Toward Greener Supply Chains by Decarbonizing City Logistics: A Systematic Literature Review and Research Pathways

Abstract

The impacts of climate change (CC) are intensifying and becoming more widespread. Greenhouse gas emissions (GHGs) significantly contribute to CC and are primarily generated by transportation—a dominant segment of supply chains. City logistics is responsible for a significant portion of GHGs, as conventional vehicles are the primary mode of transportation in logistical operations. Nonetheless, city logistics is vital for urban areas’ economy and quality of life. Therefore, decarbonizing city logistics (DCL) is crucial to promote green cities and sustainable urban living and mitigate the impacts of CC. However, sustainability encompasses the environment, economy, society, and culture, collectively called the quadruple bottom line (QBL) pillars of sustainability. This research uses the QBL approach to review the extant literature on DCL. We searched for articles on SCOPUS, focusing on analytical scholarly studies published in the past two decades. By analyzing publication years, journals, countries, and keyword occurrences, we present an overview of the current state of DCL research. Additionally, we examine the methods and proposals outlined in the reviewed articles, along with the QBL aspects they address. Finally, we discuss the evolution of DCL research and provide directions for future research. The results indicate that optimization is the predominant solution approach among the analytical papers in the DCL literature. Our analysis reveals a lack of consideration for the cultural aspect of QBL, which is essential for the applicability of any proposed solution. We also note the integration of innovative solutions, such as crowdsourcing, electric and hydrogen vehicles, and drones in city logistics, indicating a promising research area that can contribute to developing sustainable cities and mitigating CC.

1. Introduction

Scientific reports indicate that the world’s temperature has risen by approximately 1.1 °C from the late 1800s to 2020, highlighting the increasing severity and frequency of climate change (CC) impacts [1,2]. Consequences of CC include but are not limited to several disasters, sea level rise, extreme weather events, and intense droughts, all of which influence life on Earth. For instance, rising sea levels lead to floods, while wildfires result in the loss of forests and biodiversity [3]. Hurricanes and cyclones may cause loss of life and property, and drought and flooding create health issues [4]. These are only some examples of the severe consequences of CC, rampantly impacting humanity and all life on the planet. These indicate the necessity of determining the root causes of CC and acting to curb them.

Several factors contribute to global warming and, ultimately, to CC. Research demonstrates that the main drivers of global warming are GHGs produced primarily by burning fossil fuels used in conventional vehicles [5] and heat and electricity production. These GHGs comprise carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and several others, where CO₂ accounts for approximately 75% of the total amount of gases emitted as of 2023 [6]. Studies identify production, transportation, and residential life as the primary sources of GHGs. Scientists underline that the continuously increasing global temperatures seriously threaten the sustainability of human society [7]. Therefore, it is crucial to generate solutions to minimize the GHGs released into the atmosphere and take action to lessen the drivers of and mitigate the rampant and severe impacts of CC.

Transportation-related operations produce approximately one-fourth of the CO₂ emissions contributing to global warming [8]. Most of these emissions are produced during logistics operations where the primary mode of transportation is conventional vehicles that use fossil fuels [9]. This study emphasizes logistics operations in urban areas, which we will call city logistics. In Figure 1, we present a pictorial description of a typical city logistics process where high levels of GHGs are produced due to the transportation of various goods. Since the population and demand for city logistics continuously increase, these emissions are expected to rise [10,11]. This exacerbates environmental and societal well-being by increasing global warming and air pollution, raising various health concerns [12]. However, logistics operations play significant roles in cities in terms of economic development, and transportation of essential goods and passengers that meet urban life demands (cf. [13]). Therefore, removing these operations cannot be feasible in terms of the sustainability of cities. Yet, greener cities can be achieved by developing solutions that have the potential to mitigate the negative impacts of city logistics.

Reducing GHGs helps green supply chains. A supply chain encompasses the operations concerning the flow and transformation of goods, from the extraction of raw materials to final delivery to the end customers, along with the associated information flows [14]. Optimizing these operations and utilizing emerging technologies can help minimize GHGs in supply chains. For instance, greener vehicles (e.g., electric trucks, hydrogen vehicles, e-bikes, etc.) can be used for logistics operations in cities. Various green supply technologies, such as spatial modulation (SM) and index modulation (IM) can also be beneficial. These advanced communication technologies can improve the efficiency of information flow, and help monitor vehicle conditions and optimize routes [15,16]. Utilizing such innovative technologies is an essential step for achieving greener supply chains and decarbonizing city logistics (DCL).

Many recent studies on DCL have emerged. Decarbonization refers to reducing GHGs and it is subject to several standards. For instance, European Emission Standards limits GHGs per kilometer for passenger cars (e.g., CO₂ emissions limited to 95 g) [17]. California Air Resources Board Zero-Emission Mandate requires all new automobile sales to be zero-emission vehicles by 2035 in California [18]. These are some of the standards for decarbonization, and they are crucial to meet international targets set in the Paris Agreement, which includes limiting global warming to below 2 °C [19].

This study reviewed the analytical articles in the DCL literature that studied potential solutions to achieve more sustainable cities. Moreover, we discuss these articles considering the quadruple bottom line (QBL) pillars of sustainability within the supply chain framework, which involves the environment, economy, society, and culture, as introduced by Ülkü and Engau [20]. In that sense, this study aims to explore the current research efforts that propose analytical solutions for DCL by considering QBL aspects. We aim to address the following research questions (RQs):

RQ1: What is the current state of the research on DCL?

RQ2: What analytical approaches have been adopted?

RQ3: Which QBL aspects are studied in the DCL literature?

RQ4: How is DCL research evolving? What are the barriers, enablers, and future research venues?

This study contributes to the DCL literature in various ways. First, we analyze relevant analytical articles and demonstrate the current state of the research on DCL. Second, we present various analytical approaches adopted to generate potential solutions for distinct challenges associated with city logistics. We report the patterns we observe in these potential solutions that practitioners and policymakers can use in their decision-making. Third, based on our analysis, we discuss the most recent changes and innovations in city logistics in terms of barriers and enablers that create potential areas of study for future research. Finally, by adopting a QBL perspective, we collectively consider the four pillars of sustainability to examine the analytical studies in the DCL literature. The existing articles that review the DCL literature consider environmental, economic, and social sustainability, overlooking the cultural dimension (e.g., [21,22]). However, consideration of cultural aspects is also critical for developing effective and sustainable solutions, as neglecting them can lead to conflicts with cultural identities and limit the acceptance and effectiveness of proposed solutions. Thus, we posit that all QBL aspects should be taken into account while developing sustainable solutions for city logistics. Therefore, this study examines which QBL aspects are considered in the DCL literature. To our knowledge, no scholarly article used a QBL perspective to review the analytical studies in the extant literature on DCL.

The remainder of this paper is structured as follows: Following the introduction, Section 2 discusses the research background and motivation of the paper. Section 3 describes the methodology employed to conduct this literature review. In Section 4, we present and discuss the results from the bibliometric analysis. Section 5 details the selected the literature on DCL and their relation to the QBL and optimization aspects. Finally, Section 6 discusses the innovations, barriers, and potential areas for future research in the DCL literature.

2. Research Background and Motivation

2.1. Research Background

According to recent data, more than half of the global population resides in urban areas as of 2023 [23]. This percentage is poised to increase due to the migration of individuals to developed cities in search of better opportunities [24]. In 1950, only around 30% of the world’s population lived in urban areas [25]. However, experts predict that in 2050, approximately 70% of people will dwell in cities, increasing to 85% by 2100 [26]. Additionally, urban areas account for 80% of the global domestic product, highlighting their significance in the global economy [27]. City logistics is critical in driving this economic growth by facilitating the transportation and storage of various goods within urban areas to meet demand most efficiently and sustainably [26]. Nevertheless, the growing urban population adds complexities to city logistics.

City logistics deals with the inter-urban and intra-urban flow of products (recall Figure 1). Inter-urban flow represents the inflow and outflow movement of goods with either a destination or origin outside the city limits. Product inflow emphasizes transporting demanded goods to the city from external regions. In contrast, the outflow involves the transportation of product returns and waste from the city to external regions. Examples of inter-urban product inflows include but are not limited to, package deliveries associated with business-to-consumer (B2C) e-commerce and the shipment of large quantities of goods in business-to-business (B2B) deliveries. On the other hand, e-commerce product returns and the removal of waste from the cities exemplify the outflow operations. In contrast, intra-urban flow involves transporting goods where both the origin and destination points are within the city limits, such as online food and grocery deliveries [28].

The B2C e-commerce and online food and grocery delivery sectors have experienced substantial growth over the last decade and emerged as significant contributors to the city economy. The COVID-19 pandemic, in fact, further accelerated the growth of these sectors as individuals began to spend more time at home [21]. Reports indicate that global e-commerce sales reached approximately USD 5.5 trillion in 2023, up from USD 1.3 trillion in 2014 [29]. The online food and grocery delivery sector, too, saw a surge in demand, with its market size estimated at around USD 370 billion in 2019 and growing to over a trillion USD in revenue as of 2023 [30]. This growth and the subsequent impact on the city economy underline the importance of e-commerce and online food and grocery delivery industries, making the audience feel the significance of these sectors.

The delivery process of goods shipped by e-commerce companies involves city logistics. Amazon, Walmart, and Alibaba are the largest e-commerce companies globally. Amazon is the market leader, capturing 37.6% market share in the United States [31] and generating approximately USD 575 billion in annual net sales revenue in 2023 [32]. As the largest e-commerce company, Amazon’s delivery process may represent a typical e-commerce delivery in cities. The delivery process begins with the customers’ online orders, which are picked and packed at Amazon’s warehouses. After that, Amazon transports the packages to an urban distribution center located within or near the city, using large trucks and possibly additional modes of transport. Employees organize these packages at the distribution center and load them onto smaller trucks. Finally, the packages depart from the urban distribution center and are delivered to the end customers residing in the city [33]. The stages following the arrival of goods to the distribution center are considered last-mile logistics. The last-mile delivery of goods to the end customers is also considered the final of many preceding supply chain stages. B2B commerce is also a vast industry that follows a similar delivery process involving the logistics of large quantities of products.

As an example of an intra-flow city logistics operation, the online food and grocery delivery process differs from the e-commerce industry as it is entirely carried out within the city limits. UberEATS and DoorDash are the leading online food and grocery delivery companies, achieving respective market shares of 23% and 67% in the United States as of March 2024 [34]. These companies and the entire industry are poised to grow further in the following years as demand for food and groceries ordered online becomes increasingly popular among consumers living in urban areas [35]. To that end, “crowdsourcing” has emerged as a vital concept for city logistics in catching up with the rampant demand for delivery capacity. Crowdsourced delivery is a method that introduces the enlisting of public drivers as couriers to deliver customers’ online food and grocery orders. The couriers are independent contractors who use their vehicles to deliver the goods [36], mainly food and groceries.

All these operations are essential to sustaining cities’ livability and meeting urban dwellers’ demands. However, there are several sustainability concerns associated with them. In the case of B2C e-commerce deliveries, transporting goods to customers’ homes or designated pickup locations instead of retail stores leads to a significant increase in the number of freight movements. As conventional vehicles are the primary mode of transportation in city logistics, this results in a considerable increase in GHGs produced within cities. It also worsens issues such as traffic congestion and air and noise pollution, which are already significant challenges, particularly in densely populated cities. Moreover, many people in cities have started to prefer ordering meals and groceries online from platforms such as UberEATS and DoorDash instead of visiting restaurants or supermarkets. Such shifts in consumers’ lifestyles also increase vehicle use, causing significant harm to the environment. Therefore, although these operations play an essential role in cities, they also pressure city logistics and raise several environmental concerns. Hence, solutions for DCL are required to minimize the environmental impact of these and several other operations concerning city logistics. However, sustainability does not only consist of the environment but also the economy, society, and culture. Thus, consideration of all QBL aspects for any potential solution is essential.

2.2. Research Motivation

Environmental, economic, social, and cultural sustainability, which are overall called QBL pillars, are considered in this study for the discussion of articles we selected for the review. Four aspects of QBL are provided in Figure 2. We adopt a QBL perspective instead of the commonly used triple bottom line (environmental, economic, and social sustainability) because the real-life applicability of the solutions proposed in the existing DCL literature highly depends on their acceptance by the culture in which they are intended to be applied. The authorities will evaluate a solution based on their area’s cultural identity and decide whether to utilize a proposed solution. If a solution contradicts a society’s unique cultural characteristics, it may not be applicable or effective. Therefore, it is imperative to consider various cultural aspects while developing an influential solution. Given this, the traditional TBL approach falls short in accounting for cultural factors, making QBL a more comprehensive and critical approach. Hence, this study examines the proposed solutions in the DCL literature through the lens of the four QBL pillars of sustainability.

Given the urgent need for DCL to achieve more sustainable cities, we analyzed and reported the current research trends in the DCL literature. Considering our keywords, we analyzed the number of scholarly articles written in English each year between 2004 and 2023 to observe the emphasis on the DCL literature and the portion of articles that took sustainability into account. We obtained data on the number of scholarly articles from SCOPUS. First, we retrieved the number of articles published each year from the search term “city logistics” and its synonyms, represented with blue (see Figure 3 and Figure 4). Second, we also added sustainability to city logistics and recorded the data (represented in yellow). Third, we added decarbonization to “city logistics” and excluded the sustainability term (represented with green). Finally, we combined all key terms (represented in red). Figure 3 illustrates the number of published articles we obtained when the key terms were searched in all fields (title, abstract, keywords, text) of the articles. On the other hand, Figure 4 represents the number of published articles we obtained when the key terms were searched only in articles’ titles, abstracts, or keywords. We aggregated the years so that each of the five periods represents a four-year subperiod.

We observe a similar trend in all four search terms in both Figure 3 and Figure 4, demonstrating an increase in the number of scholarly articles published, especially after 2008–2011. Moreover, the number of articles has increased significantly in the final four-year period, 2020–2023. Considering these, we deduce that city logistics, decarbonization, and sustainability terms have recently drawn significant attention from researchers, which is reasonable considering the urgent need for action to minimize the impact of city logistics on CC. Second, we noticed that the concept of decarbonization has received relatively less attention in the context of city logistics. As given in the final period of Figure 4, only 86 articles used the term “decarbonization” in their titles, abstracts, or keywords, whereas there are 746 articles that used “city logistics”. This ratio is even lower in the prior sub-periods. One potential reason for this can be the unfamiliarity of the term since, based on our observations, several studies have proposed solutions to reduce CO₂ emissions in city logistics, yet they do not explicitly use the term “decarbonization” in their papers. An overall deduction from Figure 4 is that research focusing on decarbonization and sustainability in the city logistics context is still scarce despite the consistent increase in studies published throughout the last 20 years.

Although it seems from the final period (2020–2023) of Figure 3 that a great majority of studies proposed integrating a sustainability perspective into their research, Figure 4 shows that only a few considered sustainability as an essential part of their study. Specifically, looking at the final period, 2020–2023, Figure 3 shows that approximately 81% of the articles that include “city logistics” terms mentioned “sustainability”. On the other hand, this percentage is approximately 38% in Figure 4, where the key terms are only looked up in articles’ titles, abstracts, or keywords. Nevertheless, the number of articles containing the term sustainability in the context of city logistics has increased in both figures over the last two decades. This demonstrates that more researchers studying city logistics are considering sustainability in their articles. The increasing number of studies and the lack of a QBL perspective in the DCL literature motivates this study.

3. Method

In this study, we adopted the systematic literature network analysis (SLNA) (cf. [37]) and conducted a systematic literature review followed by a bibliometric network analysis. We used SCOPUS, a well-known academic database, to determine the articles for our literature review. We used Boolean operators such as “AND” and “OR” to develop our search term. Following several adjustments, we decided to use the following search term on SCOPUS: (“city logistics” OR “urban logistics” OR “urban freight transport”) AND (decarbon* OR emission) AND sustainab*. In our search term, we used two synonyms for “city logistics” to avoid missing studies that used different words to indicate it. Our initial search provided 243 results. However, we decided to include only journal articles in this study. Moreover, as we aim to review the current state of the DCL literature, we included journal articles published between 2003 and 2024. Finally, we excluded any articles that were not in English. As a result, we obtained 142 articles.

To determine articles that employed an analytical method, we analyzed the titles, abstracts, methodologies, and results of these articles. We selected relevant studies that used an analytical approach to study a solution by considering at least two of the QBL pillars of sustainability. As a result, our final sample consists of 64 scholarly articles. All studies selected for review were published in one of the following 33 scholarly journals: Renewable and Sustainable Energy Reviews, Alexandria Engineering Journal, Applied Sciences, ISPRS International Journal of Geo-Information, Uncertain Supply Chain Management, Sustainability, European Journal of Operational Research, Research in Transportation Business and Management, Sustainable Cities and Society, Complex and Intelligent Systems, Computers and Operations Research, Computers and Industrial Engineering, Journal of Heuristics, International Journal of Physical Distribution and Logistics Management, Smart Cities, IEEE Access, Tunnelling and Underground Space Technology, PLoS O.N.E., Transportation Research Part D: Transport and Environment, Journal of Cleaner Production, Transportation Research Record, World Electric Vehicle Journal, Research in Transportation Economics, Energies, Mathematics, Transportation, Transportation Research Part C, Annals of Operations Research, International Journal of Production Research, European Transport, Journal of Transport Geography, European Transport Research Review, and Expert Systems with Applications.

4. Bibliometric Analysis and Discussion

This section addresses RQ1 by analyzing and reporting the selected articles’ publication years, journals, and countries, along with the keyword co-occurrence network, presenting the current state of the DCL research.

4.1. Publication Years

Our initial search on SCOPUS has produced 142 articles. Based on this, the oldest article available using our search term was published in 2007, and only four articles were published until 2013. After narrowing our selection to 64 articles, we created Figure 5 to illustrate the yearly distribution of the published articles we selected for review. Although some studies were published between 2013 and 2019, as indicated in Figure 5, the majority (54 out of 64) of the articles were published following 2019. It is important to note that Figure 5 includes articles published before June 2024. Taking Figure 3, Figure 4 and Figure 5 into consideration, we conclude that research efforts have significantly increased in recent years, reflecting the growing interest in DCL to promote sustainable cities and mitigate the severe impacts of CC. This surge may be attributed to city logistics being a significant contributor to CC and the increasingly intense and widespread effects of CC in recent years. The growing urban population and rise in e-commerce and online food and grocery delivery industries pose further challenges in city logistics, such as increased air and noise pollution, traffic congestion, accidents, and various health issues, offering new areas for research. Considering these, despite the plethora of published articles on city logistics, we posit that DCL calls for more research, especially in the face of the climate crisis.

4.2. Publication by Journals

The articles we reviewed were published in 33 different journals: Among those, only Sustainability, Energies, Sustainable Cities and Society, and Computers and Industrial Engineering published three or more papers on DCL. Figure 6 demonstrates the top ten journals that published articles on DCL, which considered multiple aspects of QBL. 22% of the reviewed articles (14 out of 64) were published by the journal “Sustainability”. The second and third most published journals were “Energies” (6 articles) and “Sustainable Cities and Society” (5 articles).

4.3. Publication by Country

We created the graph in Figure 7 by considering the affiliations of the selected articles’ authors. Our country-wise classification of the articles reveals that China has published the most studies (12) focusing on the DCL and QBL aspects. As the world’s second most populated country with 1.41 billion people [38] and the most significant global manufacturer, city logistics plays a crucial role in major cities such as Shanghai, Beijing, and Shenzhen. Given that China is the leading emitter of GHGs, air pollution and the climate crisis are critical issues, and city logistics significantly contribute to them. These unique characteristics of China may explain the increased research efforts directed toward DCL to achieve greener cities and protect public health. Figure 7 also highlights the interest of the United States and various European countries in sustainability and DCL research.

4.4. Keyword Analysis

VOSviewer is a tool that can provide the relationships between keywords stated in a set of scholarly articles. Providing a keyword co-occurrence network can help observe the current state of the literature by visualizing the trends and patterns, such as the topics that were studied the most. Therefore, as given in Figure 8, we created a co-occurrence network with the authors’ keywords from the selected 64 articles using VOSviewer (version 1.6.20). While establishing the network, we used the complete counting method to tally the total number of keyword occurrences. The keywords are grouped in a way that each of them belongs to exactly one group. It should be noted that we included keywords used by at least four articles to obtain a more precise visualization. As a result, we obtained a total of 51 key terms and 684 links under four main groups (clusters) given in red, blue, green, and yellow colors. We analyzed the keywords used the most for each cluster and their connections.

Analyzing Figure 8, “city logistics” is the central node and stands out the most. There are other nodes such as “urban logistics”, “urban freight transport”, “urban transportation”, “freight transport”, and “freight transportation” that are shown in relatively smaller circles and different color groups. Based on our analysis, all of these terms referred to (freight) logistics in urban areas, although they were named differently. The terms “sustainability” and “sustainable development” are other prominent nodes that are connected both with “city logistics” and its synonyms, confirming the selected articles’ emphasis on sustainable city logistics.

Cluster 1 is denoted in red color in Figure 8. “Sustainable development” is the most prominent node with 18 occurrences. It is connected with 45 other key terms from different clusters. The terms “urban transportation”, “urban freight transport”, and “freight transportation” appear around “sustainable development”, and they are synonyms with a total of 34 occurrences combined. Smaller circles connected with these terms are related to vehicles, cost, energy consumption, and greenhouse gases. We observe terms such as “automobiles”, “trucks”, “electric vehicles”, “costs”, “cost reduction”, “operating costs”, “energy utilization”, and “energy efficiency”, showing that the emphasis is on different types of vehicles, cost efficiency, and energy consumption in the context of sustainable city logistics. Our full-text analysis confirms this, as we encountered several articles that integrated different vehicle types into their optimization efforts to minimize operational costs and energy consumption in city logistics. The term “last mile” also forms a meaningful connection with the nodes “electronic commerce” (cluster 3) and “electric vehicles” as we analyzed several articles that studied the potential use of electric vehicles in the last-mile deliveries of e-commerce products.

Cluster 2 is represented in green color, where “urban logistics” appears as the central node with 11 occurrences and as a synonym for “city logistics”. It is connected with the nodes “carbon emission” and “carbon dioxide”, which are also connected with “vehicle routing problem”, “vehicle routing”, and vehicle routing problems” nodes. These terms are all connected to the “sustainability” term from cluster 3 (blue). These are meaningful connections as CO₂ emissions are highly concerned with environmental sustainability as one of the main drivers of CC, and city logistics is responsible for a significant portion of CO₂ emissions produced. Thus, several articles focused on the reduction of CO₂ emissions. Our full-text analysis revealed that these articles performed optimization and emphasized vehicle routing problems (VRPs) to minimize CO₂ emissions and total costs in city logistics. This is confirmed by the key terms and their connections in cluster 2, which are related to the CO₂ emissions in city logistics and vehicle routing problems.

Cluster 3 is represented by blue and includes some of the terms that occurred the most. “City logistics” is the most prominent node in the network, with 26 occurrences and a total link strength of 143. It is followed by the node “sustainability”, which appeared 24 times and had a total link strength of 135.

“Optimization” is another essential term with 12 occurrences. We anticipated this number would be higher, considering that a majority (39 out of the 64) of reviewed articles focused on optimization and developed mathematical models. One possible explanation for this inconsistency is that some articles utilized “vehicle routing problem” and its variations as keywords instead of “optimization”. Nevertheless, all studies on vehicle routing problems were centered on optimization. Our full-text analysis and the keyword co-occurrence network demonstrate that optimization is integral to the proposed solutions in city logistics research. Another critical key term, “traffic congestion”, has appeared in 13 articles. Traffic congestion in urban areas is closely linked to QBL aspects. The growing urban population and the rise in e-commerce and online food delivery services exert additional pressure on city logistics and increase traffic congestion, resulting in higher GHGs, air and noise pollution, and reduced accessibility. The terms ”electronic commerce” and “crowd shipping” are also connected with “traffic congestion” in cluster 3, supporting this claim. As discussed in Section 4.3, China had the highest number of published articles among the selected ones. China also occurs in cluster 2 and is connected to nodes such as “city logistics”, “sustainability”, and “carbon emission”, highlighting the emphasis on sustainable city logistics and carbon emissions reduction. In summary, cluster 3 underscores the research efforts to optimize city logistics to address associated challenges and achieve sustainable cities.

Cluster 4 (yellow) comprises six key terms: “freight transport”, “urban transport”, “electric vehicle”, “last mile delivery”, “emission control”, and “land use”. As discussed earlier, “freight transport” and “urban transport” refer to the same operation as most articles studied freight transportation in urban areas. Moreover, they are connected with “electric vehicle” and “emission control”, further highlighting the objective of reducing emissions in freight logistics in urban areas by utilizing innovative solutions such as electric vehicles. Finally, we realized that the term “decarbonization” does not occur in the network. Our analysis showed that it is not a commonly used keyword, although almost all the selected articles considered emissions in city logistics.

5. A Recap of Proposed DCL Solutions and QBL Aspects in the Extant Literature

Our initial analysis demonstrated that many articles published within the DCL literature adopted a quantitative approach, from which we selected 64. These analytical papers employed distinct methods. Many studies (39) performed optimization and developed an analytical model. On the other hand, several studies (25) analytically assessed the cost and effectiveness of various innovative solutions. Regardless of their analytical method, these studies aimed to contribute to city logistics operations while referring to at least two or more QBL aspects. Table 1 presents the QBL aspects discussed, albeit weak or strong, by these studies. It also includes the objective functions of the optimization models proposed in those articles. Table 1 and the information provided in this section address RQ2 and RQ3 by presenting the analytical approaches adopted and the QBL aspects studied.

As given in Table 1, propositions made by optimization articles include vehicle route and inventory optimization for freight transportation and location selection for urban distribution centers and parcel lockers. Many of these articles have integrated various innovative solutions into their optimization models. These include crowdsourcing for food, grocery, and parcel deliveries and various transport modes such as electric and hydrogen vehicles, cargo bikes, e-scooters, and unmanned aerial vehicles for freight distribution. Additionally, some suggested using inland waterways (where applicable) [39] and using an underground logistics network for freight distribution in cities.

Many studies have aimed to improve the economic performance of city logistics while reducing the environmental damage it causes. In that sense, a noticeable pattern in these articles’ analytical models was their objective functions, which were set to minimize the total costs, GHGs emitted, or both [9,40,41]. Although several articles did not cast their objective functions, mainly to minimize GHGs/CO₂, almost all claimed an essential reduction in emissions due to their optimization model. There are also several studies with objective functions that minimize the total travel distance/time of the vehicles used for freight transportation. On the other hand, only a few studies minimized social impact in their objective functions.

Our in-depth analysis of the optimization articles provided more specific patterns regarding their analytical models. We observed that many studies emphasize the vehicle routing problem [42,43] and its different variations such as with mixed fleets [44], time windows [45], covering options/locations [46], multi-depot [47], and multi-compartment [48] in city logistics context. Although their emphasis differs, their solutions are subject to similar constraints associated with city logistics operations. These constraints include but are not limited to the load capacity of vehicles [49], supply [50], demand [51], travel distance of vehicles [52], time window [53], and flow balance [54] constraints. By considering multiple constraints, these studies developed distinct analytical models. Optimization articles that integrated the aforementioned innovative technologies into their analytical models included additional constraints based on the suggested solution. For instance, articles that integrated electric vehicles (EVs) such as e-vans, e-bikes, e-scooters, in their model included a constraint for battery levels [53].

We also noticed that common analytical modeling approaches include bi-objective/multi-objective programming, which is used mainly by studies that define multiple objective functions such as minimizing total cost and GHGs [55], integer/mixed-integer programming [52], and various heuristics/metaheuristics [41].

Based on our analysis, some aspects were missing in the proposed solutions of the articles we reviewed. These include consideration of uneven roads and missed deliveries, which may significantly influence the amount of GHGs released into the atmosphere during city logistics operations. Although integration of such aspects can be challenging, they should be considered in order to develop more effective solutions, especially when studying densely populated cities.

Aside from the optimization studies, many articles that studied innovative and practical solutions made an effort to evaluate their implementation. Innovative solutions that were evaluated by these articles include the use of electric [56] and hydrogen [57] vehicles, cargo bikes/e-trikes [58], inland waterways [59], crowdsourced delivery [60] and a metro-based delivery system [61] for transportation of various goods. Studies highlighted the potential benefits of these solutions, such as the reduction of GHGs [58] and air pollution [57], as well as challenges associated with their implementation, such as high costs [12].

Table 1. Pointers to the recent DCL literature and its relation to QBL and optimization aspects.

Paper	QBL Aspects 1				Analytical Aspects 2				Proposition
	EC	EN	SO	CU	CT	EM	TR	SI
Ahani et al. (2023) [52]	✔	✔			✔				Optimal type and number of vehicles: freight transport
Akbar et al. (2024) [62]	✔	✔	✔						Crowdshipping for inter-urban freight transport
Akkad & Bányai (2020) [53]	✔	✔				✔	✔		Optimization of freight distribution with EVs
Aloui et al. (2021) [63]	✔	✔	✔		✔	✔			Inventory, location, and routing optimization
Anderluh et al. (2021) [43]	✔	✔	✔		✔	✔			Route optimization (2E-VRP) in grey zone deliveries
Arnold et al. (2018) [64]	✔	✔	✔						Cargo bikes for freight delivery
Azad et al. (2023) [58]	✔	✔							E-trikes for delivery
Bi et al. (2020) [65]	✔	✔	✔				✔		Optimal location for crowdsourcing stations
Büttgen et al. (2021) [66]	✔	✔	✔		✔				Optimal route and location with e-vans/cargo bikes
Ceccato & Gastaldi (2023) [67]	✔	✔	✔						Cargo bikes for home deliveries
Chen et al. (2018) [68]	✔	✔			✔	✔	✔		Underground freight transport system
Chen et al. (2023) [42]	✔	✔			✔				Optimal cold chain distribution with EVs
Dupas et al. (2020) [54]	✔	✔	✔				✔		Optimization of freight transport flow
Dupas et al. (2023) [69]	✔	✔	✔		✔				Optimal location selection for urban consolidation centers
Enthoven et al. (2020) [46]	✔	✔			✔				Bikes and parcel lockers: optimal delivery (VRP)
Fan (2023) [70]	✔	✔			✔			✔	Route optimization for EVs
Fan (2024) [47]	✔	✔			✔				Route optimization for EVs
Fan et al. (2023) [48]	✔	✔			✔				Route optimization: multi-compartment vehicles
Fontaine et al. (2023) [71]	✔	✔			✔				City freighters for two-echelon freight delivery
Fraselle et al. (2021) [12]	✔	✔	✔					✔	E.V.s and cargo bikes for freight delivery
Gatta et al. (2018) [72]	✔	✔	✔						Crowdshipping using the mass transit network
Gatta et al. (2019) [60]	✔	✔							Crowdsourced delivery
Giordano et al. (2018) [56]	✔	✔	✔						EVs for freight transport
Gruzauskas et al. (2023) [73]	✔		✔						Information sharing impact on food delivery
Guo et al. (2019) [74]	✔	✔	✔						Crowdsourced delivery for last-mile logistics
Guo et al. (2022) [7]	✔	✔			✔	✔			Urban logistics enterprise
Hassouna (2022) [75]	✔	✔							EVs for freight transport
Islam et al. (2021) [9]	✔	✔					✔		Hydrogen vehicles/route optimization
Jaegler et al. (2024) [59]	✔	✔	✔						Optimization with inland waterway and EVs
Jiao et al. (2023) [10]	✔	✔					✔		Location selection for urban logistics centers
Jones et al. (2020) [57]	✔	✔							Hydrogen vehicles for freight transport
Kłodawski et al. (2024) [76]	✔	✔	✔						Intermodal terminals for reduced energy use
Kwasiborska et al. (2023) [77]	✔	✔	✔						Delivery: e-scooters and unmanned aerial vehicles
Labarthe et al. (2024) [78]	✔	✔	✔				✔		Joint use of transport modes for freight and passengers
Lee et al. (2020) [49]	✔	✔	✔		✔				Route optimization with mixed fleet
Leyerer et al. (2020) [79]	✔	✔	✔		✔				Optimal locations for refrigerated grocery lockers
Li et al. (2021) [41]	✔	✔	✔		✔	✔			Logistics infrastructure investment options
Liu et al. (2021) [80]	✔	✔			✔	✔			E-grocery delivery: optimal locations and routes
Ma et al. (2024) [81]	✔	✔			✔				EVs: route optimization
Märzinger et al. (2021) [82]	✔	✔							EVs and charging stations
Moll et al. (2020) [83]	✔	✔	✔						Electric trucks for freight delivery
Nocera & Cavallaro (2017) [84]	✔	✔							Urban distribution center GHGs assessment
Olapiriyakul & Nguyen (2019) [55]	✔	✔	✔		✔			✔	Warehouse location selection and material flow
Peng et al. (2024) [85]	✔	✔			✔				EVs for dispatching in freight transport
Peppel & Spinler (2022) [86]	✔	✔			✔	✔			Optimal parcel locker location
Perera et al. (2020) [87]	✔	✔	✔		✔	✔	✔		Link toll (A new toll-charging scheme)
Pietrzak et al. (2021) [88]	✔	✔	✔						Rail transport for urban freight delivery
Pilati et al. (2020) [89]	✔	✔							EVs for parcel delivery
Pourmohammad-Zia & van Koningsveld (2024) [39]	✔	✔			✔				Waterway based distribution/EVs in last-mile delivery
Ramirez-Villamil et al. (2022) [90]	✔					✔	✔		Route optimization (2E-VRP): stochastic travel times
Ramírez-Villamil et al. (2023) [91]	✔	✔	✔				✔		Route optimization for parcel delivery
Resat (2020) [92]	✔	✔	✔		✔	✔			Optimal cargo distribution using drones
Saeedi et al. (2018) [93]	✔	✔			✔		✔	✔	Location selection for urban distribution centers
Sayarshad et al. (2021) [44]	✔	✔	✔		✔		✔		Optimal routing and scheduling: EVs
Simoni et al. (2020) [94]	✔		✔						Crowdsourced delivery
Teimoury & Rashid (2024) [95]	✔	✔	✔		✔	✔	✔	✔	Optimal freight transport using drones and trucks
Vajihi, M & Ricci (2021) [96]	✔	✔	✔						Urban rail system for freight distribution
Villa & Monzón (2021) [61]	✔	✔	✔						Metro based delivery system with parcel lockers
Voegl et al. (2019) [97]	✔								Developed unloading infrastructure to reduce GHGs
Wang et al. (2023) [98]	✔	✔	✔		✔	✔	✔		Location selection for urban logistics centers
Wehbi et al. (2022) [45]	✔	✔					✔		Optimal routes: on-foot porter for last-mile delivery
Wei et al. (2020) [51]	✔	✔	✔		✔				Underground logistics network optimization
Wong et al. (2020) [50]	✔	✔			✔		✔		Route optimization
Zhang & Cheah (2024) [99]	✔	✔					✔		Crowdshipping for freight delivery

¹ The QBL Aspects here consider qualitative discussion of the four Economic (EC), Environmental (EN), Societal (SO), and Cultural (CU) pillars. ² The Analytical Aspects refer to the optimization (quantitative) modeling of the objectives Cost (CT), Emissions (EM), Travel distance and time (TR), and Social Impact (SI).

Table 1. Pointers to the recent DCL literature and its relation to QBL and optimization aspects.

Paper	QBL Aspects 1				Analytical Aspects 2				Proposition
	EC	EN	SO	CU	CT	EM	TR	SI
Ahani et al. (2023) [52]	✔	✔			✔				Optimal type and number of vehicles: freight transport
Akbar et al. (2024) [62]	✔	✔	✔						Crowdshipping for inter-urban freight transport
Akkad & Bányai (2020) [53]	✔	✔				✔	✔		Optimization of freight distribution with EVs
Aloui et al. (2021) [63]	✔	✔	✔		✔	✔			Inventory, location, and routing optimization
Anderluh et al. (2021) [43]	✔	✔	✔		✔	✔			Route optimization (2E-VRP) in grey zone deliveries
Arnold et al. (2018) [64]	✔	✔	✔						Cargo bikes for freight delivery
Azad et al. (2023) [58]	✔	✔							E-trikes for delivery
Bi et al. (2020) [65]	✔	✔	✔				✔		Optimal location for crowdsourcing stations
Büttgen et al. (2021) [66]	✔	✔	✔		✔				Optimal route and location with e-vans/cargo bikes
Ceccato & Gastaldi (2023) [67]	✔	✔	✔						Cargo bikes for home deliveries
Chen et al. (2018) [68]	✔	✔			✔	✔	✔		Underground freight transport system
Chen et al. (2023) [42]	✔	✔			✔				Optimal cold chain distribution with EVs
Dupas et al. (2020) [54]	✔	✔	✔				✔		Optimization of freight transport flow
Dupas et al. (2023) [69]	✔	✔	✔		✔				Optimal location selection for urban consolidation centers
Enthoven et al. (2020) [46]	✔	✔			✔				Bikes and parcel lockers: optimal delivery (VRP)
Fan (2023) [70]	✔	✔			✔			✔	Route optimization for EVs
Fan (2024) [47]	✔	✔			✔				Route optimization for EVs
Fan et al. (2023) [48]	✔	✔			✔				Route optimization: multi-compartment vehicles
Fontaine et al. (2023) [71]	✔	✔			✔				City freighters for two-echelon freight delivery
Fraselle et al. (2021) [12]	✔	✔	✔					✔	E.V.s and cargo bikes for freight delivery
Gatta et al. (2018) [72]	✔	✔	✔						Crowdshipping using the mass transit network
Gatta et al. (2019) [60]	✔	✔							Crowdsourced delivery
Giordano et al. (2018) [56]	✔	✔	✔						EVs for freight transport
Gruzauskas et al. (2023) [73]	✔		✔						Information sharing impact on food delivery
Guo et al. (2019) [74]	✔	✔	✔						Crowdsourced delivery for last-mile logistics
Guo et al. (2022) [7]	✔	✔			✔	✔			Urban logistics enterprise
Hassouna (2022) [75]	✔	✔							EVs for freight transport
Islam et al. (2021) [9]	✔	✔					✔		Hydrogen vehicles/route optimization
Jaegler et al. (2024) [59]	✔	✔	✔						Optimization with inland waterway and EVs
Jiao et al. (2023) [10]	✔	✔					✔		Location selection for urban logistics centers
Jones et al. (2020) [57]	✔	✔							Hydrogen vehicles for freight transport
Kłodawski et al. (2024) [76]	✔	✔	✔						Intermodal terminals for reduced energy use
Kwasiborska et al. (2023) [77]	✔	✔	✔						Delivery: e-scooters and unmanned aerial vehicles
Labarthe et al. (2024) [78]	✔	✔	✔				✔		Joint use of transport modes for freight and passengers
Lee et al. (2020) [49]	✔	✔	✔		✔				Route optimization with mixed fleet
Leyerer et al. (2020) [79]	✔	✔	✔		✔				Optimal locations for refrigerated grocery lockers
Li et al. (2021) [41]	✔	✔	✔		✔	✔			Logistics infrastructure investment options
Liu et al. (2021) [80]	✔	✔			✔	✔			E-grocery delivery: optimal locations and routes
Ma et al. (2024) [81]	✔	✔			✔				EVs: route optimization
Märzinger et al. (2021) [82]	✔	✔							EVs and charging stations
Moll et al. (2020) [83]	✔	✔	✔						Electric trucks for freight delivery
Nocera & Cavallaro (2017) [84]	✔	✔							Urban distribution center GHGs assessment
Olapiriyakul & Nguyen (2019) [55]	✔	✔	✔		✔			✔	Warehouse location selection and material flow
Peng et al. (2024) [85]	✔	✔			✔				EVs for dispatching in freight transport
Peppel & Spinler (2022) [86]	✔	✔			✔	✔			Optimal parcel locker location
Perera et al. (2020) [87]	✔	✔	✔		✔	✔	✔		Link toll (A new toll-charging scheme)
Pietrzak et al. (2021) [88]	✔	✔	✔						Rail transport for urban freight delivery
Pilati et al. (2020) [89]	✔	✔							EVs for parcel delivery
Pourmohammad-Zia & van Koningsveld (2024) [39]	✔	✔			✔				Waterway based distribution/EVs in last-mile delivery
Ramirez-Villamil et al. (2022) [90]	✔					✔	✔		Route optimization (2E-VRP): stochastic travel times
Ramírez-Villamil et al. (2023) [91]	✔	✔	✔				✔		Route optimization for parcel delivery
Resat (2020) [92]	✔	✔	✔		✔	✔			Optimal cargo distribution using drones
Saeedi et al. (2018) [93]	✔	✔			✔		✔	✔	Location selection for urban distribution centers
Sayarshad et al. (2021) [44]	✔	✔	✔		✔		✔		Optimal routing and scheduling: EVs
Simoni et al. (2020) [94]	✔		✔						Crowdsourced delivery
Teimoury & Rashid (2024) [95]	✔	✔	✔		✔	✔	✔	✔	Optimal freight transport using drones and trucks
Vajihi, M & Ricci (2021) [96]	✔	✔	✔						Urban rail system for freight distribution
Villa & Monzón (2021) [61]	✔	✔	✔						Metro based delivery system with parcel lockers
Voegl et al. (2019) [97]	✔								Developed unloading infrastructure to reduce GHGs
Wang et al. (2023) [98]	✔	✔	✔		✔	✔	✔		Location selection for urban logistics centers
Wehbi et al. (2022) [45]	✔	✔					✔		Optimal routes: on-foot porter for last-mile delivery
Wei et al. (2020) [51]	✔	✔	✔		✔				Underground logistics network optimization
Wong et al. (2020) [50]	✔	✔			✔		✔		Route optimization
Zhang & Cheah (2024) [99]	✔	✔					✔		Crowdshipping for freight delivery

¹ The QBL Aspects here consider qualitative discussion of the four Economic (EC), Environmental (EN), Societal (SO), and Cultural (CU) pillars. ² The Analytical Aspects refer to the optimization (quantitative) modeling of the objectives Cost (CT), Emissions (EM), Travel distance and time (TR), and Social Impact (SI).

Based on our analysis of the selected articles, keyword trends, and the output of the keyword co-occurrence network, we deduce that the term “sustainability” is increasingly appearing. Moreover, we observed that sustainable approaches have been developed and integrated into the analytical models in most optimization studies. On the other hand, studies that did not perform optimization yet analytically analyzed and evaluated potential solutions mostly had a sustainability criterion. We analyzed the articles to see which QBL aspects were studied, as reported in Table 1. However, we observed that the environmental and economic aspects of QBL were mainly considered.

Albeit weak or strong, all articles we analyzed considered environmental aspects. Studies that developed an analytical model either set the reduction of CO₂ emission as their objective function or argued that implementing their model would help reduce CO₂ emissions based on their case study results. For instance, Akkad and Bányai [53] minimized GHGs specifically and found that up to 92% emission reduction is possible in some scenarios. On the other hand, Büttgen et al. [66] did not minimize GHGs in their objective function. Still, they showed that emissions could be reduced by around 96% by minimizing total costs, including operational (travel) and emissions costs. Several studies assessed the environmental implications of various practical solutions [58]. Such emphasis is crucial for environmental sustainability, and there is a need for further research to achieve greener cities and mitigate the consequences of CC.

In terms of economic sustainability, the articles made valuable contributions. Almost all the articles (60 out of 64) we analyzed considered economic aspects in their proposals. Many optimization studies set their objective functions to minimize the total costs that occur during the operations related to urban freight transport, including operational and environmental costs. These studies aimed to optimize freight transportation by vehicles so that the operations can be completed with minimum use of resources such as money, time, and fossil fuels. For instance, Enthoven et al. [46] minimized total travel and connection costs for freight distribution in their analytical model.

Studies that did not conduct optimization analytically assessed the economic impacts or benefits of applying different innovative solutions, as given in Table 1. Thus, these studies frequently referred to the environmental and economic aspects of QBL These two QBL aspects are also highly related to each other as studies that focused on minimizing costs also found significant reductions in emissions, and vice versa. Nevertheless, more research focusing on cities’ environmental and economic sustainability is required to explore ways to minimize the impacts of CC in the most economical way possible.

Only 34 of 64 studies considered social sustainability, unlike environmental and economic sustainability. The solutions proposed in these articles contribute to social sustainability. These include but are not limited to using electric and hydrogen vehicles in urban freight transportation, which may reduce air and noise pollution and provide a healthier environment for city dwellers. Similarly, optimization of routes in logistics operations may ease traffic congestion and save time, creating a more accessible environment. Other innovative solutions, such as crowdsourcing, can create new city job opportunities. On the other hand, applying proposed solutions may impact society negatively unless social aspects are considered thoroughly.

On the other hand, consideration of the cultural aspect of QBL is scarce in the DCL literature as we have not encountered any studies considering cultural sustainability. This indicates a gap in the DCL literature, as cultural aspects are significant determinants of the applicability of any potential solution. City logistics is a common and indispensable aspect of all urban areas. However, cultural norms and values may significantly vary from city to city. Therefore, additional cultural constraints may be present that can limit the applicability of a solution. For instance, cargo bikes are integrated into the solution proposals made by several articles for last-mile deliveries in cities. Using cargo bikes could be a viable and effective solution in various European countries, like Denmark, where bicycles are already a popular mode of transportation. However, in countries such as Türkiye, where bicycles are not as commonly used, cultural attitudes may hinder the widespread adoption and effectiveness of cargo bikes as a solution. For proposed solutions to be effective, they should not conflict with the related area’s culture. This is also crucial for sustaining the cultural identities of these areas (cf. [100]). Thus, studies should consider various cultural aspects throughout the development of a solution. This indicates that all four QBL pillars of sustainability are crucial to consider when addressing the challenges in DCL.

6. Evolution of DCL Research and Concluding Remarks

This section addresses RQ4 by presenting enablers, barriers, and future research directions in the city logistics context. Enablers include the aforementioned innovative technologies, which are expected to be integral to DCL efforts. Based on our analysis, these technologies are increasingly becoming the focus of several studies in the DCL literature and emerge as a viable factor integrated into various optimization efforts in the articles analyzed. Crowdsourcing, inland waterways, and underground networks are other innovative solutions for city logistics, promising results in terms of QBL aspects.

Some technologies can be greener substitutes for conventional vehicles, which cause more noise and air pollution. For instance, unlike conventional vehicles, electric and hydrogen vehicles do not burn fossil fuel to operate, reducing the GHGs released into the atmosphere. Since city logistics operations account for a significant portion of the total GHGs emitted, transitioning to greener transportation modes from conventional vehicles can significantly reduce GHGs, which may slow down CC. We posit that integrating these technologies into optimization efforts is a significant area of research with the potential to achieve more sustainable cities.

Researchers and practitioners can utilize this study as we provide an overview of the solutions studied in the DCL literature. However, our analysis shows several barriers exist to employing such technologies. For instance, the upfront cost of electric and hydrogen vehicles is higher than conventional ones. Moreover, a lack of infrastructure for charging/refueling stations may limit the sustainable use of electric and hydrogen vehicles, especially in developing and underdeveloped countries. The financial burden of high upfront costs and infrastructural limitations would undoubtedly influence the economic sustainability of cities and related organizations. Future studies integrating these technologies into their optimization efforts should consider the barriers since they require adding new constraints, among others, due to possible public-private partnerships.

Analyzing the studies, we observed that the impact of uneven roads on city logistics efficiency is lacking. As conventional vehicles are currently the primary mode of transportation in urban logistics, uneven roads increase fuel consumption, resulting in higher GHGs. Therefore, future studies should consider uneven roads in their emission calculations. Another gap in the DCL literature is the impact of missed deliveries on supply chain performance, which increases freight movement and, consequently, GHGs. Future studies can emphasize developing analytical solutions to minimize missed deliveries in cities and their associated externalities by considering QBL aspects.

Based on our findings, most studies emphasize environmental, economic, and occasionally social sustainability, overlooking the cultural aspect of QBL However, various cultural aspects are crucial determinants for the applicability and effectiveness of a solution. Therefore, future research should consider the unique cultural identities of different cities while proposing a solution. Furthermore, because only about half of the reviewed articles consider social sustainability, we encourage future studies to emphasize it so that life quality and accessibility of city dwellers can be enhanced.

This study analyzes the analytical solutions proposed in the DCL literature by considering QBL pillars of sustainability. First, we found that an increasing number of articles emphasize decarbonization and sustainability of city logistics. However, more research is needed to achieve sustainable cities and mitigate the severe consequences of CC. We demonstrated the current state of the research by presenting the publication years, journals, and countries. Results indicate the increasing interest in sustainable city logistics in the last decade, especially in countries such as China, the United States, France, and Italy. The keyword co-occurrence network shows the topics most emphasized and the keywords commonly used in the DCL literature. Some topics that stand out the most include sustainable development, optimization, and traffic congestion in city logistics. Second, we discussed the analytical solutions proposed by the articles we reviewed. We reported patterns in their analytical models and calculations. Our findings show that route optimization and location selection are common solution approaches in the DCL literature. Several studies integrated the aforementioned innovative solutions into their analytical approaches such as the vehicle routing problem with electric vehicles. Future research can utilize these patterns while developing their solution approaches. Practitioners and policymakers can also utilize these findings to enhance their decisions concerning city logistics. Finally, we discussed the potential benefits and barriers to implementing various innovative solutions that influence DCL’s efforts toward greening supply chain logistics.