## 4 Discussion and Future Research  
Previously, a simulation model was developed to examine a geometrical model-based new city logistics approach where cargo bikes were used in the logistics system; the theoretical model, the symmetrical model, and the model of the real system were developed, and this model was applied for the current network of the shopping malls of Budapest. Such an approach can be very useful in the future to plan new cargo bike-based city logistics systems and to integrate the use of cargo bikes into smart cities. The primary purpose in the next steps is to define the reference geometrical model, which could be used as a benchmark in the future for the examination of any similar city logistics system. Knowing this reference geometrical structure, it will be possible to determine to what extent the reference structure approximates a given model, so it will be possible to see to what extent it is worthwhile to apply this geometric model-based city logistics approach for a given urban area. First, the plan is to define this structure by use of the presented simulation model in a heuristic way, and later we would like to prove its optimality in a mathematical way. There can be many aspects to define the reference structure; the proper selection of the objective function will be critical. It seems clear from the previous studies that multi-criteria examination will be necessary. Some ideas were already formulated (see Figure 15) for defining the reference geometrical model, along which the next research phase will be able to get started. The plan is to estimate the central ring with the best fitting circle/ellipse, so in the reference structure, the CSDLs will be moved to this circle; adding the rays to this, we are going to look after the smallest total squared deviation between the original and the new positions of the CSDLs, by rotation of the best fitting circle and the rays. In this model, the CSDLs can be moved to other rays if that would give the smallest deviation. Additionally, the optimal position of the consolidation center in the examined structure and the optimal number of consolidation centers will be important questions, too. In this paper, primarily the case of Budapest and the theoretical model were examined, but in the future, the examination of other cities will be important too, and we would like to develop a ranking model which can help to decide, in the case of a given city, if it is worth implementing this geometrical model-based thinking or not. For this, the reference model will be able to help us. For some European cities, the examinations are already started, and based on them, Vienna and Warsaw seem to be good options for a similar concept. For this research, the analysis of the current logistics system and the estimation of the goods amounts will be very important for them, as based on our results, for example, in the case of Vienna, the characteristics of the shopping malls are much different to Budapest, and this must be considered at the estimation of delivery demands in the model. It should be highlighted that in the case of the cities examined so far, there was no opportunity to evaluate the suitability precisely, so in the future, it will be very important to develop a ranking methodology to decide whether it is worthwhile to study such concepts in each city. Such a multicriterial method could significantly help in the future the application of this approach for a given city. Of course, in such a model, several indicators should be considered, and these indicators can have two main groups: geometrical model-based indicators and technology-based indicators. In the case of the geometrical model-based indicators, the urban structure, the area to serve, the number of CSDLs, the properties of the model, and the network-related entropy-type indicators can be interesting.