced feedback from citizens compared to the use of other forms of visualization and representation [37; 41; 42; 43]. Likewise, immersive VR and using 3D modeling platforms for urban environment representation have shown an increased quality of citizen engagement [44; 45]. In architecture, VR has been used for communication between the stakeholders in actual projects [46; 47], showing cost benefits and supporting evaluations based on increased spatial awareness by stakeholders even though the implemented modification is only made virtually [40]. Furthermore, to facilitate two-way communication between the urban planners and designers and the stakeholders, there is a need for possibilities for non-expert stakeholders to put forward their vision [37; 48; 49; 50; 51]. Here, non-expert stakeholders include those who are not familiar with architectural and urban planning visualization methods, including the use of 2D and 3D drawing and modeling software. They would typically have difficultieseasily understanding and interpreting architectural drawings and urban plans and lack the skills to fully bring forward their vision of spatial design using the digital visualization medium. While immersive technologies seem to provide stakeholders with increased spatial perception [37; 38; 41; 51; 52] and heightened engagement [45; 50; 51], stakeholder input is still often created through verbal or written feedback [50]. Some 3D-modeling software and VR platforms directly reflect user feedback by modifying the spatial design [40; 45; 53]--changes experienced by all stakeholder simultaneously. Still, in these cases, a designer or a 3D modeler is present at the sessions with the stakeholders, modifying the spatial design using 3D-modeling software based on the stakeholder feedback. The stakeholders can then experience the changes while being immersed in the VR environment. For example, if the width of a corridor is experienced to be too narrow by a stakeholder, the 3D modeler increases the width, and the impact of the stakeholder input is directly experienced. This has shown an increased efficiency in communication between the designers and end-users, as well as cost benefits in design communication and modification [40; 54]. Multiple-user collaboration in shared virtual environments--especially when combined with immersive visualization technologies--has the potential to facilitate simultaneous design collaborations where stakeholders can collectively plan, design and create virtual environments [55; 56]. Such multiple user virtual environments (MUVEs) allow users \"to access virtual contexts, to interact with digital artifacts, to represent themselves through 'avatars,' (and) to communicate with other participants\" [57]. From the planning and design collaboration perspective, MUVEs have been seen to support remote collaboration and education in architectural designing [56; 58; 59]. Moreover, collaborating while observing and visualizing other stakeholders' movements and design trials enhanced explorative creativity compared to working alone [59]. Desktop-based MUVEs, where diverse stakeholders collaborate and communicate through VR and AR media, might, thus, provide efficient communication and collaboration platforms in the context of complex spatial issues related to the urban environment [59; 60]. Especially bringing mobile (e.g., smartphone-based) AR (MAR) into such MUVEs would facilitate taking the task of designing built objects away from the 3D virtual environment out to the actual project site [37], with benefits of providing the site-specific \"auditory, olfactory, haptic and kinetic experiences\" [28] inherent in urban spaces.