## 4 Discussion  
This study identified three main categories of sleeping assets: neglected routes, idle real estate, and underused resources. Using these assets for city logistics can help to increase the sustainable flow of goods. Furthermore, shifting transportation from road to waterways or railways relieves congested urban road systems. Based on our analysis, we have identified seven main categories of implementation barriers with respect to user resistance, stakeholders, financing, politics and legislation, infrastructure, technicality and technology, and situations that cannot be influenced. It is crucial to consider these barriers in their interconnected context. For instance, infrastructure barriers often coincide with technical and technological barriers. In many cases, the existing infrastructure requires adaption to meet the needs of logistics, and it may lack necessary technical devices for logistics processes. The provision of specific technical equipment is dependent on financial investment, necessitating long-term planning security for logistics providers. These interconnections illustrate the strong interaction and dependencies among the barriers. Nevertheless, addressing these barriers during the planning phase of innovative logistics concepts can significantly contribute to successful implementation. In addition to these aspects, the planning phase of such concepts frequently encounters resistance barriers from users. This resistance can be mitigated by (1) involving stakeholders, (2) effectively communicating the benefits of the logistics concept, and (3) offering additional services. By overcoming these barriers and successfully implementing logistics concepts, the reluctance of users and stakeholders can be diminished. Furthermore, it is essential to address barriers related to financing, particularly due to a disparity between actual social costs and the visible costs associated to delivery services. This discrepancy fails to accurately reflect the true social costs incurred by such services. This factor plays a decisive role in determining the economic viability of implementing a sleeping asset model. It also represents a significant opportunity for state and city authorities to effect change through legislation, creating economic incentives for the efficient utilization of available urban resources. Political and legislative barriers emerged as the most frequently discussed subjects during the interviews, indicating the great potential for establishing an appropriate legislative framework that fosters innovation in urban logistics. Consequently, it is crucial to continuously review and adapt existing laws. Engaging in a dialogue with practitioners is highly recommended to develop regulatory solutions that facilitate sustainable innovations in this field. Long-term planning and a comprehensive urban development strategy encompassing the integration of urban logistics serve as supportive measures in this regard. Although most implementation barriers apply to all three categories of sleeping assets, some relate more to one specific category. For neglected routes, our interviewees saw the installation and maintenance of certain technical equipment as the main barrier. Cities or states could use this knowledge as leverage. By investing in such infrastructure, the use of neglected routes, such as waterways, can be promoted. Furthermore, expanding the infrastructure has the potential to significantly reduce the entry barriers for transportation operators to transition their operations to alternative modes. This, in turn, would enable a smaller portion of the route to be covered by road transportation. Several European cities have already implemented different usage concepts for neglected routes, demonstrating the successful shift of transportation from road to rail- or waterways. The reduction in infrastructural barriers and the increased utilization of waterways and railways also contribute to advancements in technology and techniques in this domain, gradually eliminating these barriers.