## 5 Conclusions and Research Outlook  
While cities face unique challenges, this research has identified common patterns, implementation barriers, and potential solutions for sustainable urban logistics. Based on these findings, we have assigned (partially) unused urban infrastructures to the three main groups of sleeping assets, namely (1) neglected routes, (2) idle real estate, and (3) underused resources. We conducted a comprehensive literature study and several interviews with experts from four European cities to examine the opportunities and implementation barriers associated with utilizing each category. Within the selection process of the interviewees, we focused on cities that meet specific requirements, i.e., there is access to water, pressure on public spaces, and the political will to implement innovative logistics concepts, as well as people who have a broad knowledge of underused urban resources. Our primary focus was interviewing city authorities from different departments and real estate companies. We further recognize the value of enhancing our results by exploring different participatory approaches employed in innovative logistics projects and incorporating knowledge of best practice from them. Future research will expand the scope by engaging with interviewees from other European cities and cities from different continents as well as from multiple sectors to enhance the generalizability of insights. We mainly experienced constraints regarding the conducted expert interviews concerning participants' willingness and temporal availability. Nevertheless, in addition to the literature study, we conducted 13 in-depth interviews with relevant stakeholders. This allowed us to gain crucial insights from different perspectives on the status quo of sleeping asset utilization in cities and relevant implementation barriers. The analysis revealed political and legislative framework conditions as one of the major impeding factors in initiating the utilization of urban sleeping assets. Thus, this paper points towards crucial leverages for cities. The adaption of legislative frameworks can facilitate sleeping asset implementation, fostering innovation in this area. In addition, the results underlined the importance of holistic strategies aligning state and city requirements with long-term political objectives to provide a clear direction for city logistics actors to set innovative actions for the sustainable transition and plan investments accordingly. Thus, this work is relevant for practitioners from city administrations and organizations, particularly but not exclusively urban logistics actors, as it can act as a guideline that reveals the untapped potential of increased sleeping asset utilization. Simultaneously, it encourages exploiting this unused potential and avoiding the pitfalls encountered in practice by outlining critical barriers that should be considered during project planning and circumvented during implementation. In this context, the present work constitutes another step towards developing sustainable city logistics systems. Further studies can utilize the presented categorization of sleeping assets and the evaluation of implementation barriers for future case studies in urban logistics practice. This work presents a framework which (1) serves as a starting point for further studies in this research field, and (2) provides vital suggestions for urban logistics stakeholders intending to use sleeping assets within the scope of specific projects. Conceptualization, L.S., Y.K. and P.H. ; methodology, L.S. ; validation, L.S., Y.K. and P.H. ; formal analysis, L.S. ; investigation, L.S. and Y.K. ; resources, P.H. ; data curation, L.S. and Y.K. ; writing--original draft preparation, L.S. and Y.K. ; writing--review and editing, L.S., Y.K. and P.H. ; visualization, L.S. and Y.K. ; supervision, P.H. ; project administration, P.H. ; funding acquisition, P.H. All authors have read and agreed to the published version of the manuscript.