The Adoption of Green Vehicles in Last Mile Logistics: A Systematic Review

<jats:p>Widespread adoption of green vehicles in urban logistics may contribute to the alleviation of problems such as environmental pollution, global warming, and oil dependency. However, the current adoption of green vehicles in the last mile logistics is relatively low despite many actions taken by public authorities to overcome the negative externalities of distributing goods in cities. This paper presents a comprehensive literature review on studies investigating the adoption of green vehicles in urban freight transportation, paying specific attention to e-commerce. To shed light on the adoption of green vehicles in city logistics, the paper conducts a systematic review of the empirical literature on the topic. The 159 articles reviewed were classified into the following: (a) Optimization and scheduling (67 papers); (b) policy (55 papers); (c) sustainability (37 papers). Among the 159 articles, a further selection of 17 papers dealing with e-commerce, i.e., studies that highlight the most relevant aspects related to the integration of green vehicles in e-commerce urban logistics, was performed. Our findings indicate that green vehicles are competitive in urban deliveries characterized by frequent stop-and-go movements and low consolidation levels while incentives are still necessary for their adoption. The use of autonomous vehicles results the most promising and challenging solution for last-mile logistics.</jats:p>

section_content: The demand for urban freight transportation has increased considerably owing to urbanization and demographic growth, along with the increased diffusion of e-commerce, new management principles (e. g., just-in-time), and the introduction of new pervasive technologies [1]. 

The ever-increasing trade volumes of e-commerce, which still showed a worldwide growth rate of 23. 3% in 2018 and have drastically increased during the COVID-19 pandemic, have led to huge parcel volumes that need to be delivered each day especially in large urban areas. More and more delivery vehicles are required to bridge the last mile towards the customers [2]. 

Due to the increasing number of goods vehicle movements in urban areas, modern cities are facing congestion, lack of public space, air pollution, noise, etc., which are reducing life quality. In fact, as mentioned by Janjevic et al. [3], urban logistics operations have a considerable impact on three different aspects of sustainability: Economic (e. g., efficiency and costs of deliveries), environmental (e. g., CO 2 emissions), and social (e. g., congestion). 

To cope with this situation, city municipal administrations are implementing several sustainability initiatives such as urban consolidation centers, intelligent fleet management systems, use of green vehicles, and putting in place various freight regulations such as vehicle sizing, access timing restrictions, and congestion pricing [4] [5] [6]. The efforts made by public authorities are also in line with the goals that the EU is pursuing with respect to this sector as indicated in the white paper 2011 on transport [7] where it is stated that a reduction of at least 60% of GHGs by 2050 with respect to 1990 is required from the transport sector. Moreover, in the second point of the white paper 2011 it is affirmed that, in order to reach the energy and environmental target set, technological innovations able to raise vehicle energy efficiency are required along with the implementation of new technologies and engines for clean road transport. Among these concepts, this paper focuses on green vehicles adoption due to its relevance in a context where the increase in the e-commerce market and the rise of on-demand logistics imply poor efficiency and might cause additional negative environmental consequences. 

The innovations introduced have been disruptive, especially with respect to new engine technologies and driverless vehicles [8, 9]. The European Commission considers green vehicles (a 'green' vehicle is defined as one with emissions intensity that does not exceed 120 g of CO 2 emissions per km [10] ) relevant for decarbonizing transport, reducing air pollution, and increasing system efficiency. Green vehicles such as electric vehicles (EVs), autonomous vehicles (AVs), and drones will be evaluated in terms of their current integration in city logistics. 

The paper aims at providing a literature review concerning the introduction of green vehicles in the last mile distribution to identify the main issues and challenges. Moreover, this study proposes some research directions and approaches for the advancement of sustainable freight transportation in urban logistics. 

As observed by Hu et al. [11], achieving green logistics goals relies on two dominant strategies: (1) To promote the introduction of green vehicles; (2) to develop and implement policy measures and regulations to reduce the negative effect of city logistics. The research presented in this paper attempts to perform a systematic review on the first of these aspects, distinguishing from previous similar works for this specific focus. The state of the art and trends of the growing research on the integration of green vehicles in urban freight distribution are explored. 

A recent review reporting a comprehensive analysis from a broad perspective of the research on green logistics has been proposed by Ren et al. [12]. They found that the number of papers published in this field has been rapidly growing in the past two years as well as the multitudinous research directions. 

An integrated view of the literature published on the research area of last mile logistics, has been also proposed by Olsson et al. (2019) [13]. They report four previous systematic reviews focusing on specific elements within the area of last mile logistics [14] [15] [16] [17]. Among these studies, only the review proposed by de Oliveira et al. [17] focused on vehicles alternatives for last mile distribution in urban freight. 

In this light, the purpose of this study is to update and consolidate the current understanding with respect to the adoption of EVs, AVs, and drones in urban logistics, shedding light on the e-commerce sector. 

The reminder of the paper is organized as follows. Section 2 outlines the methodology used to perform the literature review, with results presented in Section 3. Section 4 provides an overview of the most relevant aspects related to the integration of green vehicles in e-commerce urban logistics. Section 5 concludes, summarizes the paper, and provides some suggested future research paths.
section_name: Introduction

section_content: The literature selection procedure is shown in Figure 1. The keywords selected in the initial thematic search were: 'autonomous vehicles', 'electric vehicles', 'green vehicles', 'last mile logistics', 'urban logistics', 'city logistics', and 'urban freight'. The query string used for database searches was: ((«autonomous vehicles» OR «electric vehicles» OR «green vehicles» OR «drones») AND («last mile logistics» OR «urban logistics» OR «city logistics» OR «urban freight»)). To ensure homogeneous quality levels of the contribution investigated, we have only included papers published in peer-reviewed journals in English. Consequently, the literature review excludes sources falling outside the perimeter of peer-reviewed articles available online (e. g., conference proceedings, book chapters, and white papers) or not written in English, since this represents the dominant language in last mile logistics research (box C in Figure 1 ). Although some papers were selected since they matched the keywords used in the search process, after reading the abstracts we excluded 48 since the key topic was not aligned with the research focus of our paper (box D in Figure 1 ). As mentioned in the Introduction, the paper aims at improving the understanding of the underlying motivations with respect to green vehicles adoption in last mile logistics. 

A final selection of 159 papers was included in this review. Among these articles, a further selection of 17 papers which provide significant insights into the introduction of green vehicles in urban freight distribution, are discussed in Section 4.
section_name: Methodology

section_content: Urban logistics is a research area that encompasses different fields ranging from engineering to transportation policy and human studies. This work proposes a systematic literature review of articles belonging to different research areas. A classification of the papers reviewed is introduced to better evaluate and discuss their added value and their potential integration in the future research directions about the diffusion of green vehicles in e-commerce and urban logistics. Three main categories were identified: Optimization and scheduling (O), policy (P), and sustainability (S). A similar categorization was also found in other literature review papers [18, 19]. The following criteria were employed for the classification of the papers.
section_name: Results

section_content: Optimization and scheduling: The works investigating operation research problems were included in this category. The methodologies that are generally applied are based on mixed integer mathematical problems formulated to study vehicle routing problems and solved using algorithms aiming at minimizing delivery time, total costs of the activities, or the generated emissions. Neural networks, mathematical optimization techniques, and stochastic modeling are further characteristic applications regarding this category.
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section_content: Policy: The works classified in this category focus on governance, planning, regulations, and incentives for innovative technologies. The topics are centered on:

-Evaluation of policy interventions effectiveness/acceptability before and/or after their deployment. -Innovative incentivization schemes in order to facilitate the spread of 'green vehicles'. 

-Coordination among stakeholders having contrasting interests. 

-Recommendations for policy or decision makers, both public and private, aiming at improving urban logistics. 

The methods that are commonly applied are based on econometric or statistical analyses of empirical data acquired via questionnaires or stakeholders' interviews.
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section_content: Sustainability: Environmental, economic, and social sustainability considerations characterize this group. The papers included usually evaluate some logistics configurations or future scenarios comparing their economic and environmental performances. Social sustainability focuses on quality of life and conflicts concerning the use of space in urban cities. The methodologies that are often applied in this section make use of life cycle analysis, energy/fuel consumption modeling, economic analysis, or performance indicators. 

Some review articles [18-21], employing a multidisciplinary approach, investigated aspects belonging to different categories (e. g., policy and sustainability). In these few cases the papers were classified following the topic that was considered characteristic and prevailing. Finally, the works about e-commerce were simply retrieved verifying that the topic was adequately dealt with in the text. 

Tables A1-A3 in the Appendix A summarize the classification of the main contributions of the relevant papers included in this review sorted by the number of citations; a similar approach has been already proposed by Hilmola [22]. Figure 2 plots the publication year of the works selected. It can be noted that most of the works are quite recent and belong to the 2010-2020 decade. Figure 3 shows the distribution of the works per category: 42% belongs to the optimization and scheduling group (O), 34% is related to policy (P), and 24% is sustainability centered (S). The total percentage of papers containing references to e-commerce is 11% with 5. 7% belonging to the policy group (see Figure 3 ). Figure 4 shows a word map colored for the number of authors per country: As can be expected, the most of them are affiliated with research institutions settled in developed countries (e. g., Europe and North America). Table 1 shows the most active authors with more than three papers in the sample analyzed. Friedrich-Schiller-Universität Jena 3 68
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section_content: As reported in Figure 2, the academic interests in urban freight distribution from the operation research point of view is significantly increased through time. Most of the literature focus on green vehicles routing and scheduling problem (GVRSP). This aims to minimize green-house gas (GHG) emissions in logistics systems through better planning of deliveries/pickups made by a mixed fleet of vehicles (green and traditional). The optimization techniques can be applied to several aspects capable of reducing externalities in smart logistics [14]. Cattaruzza et al. [23] provide a general overview of the most relevant studies of the GVRSP for city logistics. 

In the mathematical formulations of the GVRSP the charging time is considered a downtime and the insufficiency of charging infrastructure as well as the battery limitations are considered a barrier for the integration of green vehicles in urban logistics. 

Many studies propose models and applications for the optimal location of charging stations for electric freight vehicles [24] [25] [26]. A recent article by Cortés-Murcia et al. [27] propose a novel variant of the electric vehicle routing problem (EVRP) where the customer visits are allowed by an alternative means of transport while the EV is at a recharging station. 

Recent studies have proposed models for the GVRSP considering a mixed fleet of traditional manually operated vehicles and autonomous vehicles (AVs). This represents a substantial technological advance in the field of transportation [28] and is expected to play an important role in supporting the last mile logistics within intelligent transportation systems [29]. Bucsky [30] highlights that autonomous vehicles can navigate less efficiently than human driven trucks and special drop-off places should be created since parking for a longer period and lack of human communication with other drivers could generate traffic congestion in mixed fleet scenarios. 

To reduce excessive road traffic from last mile deliveries, Boysen et al. [31] propose the use of unmanned aerial vehicles (also known as drones) launched from trucks. The authors provided an optimization model where the truck serves both as a mobile depot for the goods and as a mobile launching platform for one or multiple drones based on the top of the truck. The en-route launch of the drone, i. e., the drone is launched from the truck and later re-joins the truck, and its relative optimization model has been also proposed by [32] [33] [34]. 

Since the standard GVRSP does not allow multiple trips to the depot, Dorling et al. [35] suggests solving drone delivery problems with a multitrip vehicle routing problem to overcome the difficulties caused by the short operating ranges of drones. Other studies [36, 37] are aimed at finding the best spatial distribution of facilities of launch and recharge stations for a drone delivery system. 

It is worth mentioning that recent and comprehensive review on GVRSP from various perspectives can be found in Moghdani et al. [38] and Ferreira et al. [39].
section_name: Optimization and Scheduling Group

section_content: One of the main issues regarding policy applied to urban freight logistics is to develop methods to support decision makers for the ex-ante evaluation of possible alternative solutions taking into account the interests and preferences of all different stakeholders involved [40, 41]. The integration and coordination of the different stakeholders (public and private) is a valuable solution to improve urban freight logistics reducing costs and negative environmental externalities [42, 43]. The creation of urban consolidation centers (UCCs), for example, can result in a great commitment for the stakeholders involved and the interplay among them is often characterized by conflicting objectives [44]. As observed by Cleophas et al. [45], the key challenge to developing sustainable urban logistics is collaboration between businesses, logistics service providers, citizens, and the public sector. 

The horizontal and vertical collaboration among courier companies in load consolidation can provide consistent benefits for all the subjects involved [46, 47] also reducing urban congestion and pollution problems. Subcontracting and partnership should be preferred over creating new on-demand dedicated services [48] that would imply a huge investment on warehouse facilities and large delivery fleets. The use of sub-contractors is particularly desirable in urban contexts with strong regulatory constraints, high flexibility, and seasonal variations of flows. The spread of cross borders parcel distribution is boosting the cooperation of different distribution actors in the use of a common information technology platform and in the establishment of cross border settlements. The awareness of political authorities about the potential of e-commerce is growing and new regulations are developing aiming at improving the convenience of the service for customers [48]. 

The usage of small electric vehicles for urban deliveries is incentivized by public authorities because of their capacity for local pollution reduction [49]. For this reason, electric vehicles are often free from restrictions of access to inner city centers [5] ; moreover, their ease of movement through the narrow streets of historic centers facilitates the delivery processes.
section_name: Policy

section_content: The claim for the reduction of negative externalities deriving from urban freight logistics is growing and the attention is focused on economic and environmental sustainability. This is strongly linked to the reduction of greenhouse gas emissions and other air pollutants in urban areas. The strategies that are often considered in order to improve the logistics system from both the environmental and costs perspectives are the reduction of the vehicles size and their electrification [14, 17, 50], conversion of a homogeneous fleet to a mixed one [1, 51], creation of UCCs that work in combination with a mixed fleet for deliveries [14, [52] [53] [54]. 

A recent solution refers to the use of crowdshipping especially for last-mile deliveries, however, its impacts mostly depend on the mode employed, length of detour, parking behavior, and daily traffic variations [55]. Gatta et al. [56] investigate a green-type of crowdshipping based on public transportation, estimating its environmental and economic impacts. 

The use of light electric vehicles, drones, unmanned aerial vehicles, or cargo bikes constitute a solution that can reduce the operational energy requirements in most of the cases analyzed [52, 57] even if the mileage toured is higher than that characterizing traditional vans. The capacity of new delivery vehicle types is lower, and a higher number of tours are necessary to serve the same number of customers. The spread of renewable electricity production is able to further reduce operational well-to-wheel energy consumption increasing the sustainability level of the electrification of the fleets employed for last mile deliveries [58]. The reduction of the operational energy is always coupled with a reduction of some environmental impacts, such as the amount of greenhouse gas emissions. 

Life cycle assessment (LCA) applications are performed by different authors [59] [60] [61] to evaluate the overall benefits when the reduction of the operational energy consumption of urban freight vehicles is followed by an increase of the use of energy storage technologies or the necessity of new urban facilities. The importance of a life cycle approach is also underlined by Taefi et al. and Patella et al. [62, 63]. This article, analyzing the economic sustainability of the application of electric vehicles in urban logistics, stresses the importance of long mileages and long battery warranties as the precondition for their real competitiveness in substitution with traditional internal combustion engines. Moreover, the high cost of the battery is still a strong limit for the diffusion of electric vehicles and incentives are fundamental to guarantee their economic competitiveness [37, 64]. Furthermore, some literature works [65] [66] [67] showed a negative correlation between the reduction of greenhouse gas emissions and the life cycle costs of the delivery system: only in some specific cases it is possible to obtain a win-win scenario. 

The presence of battery charging restrictions and reduced vehicle speed cause the design of shorter routes, the increase of the total daily mileage of tours, and the multiplication of urban facility requirements (e. g., UCC, satellites, micro-consolidation centers, charging stations). Melo et al. [68] provided an example showing that cargo bikes, which have a strong potential in emission and environmental impacts reduction, need to establish an urban facility in city centers to operate adequately. The reduction of the operational energy use and of the related environmental impacts linked to the delivery vehicles causes an increase in the embodied components characterizing the supporting logistics facilities. Moreover, the increase in the number of warehouses causes an increment of the energy required for their operation. Stalaroff et al. [61] show that, in some cases, the proliferation of new urban depots and facilities can result in an increase of the overall life cycle environmental burdens so that their implementation should be carefully evaluated. The application of LCA is strongly recommended since the methodology can evaluate the right compromise between environmental impacts belonging to different life cycle phases avoiding burden shifting. Moreover, the localization of depots in the inner parts of the cities is becoming complicated and scarcely affordable because of increasing land use values [69]. The sprawl of freight facilities in the suburbs or in the external parts of the cities, where the price of land is lower, contributes to increasing the length distances of the journeys. The high competition for land use in the core of the cities also involves the reallocation of space for social activities, green mobility, public transport, and pedestrians. The increasing congestion on the streets is, in fact, reducing the space required for socialization due to the deterioration of the quality of livable areas. Furthermore, the absence of bike lanes and pedestrian safe paths hinders sustainable mobility and provokes road accidents [70]. Dedicated infrastructure for walking, cycling, and recreational activities are necessary in modern sustainable cities and sustainable lifestyles are demanding a reduction of the amount of space for traffic [71].
section_name: Sustainability

section_content: Different studies [61, 72, 73] showed that e-commerce is more sustainable than retail pick-up or traditional shopping. In a life cycle perspective, Weber et al. [74] showed that e-commerce delivery systems are characterized by 30% lower primary energy consumption and CO 2 emissions when considering the warehouses energy demand, energy used in retail stores and headquarters, electricity used for home computer shopping activities and data centers, fuel necessary for transportation (from manufacturer to wholesale warehouse, from the wholesale warehouse to the retail stores or distribution centers, for last mile deliveries), and packaging. The last mile delivery represents, on average, the most important contribution with respect to total CO 2 emissions associated with e-commerce (32%). The wholesale warehousing energy use represents the second contribution (31%) of the total emissions linked to the e-commerce delivery system, but a similar share is registered also for traditional retail (26%). The main difference between the two systems is linked to the increase of the packaging impacts in e-commerce and to the reduction of the energy necessary for customers' transport to retail stores that, in traditional retail shopping, plays a preponderant role. The significant role played by last mile delivery in the definition of the global impacts linked to e-commerce activities explains why the topic attracts such attention by the scientific literature. 

One of the main characteristics of e-commerce deliveries is their small size and light weight. The former property makes the consolidation of traditional diesel vans very difficult [75] and pushes towards the reduction of the size of the vehicles used for last mile activities in response to smaller, fragmented, and more frequent deliveries. Boosted by the spread of e-commerce, the number of light goods vehicles circulating in the inner city areas is increasing more than the number of heavy goods vehicles or cars [69]. The reduction of the capacity of fleet vehicles is, however, necessary for deliveries in historic centers characterized by narrow streets. Electrification is pursued since the efficiency of electric engines at low driving speeds (about 50 km/h [67] ) is higher than that of fossil fuel-propelled engines due to regenerative braking. The diffusion of renewable energy generation can furthermore boost the competitiveness of green vehicles in last mile logistics both from an economic and environmental perspective. The drawback of the use of light weight green vehicles is the increase of the travelled distance as shown by different authors [47, 49, 76]. The higher mileages travelled by green vehicles can increase traffic congestion and its negative externalities; off-peak deliveries are suggested by different authors [49] to avoid worsening scenarios. 

The possibility of choosing different delivery options is often given by online retailers with time windows that can be lower than 24 h (same-day delivery) [19]. Customers are demanding easy and fast deliveries and companies are experiencing a high pressure on delivery costs. The on-demand market is characterized by several independent players aiming at increasing the sales of their products and gaining market shares from their competitors also accepting to operate without covering the cost of last mile delivery operations. In order to increase online sales and meet customers' expectations, retailers guarantee 'free' delivery or offer different delivery options with a wide range of time windows. The guaranty of narrow time windows as a delivery option can furthermore stress the system reducing the possibility of demand aggregation and lowering the consolidation level of freight vehicles. The lower aggregation of demand produces an increase of the number of vehicles in circulation, of the distance travelled and, consequently, of the operational energy requirement and related environmental burdens [69, 76]. In these cases, green vehicle or drone deliveries, replacing vans that are poorly consolidated, can play an important role in optimizing the system from both an environmental and economic perspective. However, even in case of drone usage, Dorling et al. [35] showed an inverse exponential relationship between the time limit and the total cost of the deliveries.
section_name: The Adoption of Green Vehicles on E-Commerce City Logistics

section_content: This systematic literature review showed that academic interest in the adoption of green vehicles in urban logistics has significantly increased as documented by the increasing number of publications on this topic in recent years. 

This study confirmed that city logistics is intertwined with many aspects belonging to different fields related to urban planning, especially when considering sustainability. Three main categories were identified with respect to the various perspectives of the papers dealing with urban logistics: Optimization and scheduling (O), policy (P), and sustainability (S). 

Green vehicles routing problems (GVRP), which represent the most relevant contribution of operations research to green logistics, are included in the "optimization and scheduling" category. Our analysis confirmed findings from Moghdani et al. [38], showing that the number of research studies interested in GVRP has grown rapidly. 

Policy actions aiming at increasing the sustainability level of the system and the collaboration between stakeholders are discussed in the second category. The collaboration has been identified by many authors (e. g., Gatta et al. [40], Cleophas et al. [45] ) as the key factor to increase urban freight sustainability. 

The third category includes studies about environmental (employing LCA methodologies), economic, and social sustainability. The category subdivision resumes three different approaches developed by the literature analyzed in the study of the adoption of green vehicles in last mile logistics. 

The issues connected to the rise of e-commerce are addressed by papers classified in each category identified. E-commerce is more sustainable than traditional retail shopping [61, [72] [73] [74] : last mile delivery and warehouse management represent the trickiest phases but an important reduction of the environmental impacts linked to customers travelling to retail stores, that in traditional shopping are the highest, can be reached. However, the effort in market shares acquisition by independent online sellers, the guaranty of narrow delivery time windows options, and the small size of the largest part of delivered items challenge the consolidation level of traditional diesel vehicles stressing the system and increasing its negative externalities [19, 76] : The higher resulting number of vehicles in circulation and the higher distance travelled increase the energy use and the connected environmental burdens related to the delivery phase. In order to respond to this challenge, the adoption of green vehicles (lightweight electric vehicles, cargo bikes, unmanned aerial vehicles (UAVs), etc. ) are being evaluated because of their potential to reduce the environmental burdens connected to last mile delivery, particularly in urban environments [52, 57, 58, 68]. Their adoption is also incentivized by administration authorities to control local pollution [5]. The life cycle economic and environmental sustainability of the process is however still debated since the high impacts of batteries [62] and the necessity of new depots and facilities characterized by not negligible energy requirements [61, 68, 77] contribute to worsen this scenario. 

The development of vehicle technologies such as electric vehicles, connected and automated vehicles, and drones, as well as the introduction of new business models for freight transportation, requires new strategies to control the urban transportation system (freight and travelers) effectively and globally. There is a need for a system-based vision, and therefore researchers are encouraged to chart new territory in the literature by exploring ways in which overall urban transport sustainability can be enhanced. Even more important, whenever innovative solutions or disruptive changes, such as those related to the introduction of green vehicles, are tested and implemented, accurate behavioral analyses based on stated preference methods are needed to investigate stakeholders' acceptability and reactions. 

As to the limitations of the research presented in this paper, this review is subject to the potential omission of relevant studies which have not been covered by the keywords. Moreover, the search excludes records which may potentially be relevant such as books, chapters, conference proceedings, editorials, reports, and articles not in English.
section_name: Conclusions

section_content: On behalf of all authors, the corresponding author states that there is no conflict of interest. Model for the evaluation of the logical and environmental performances of three types of urban distribution systems: the traditional system and two concepts using urban distribution centers (one using vans, the other using automated underground vehicles).
section_name: Conflicts of Interest:

section_name: Nomenclature

section_content: Comparison of the emissions reduction potential of two delivery modes: one using UAV and the other using trucks. 

Stolaroff et al. [61] 2018 82 S

LCA of an urban logistics system employing UAV: drones consume less energy per package-km than delivery trucks but the additional energy for warehouse requirements and the longer distances per package travelled greatly increase the life-cycle impacts. Small drones are more environmentally friendly. 

Arvidsson [ Regressive model for the evaluation of the reduction potential of the environmental impacts (CO, NO x, SO x, PM) resulting from the implementation of some sustainable measures in urban logistics. 

Melo and Baptista [68] 2017 47 S The use of electric cargo bikes is a sustainable option for last mile delivery: when replacing all the conventional vans, the well-to-wheel CO 2 emission saving is 73%.
section_name: Goodchild and Toy