# Urban Logistics through River: A Two-Echelon Distribution Model
## 2 Literature Review
### Transportation Modes in Urban Parcel Delivery Systems  
Existing frameworks for last-mile delivery planning lack important parameters like those that drive customer decisions or the sustainability of the delivery network. Currently, sustainable practices are very important and have special influences on shoppers' decisions. So, some of the largest retailers are driving the shift to fossil fuel-free deliveries [4]. New variants of VRP have been studied to have more efficient and sustainable applications for e-commerce, parcel delivery, and city logistics distribution in the retail sector. Different transportation modalities are taken into consideration in the literature for urban parcel delivery systems; some of these are UAVs [23], AVs such as delivery robots [24; 25], and autonomous mobile lockers (AMLs) [26]. The use of public transport combines people and goods using the metro [27], bus, tram, subway [7], water bus [28], EMVs [9], cargo bikes [26; 29; 30], and finally, EVs [31].  
Results obtained in scenarios using cargo bikes for freight transportation in inner-city areas positioned this transportation mode as a promising alternative to conventional vans to make last-mile delivery; experiments show cost and CO2e emissions savings [32; 33]. The results of these studies promote zero-emission strategies for last-mile deliveries, especially in low-emission zones in inner-cities areas. The adoption of cargo bikes has become popular in several cities since it is a sustainable alternative to the current unsustainable model of freight transport. Pedal Me, in London, is one of the successfully executed projects. This initiative is approximately 1.61 times faster than the one performed by a van. In 98 days, a total of 3896 kg of CO2e was saved [34]. With such scenarios, tonnes of CO2e can be saved per year.  
Due to the objective of reaching free CO2e city logistics in inner-cities areas [35], many European cities, as in other countries, have already been regulating the flows of non-sustainable freight vehicles (mostly with diesel engines) into defined inner-city areas, with for example low-emission zones, or city tolls [31]. According to [36], EVs could be an important part of the solution to achieve this goal. Ref. [37] evaluated some policies that encouraged the use of EVs and measured the impact of the use of this transportation mode on the environment.  
From this partial literature review, we can derive the importance and relevance of increasing the efficiency of urban parcel distribution systems due to the different public policies and city configuration typologies. To design sustainable distribution networks with lower emission levels using the great diversity of transportation modes that can be adapted to each situation, with reasonable costs and efficient routes to ensure customer satisfaction. In the literature reviewed so far, it was found that there are different transportation modes applied to urban parcel delivery. Although vessels are used for long distances, few studies in the scientific community consider making parcel distribution through the city's waterways with small boats. However, some studies in Amsterdam proved that this initiative could reduce congestion in the city center, satisfying the delivery needs of retailers without interfering with other fluvial activities [38; 39]. Consequently, we want to address the 2E-VRP in two different scenarios that consider multimodal transportation to first determine the allocation of customers to the satellites, then to find a set of routes for the first and second echelon, so electric cargo bikes (e-cargo bikes) perform the last-mile delivery. The problem objective is the minimization of the total travel time of the delivery network. So, we can obtain very useful results in terms of CO2e emissions, distance, land use, energy consumption, and fixed cost that can be applied to real cases in cities that have waterways in their configurations.