Integrated People and Freight Transportation: A Literature Review

Abstract

Increasing environmental and economic pressures have led to numerous innovations in the logistics sector, including integrated people and freight transport (IPFT). Despite growing attention from practitioners and researchers, IPFT lacks extensive research coverage. This study aims to bridge this gap by presenting a general framework and making several key contributions. It identifies, researches, and explains relevant terminologies, such as cargo hitching, freight on transit (FoT), urban co-modality, crowd-shipping (CS), occasional drivers (OD), crowdsourced delivery among friends, and share-a-ride, illustrating the interaction of IPFT with different systems like the sharing economy and co-modality. Furthermore, it classifies IPFT-related studies at strategic, tactical, and operational decision levels, detailing those that address uncertainty. The study also analyzes the opportunities and challenges associated with IPFT, highlighting social, economic, and environmental benefits and examining challenges from a PESTEL (political, economic, social, technological, environmental, and legal) perspective. Additionally, it discusses practical applications of IPFT and offers recommendations for future research and development, aiming to guide practitioners and researchers in addressing existing challenges and leveraging opportunities. This comprehensive framework aims to significantly advance the understanding and implementation of IPFT in the logistics sector.

1. Introduction

Logistics processes that support freight transportation are changing rapidly due to unprecedented growth. This development also leads to changes in freight transport flows in all transport modes [1]. In this context, many studies and suggestions regarding the future of transportation are put forward. The integration of people and freight transport is one of the main current transportation issues discussed, and there are many studies [2,3,4,5] that address this issue as a current issue. The Commission of the European Communities [6] emphasized that for free-flowing cities and towns, local governments should handle all urban logistics related to people and freight transport as an integrated logistics system. Promoting urban mobility by integrated people and freight transportation (IPFT) has recently attracted increasing attention [4]. However, the scarcity of studies in this field still attracts attention. Given the relatively recent interest in the topic, most authors have noted the lack of systematic analysis [3]. It has been suggested that integrating people and freight flows can create advantages to meet the same transportation needs with fewer vehicles and emissions [7].

Furthermore, integrating public and goods flows has been proposed as a valid approach to promoting efficient and sustainable transportation [8]. Collaborative urban public transport services ensure the seamless movement of people and goods, and the negative impacts of existing urban public transport systems can be reduced [9]. This integration can reduce the adverse effects of urban logistics activities, such as noise, traffic congestion, and pollution, by reducing the number of fossil fuel vehicles traveling for goods transportation [10].

Figure 1 shows the IPFT system. This example specifies a distribution center (DC) and public transport routes (train and bus connections). Packages are transported to the stops/stations by trucks from DC. It is then transported by bus or train. The goods unloaded at the stops where customer delivery can be made are delivered to the customers by urban vehicles. This system can be expanded with different transport modes.

The study aims to systematically examine and describe integrated freight and people transportation and present its opportunities and challenges. While examining within the scope of IPFT in the study, many terminologies such as cargo hitching, freight on transit, urban co-modality, crowd-shipping, occasional drivers, crowdsourced delivery among friends, and share a ride are discussed. Strategic, tactical, and operational decision problems are considered as review criteria. In the study, the literature on the IPFT is comprehensively examined, an overview is presented, and it is aimed to create a road map for future studies. In particular, no study has been found in which terminologies are explained in such detail and the entire research scope is discussed together. We can summarize our main contributions as follows:

• We review terminologies related to IPFT. Terminologies related to IPFT, which is still a new concept, continue to develop. Therefore, we present the terminologies (cargo itching, freight on transit, urban co-modality, crowd-shipping, occasional drivers, crowdsourced delivery among friends, and share a ride) mentioned so far.
• We examine the studies carried out for IPFT in the context of strategic, tactical, and operational decision problems. We also search and classify studies dealing with uncertainty under a separate heading for these three problem dimensions.
• Based on a comprehensive review of existing research, we address the opportunities and challenges of IPFTs. While we evaluate opportunities from an economic, social, and environmental perspective, we address challenges from a PESTEL perspective. We present real studies conducted to provide application examples of the IPFT concept and a user background.
• We present suggestions regarding IPFT for future studies and practitioners. The suggestions aim to eliminate the difficulties identified within the scope of PESTEL and to become widespread. In this context, we suggest promising directions for further research.

Section 2 provides the various definitions and terminologies. Section 3 and Section 4 include the literature on passenger and freight transportation, IPFT studies, and details of strategic, tactical, operational, and real-time studies of IPFT, respectively. Section 5 presents the applications. Section 6 presents opportunities and challenges in IPFT, and Section 7 presents suggestions for future work. In Section 8, results regarding IPFT are given.

2. Terminologies of Integrated People and Freight Transportation (IPFT)

Urban transportation systems for people and goods are increasingly influenced by emerging on-demand mobility solutions that rely on shared and increasingly crowdsourced resources [11]. Different words and terminology are used regarding IPFT. It is observed that words such as goods or cargo instead of freight; public, last-mile, or commute instead of passenger; transit, mobility, or logistics instead of transport; and mix, collaborative, combined, or shared instead of integrating can be used. When commonly used terminologies are examined, concepts such as integrated people and freight transportation [3,7,8,12], collaborative urban transportation [13,14], synergy in passenger and freight mobility [15,16], cargo hitching, freight on transit (FoT), urban co-modality, sharing economy, crowd-shipping (CS), occasional drivers (OD), crowdsourced delivery among friends, share a ride (SaR) are observed. The terminology structure is shown in Figure 2. When examined structurally, it is seen that it is directly related to basic terminologies such as sharing economy and co-modality and many sub-terminologies (cargo hitching, FoT, urban co-modality, CS, OD, crowdsourced delivery among friends, SaR). The definition of some mentioned terminologies can be given as follows:

Cargo hitching is a concept in which passenger and freight transportation integration plays a key role in efficient and reliable delivery services [17]. Cargo hitching, wherein the IPFT plays a pivotal role, is crucial to ensuring efficient and dependable delivery services [17]. Romano Alho et al. [18] defined it as taking advantage of spare capacity in passenger transportation modes. Van Duin et al. [19] state in their study that the cargo hitching project was initiated five years ago by a consortium consisting of several government agencies, private companies, and Dutch universities with the financing of the national government (Dinalog) and was implemented in an area close to the city called Nijmegen.

Freight on Transit (FoT) refers to an operational strategy in which public vehicles or/and infrastructure are utilized for the transportation of freight [20,21]. It is stated that this may mean transporting goods with passengers on buses, connecting cargo trailers to public transport, operating freight vehicles between public transport trips on metro lines, etc. [22]. Delle Donne et al. [23] emphasize in their study that they aim to use the remaining capacity in public transport vehicles to transport freight within the city, and this system is known as FoT.

Urban Co-Modality concept is a sub-branch of Co-Modality. Co-modality means using alternative modes to increase the sustainability and efficiency of transportation systems [24]. From a broader perspective, co-modality means the utilization of different modes, uni and in combination, to achieve the optimum mobility outcome in terms of transport sustainability, supply efficiency, and travel effort. The utilization of various means of transport should consistently yield the optimal combination of ecological, economic, and social considerations. From the co-modality perspective, the optimal relation between two regions does not necessarily entail the finest service offered by any single mode (such as a direct connection), but rather the best service achievable between the two regions [25]. In some studies, co-modality transportation aims to integrate people and freight flows in a single system by sharing public transportation between people and freight [26]. However, the concepts of co-modality in city logistics or urban co-modality often include definitions based on passenger and freight integration. Zhu et al. [27] defined co-modality in city logistics as integrated freight transportation with public transit. They stated that it is widely applied in medium- and long-distance transportation by offering an integrated transportation solution for people and goods. Thompson [28] stated that the term ’Urban Co-modality’ was developed due to increasing air pollution, traffic congestion, and fuel consumption and was based on the idea of using the idle capacity in public transportation systems such as trains, trams, and buses during off-peak hours for freight transportation.

Sharing Economy is a widely adopted term in recent years, encompasses the collaborative creation, distribution, production, consumption, and trade of goods and services among various individuals and organizations [29]. Two main forms of sharing within transportation are passenger sharing and load sharing [30].

Crowd-shipping (CS) is also known as crowdsourced delivery [31]. CS is a transitional phase of the emerging sharing economy phenomenon, and researchers have defined this field in various ways [17]. Frehe et al. [32] define it as outsourcing logistics services to an audience where coordination is supported by a technical platform managed and hosted by the mass logistics provider. Rai et al. [33] define CS as an information link-enabled market concept facilitating the alignment of supply and demand for logistics services with an external crowd possessing available capacity in time and/or space. This system is voluntary and is charged accordingly.

Similarly, occasional drivers (OD) can be defined as the inclusion of vehicles of ordinary people who have a planned trip to service the transportation demands [34]. Dahle et al. [34] stated in their study that OD can lead to better use of vehicles on the road, reducing road congestion and emissions. OD is also known as occasional couriers (OC) [35]. More specifically, due to security and privacy problems, crowdsourced delivery among friends is also used as a current approach. This approach uses social networks to coordinate deliveries and increase flexibility [36].

Share a ride (SaR) or ridesharing, a component of the sharing economy, embodies sharing car journeys, aligning with sharing goods, experiences, knowledge, and collaboration to optimize shared resources. There is a need for a change of mindset so that everyone can gain more benefits and that sharing can bring more value than owning [37]. Traditionally, ridesharing refers to a transportation model wherein individual travelers share a vehicle for a journey, dividing travel expenses such as gasoline, tolls, and parking fees with others with similar routes and schedules [38]. Cheng et al. [4] state that a vehicle such as a shared autonomous vehicle or shared taxi can provide door-to-door service for both people and freight. Li et al. [39] discussed a SaR problem in which people and packages share the same taxis.

When all the concepts are examined, it can be said that there is not yet a well-established definition, but a general logic has emerged and developed to provide benefits such as utilizing idle capacities to reduce costs, reducing traffic congestion, and minimizing environmental and noise pollution. Integrated people and freight transportation can be summarized as collaborative transportation activities that yield various advantages within the same vehicles.

3. Overview of the Literature

Selected literature on passenger and freight transportation is examined separately, and the IPFT is examined. This section is prepared to provide a general overview of passenger and freight transport studies.

3.1. Passenger Transportation

Many studies on passenger transportation [40,41,42,43,44,45,46,47,48,49,50] are available. In their study, Pratelli and Schoen [51] address the design problem of bus route deviation. Tzeng et al. [52] examine alternative fuel vehicles for passenger transportation. Tavares-Pereira et al. [53] deals with the districting problem for passenger transportation in Paris. In their study, Yang [54] examines the success factors affecting passenger transportation in Taiwan. Labbouz et al. [55] use multi-criteria decision-making (MCDM) methods to reconcile technical interests with local expectations regarding past transportation projects.

Celik et al. [56] evaluate the satisfaction level with passenger transportation in Istanbul. Camargo Pérez et al. [57] conducted a literature review examining studies on urban passenger transportation between 1982 and 2014 that used MCDM approaches. The study states that there has been an increasing interest in this issue after 2007. Cancela et al. [58] developed a mathematical model for passenger transportation network design. In their study, Van Lierop et al. [59] examine loyalty and satisfaction in passenger transportation. In this study, Gkiotsalitis and Cats [60] examine the adaptation of passenger transportation during the COVID-19 period. Dugan et al. [61] recommend policies for low-carbon passenger transportation in this study. Filabadi et al. [62] addresses a stochastic bus scheduling problem in this study. Gkiotsalitis et al. [63] conduct a systematic review of real-time control strategies for transfer synchronization in public transportation. By conducting comprehensive literature research, Kuo et al. [64] examine passenger transportation for smart cities. Liu et al. [65] aim to create a method to estimate and analyze carbon emissions from urban passenger transportation.

3.2. Freight Transportation

Many studies on freight transportation [66,67,68,69,70,71,72,73,74,75,76] are available. Kang et al. [77] examine the value of rail transportation time savings, service frequency, and reliability for manufacturers and transportation operators in rail freight shipments in South Korea. Bulis and Škapars [78] describe Latvia’s comparative position in the Logistics Performance Index and assess the international competitiveness of freight transport services. Dua and Sinha [79] conduct a literature review for multimodal freight transportation. Izadi et al. [80] conduct a literature study on the costs related to freight transportation.

In their study, Meyer [81] performs bibliometric and network analysis on the decarbonization of freight transportation. Tavasszy [1] examines the impact of the change in logistics processes on freight transportation. Xie et al. [82] formulate an optimal service problem to analyze the welfare effects of pricing strategies in urban railway public transportation. Ghisolfi et al. [83] conduct a literature review on the decarbonization of freight transportation in this study. The study examined several strategies, including transitioning freight to low-carbon intensity modes, improving energy efficiency, enhancing vehicle utilization, reducing freight transport demand, and promoting alternative energy sources, with related recommendations formulated. Kiani Mavi et al. [84] systematically examine the literature on current innovations in freight transportation. Miklautsch and Woschank [85] present measures that can be implemented to decarbonize transportation operations and examine the mitigation potential of these measures. Tsolaki et al. [86] examine the latest developments in the application areas of freight transportation and supply chain, focusing on the problems of arrival time, vehicle routing, optimization of industrial processes, and demand forecasting. This review categorizes relevant studies according to machine learning methodologies.

4. Integrated People and Freight Transportation (IPFT)

Although people and freight transportation issues are discussed separately in the literature, IPFT has started to take its place in research in recent years. In this section, the studies carried out under the title of IPFT are examined under the subheadings of strategic, tactical, and operational level decisions. Under these headings, studies dealing with uncertain situations are detailed within the scope of SDL, TDL, and ODL. Uncertainty situations include dynamic structures, stochasticity, and real-time activities. The general framework and classification of the flow in the study are shown in Figure 3.

When comprehensive studies and literature reviews on IPFT are examined, Bruzzone et al. [8] propose performance indicators to evaluate potential improvements of IPFT flows in environmental, operational, and social contexts. In their study, Cavallaro and Nocera [3] conduct a literature review and a descriptive analysis that includes publication types, geographical regions, publication sources, research methods, and research design. Additionally, the study describes the main content-related aspects, including text mining analysis, semantic analysis, clustering, and regional scales. Elbert and Rentschler [87] conduct a comprehensive investigation of IPFT through a systematic literature review. In the study, IPFT is analyzed by classifying it into quantitative and qualitative approaches regarding the mode of transportation, shared directions, basic network, and data used.

Additionally, opportunities and barriers related to IPFT are identified. Cavallaro et al. [12] present the results of a Delphi method of international participants evaluating the design of service features and operational performances. According to the results, IPFT shows the necessity of an efficient service regarding information, security, environmental performance, and space sharing. The study states that policymakers and practitioners can use it to define performance requirements and limits regarding the design of the service. Cheng et al. [4] propose a general framework to realize the planning and operation of new forms in the future. The study focuses on route planning, facility location, and pricing under SaR, FoT, and crowdshipping headings. Jazemi et al. [88] review the vehicle routing literature from the perspective of last-mile delivery and scope crowd shipping. The study identifies the successes and limitations of research in the areas and suggests a future research agenda. de Oliveira et al. [89] define and evaluate the key factors for developing IPFT systems and classify the benefits, barriers, challenges, and strengths.

4.1. Strategic Decision Level (SDL) for IPFT

The strategic decision level (SDL), which includes long-term decisions and is difficult to change, includes high-level decisions and important investment decisions such as network design, plant locations, plant dimensioning, and capacity planning. Previous research has typically been examined at the operational planning level and neglected the strategic perspective. For this reason, studies conducted in this context are quite limited [90].

Cheng et al. [91] considers the citywide package delivery problem using CS transportation systems. The distribution scheme is modeled as an example of a multi-product flow problem and formulated as MILP. Ji et al. [92], who consider a similar problem and solution approach, consider the hub location problems by considering the proportion of taxi drivers willing to carry packages.

Dong et al. [93], Zhao et al. [94], and Ma et al. [95] aim to plan a network for a metro-based logistics system. Dong et al. [93] study results show that the proposed system can significantly reduce metro construction costs and alleviate traffic congestion. In contrast, Zhao et al. [94] provides strategic planning for the location selection of Shanghai metro distribution centers. Ma et al. [95] also measure system performance to understand strategic interactions and resulting system effects in the context of metro-integrated systems.

Azcuy et al. [96] evaluate location decisions and system performance using MILP and a heuristic method. The result shows up to 7.1 percent of savings are possible by using public transport capacity to promote urban transportation. Kiba-Janiak et al. [97] study presents an approach to assess maturity in formulating and implementing sustainable IPFT strategies. It is stated that the study was conducted and that the developed model can be used by local governments operating in various environments. This study shows that cities that achieve the best safety and environmental degradation outcomes by implementing the most comprehensive passenger and freight transport measures have strong, ongoing cooperation with stakeholders and integrated transport strategies. Delle Donne et al. [23] address strategic decisions for last-mile deliveries in IPFT. In this study, where different formulations and solution approaches for the problem are proposed, managerial information about the performance and efficiency of the system is also presented.

Uncertainty at SDL for IPFT

This title presents studies addressing uncertainty (such as dynamic structures, stochasticity, and real-time activities) within the scope of SDL.

Li et al. [98] is developed as a two-stage stochastic programming model, and the study includes stochastic delivery locations and stochastic travel times. The results show that stochastic information is valuable in real life and can significantly improve the performance of taxi-sharing systems. Fatnassi et al. [99] study investigates the potential of integrating common freights and passenger on-demand rapid transportation systems in urban areas. The study examines how a rapid public transport network can be shared and how a city’s existing transport capacity can be used more efficiently and interconnectedly.

Behiri et al. [100] propose an environmentally friendly urban freight transport alternative using the passenger rail network. The study provides decision makers with a decision support tool to assess technical feasibility, impact on passenger services, infrastructure needs, and investment. The study addresses issues that must be addressed at strategic, tactical, and operational levels by examining different models. The problem of storage space sizing at stations is strategically addressed. Mousavi et al. [101] and Nieto-Isaza et al. [90] address the warehouse location problem in the last mile context in two stages and present a stochastic approach. Mousavi et al. [101] state that the stochastic model results provide between 3.35% and 6.08% better solutions compared to the deterministic model.

4.2. Tactical Decision Level (TDL) for IPFT

The tactical decision level (TDL) includes medium-term decisions and can be defined as the reflection of SDL in the medium term. It covers production planning, distribution planning, procurement planning, purchasing, sales planning, inventory management, pricing, and forecasting.

Ghilas et al. [102] address the problem of pickup and delivery with time windows and scheduled lines, aiming to route a given set of vehicles to transport freight demands from their origins to their respective destinations. In the study, an arc-based mixed integer model is developed. Pimentel and Alvesol [10] propose and develop a mathematical model for urban logistics distribution planning for a passenger transport network. The study concludes that the IPFT flow will support higher efficiency rates for the passenger transport network and improve living conditions in large urban centers. Dahle et al. [34] develop a load and flow formulation for the pickup and delivery problem associated with ODs.

Additionally, this study focuses on the design of compensation plans for ODs. Gatta et al. [103] examine the demand and supply of green crowd-shipping potential. The study includes estimating and investigating the mobility of crowd shippers and the population using the subway for travel. Li et al. [104] examine the optimization problem of train unit scheduling, space–time–state network construction and lower bound estimation structures, and addressing IPFT. The study results considered freight and passenger flows, as well as train unit inventories, to minimize the sum of operational and travel costs and provide appropriate and effective information for the IPFT system in underground logistics systems.

Uncertainty at TDL

This section presents studies addressing uncertainty (such as dynamic structures, stochasticity, and real-time activities) within the scope of TDL.

Li et al. [39] deal with the static and dynamic model for human-freight taxi sharing by developing the MILP formulation. The obtained numerical results provide valuable information for the successful implementation of taxi-sharing with multi-commodity structure. Kafle et al. [105] propose a crowdsourcing-enabled system for urban package pick-up and delivery. Initially, delivery information is published online, and offers are collected from crowd sources such as cyclists and pedestrians. Then, the offer is selected, and coordination is ensured. Behiri et al. [100] tactically address the problem of frequency and sizing of trains.

Ulmer and Savelsbergh [106] present a mathematical model for workforce planning. The study covers workforce scheduling problems at the tactical level and dynamic routing problems at the operational level. Zhou et al. [107] propose a dynamic and domain-based pricing strategy for crowdsourced delivery. The results show that the average delivery time has improved, and the economic benefits and stable assignment rate have increased. Elbert et al. [108] consider the network flow problem for IPFT within the scope of stochastic demand.

Silva and Pedroso [109] plan flexible pricing for last-mile delivery, where the company provides part of delivering products to its customers through ODs. The problem is based on a stochastic and dynamic environment within time windows. Hörsting and Cleophas [5] present the MIP model that optimizes the train schedule and allocates cargo to investigate the effects of integrated planning under different operational modes. The study analyzes the interaction of operational modes, planning solutions, and demand settings. It presents a model that integrates the tactical planning of cargo and passenger trains and cargo allocation to vehicles.

These studies collectively showcase a diverse range of solutions—from crowdsourcing and dynamic pricing to integrated passenger-freight systems—aimed at addressing the complex logistics challenges in urban environments. The tactical models presented across these works offer valuable strategies for improving efficiency and flexibility in urban transport and delivery systems.

4.3. Operational Decision Level (ODL) for IPFT

The operational decision level (ODL) includes short-term plans and results from SDL and TDL. Activities such as vehicle routing, vehicle loading and unloading, and scheduling are within the scope of this decision level.

Arvidsson et al. [110] synthesize the basic issues regarding the first and last mile in IPFT and, based on this, investigate time-based sharing of resource use. The article concludes that IPFT in urban areas is a promising approach to the last-mile problem. In the first phase of the study, Masson et al. [111] consider the transportation of goods from a distribution center to bus stops by urban buses, using the spare capacity of the buses. In the second stage, final customers consider deployment by a fleet of near-zero-emission urban freighters. Macrina et al. [112] serve some customers by picking up packages from the central or intermediate warehouse, considering the OD. The study aims to minimize the total cost, the conventional vehicle cost, and OD compensation. The computational results in the study show that the proposed heuristic is quite effective and can solve large-sized examples in short computational times.

Li et al. [14] develop an operational strategy in which urban rail transport is used for freight transportation. An environmentally friendly urban freight transportation alternative is analyzed using optimization techniques to support IPFT. A combined optimization model is proposed to maximize the profit from the balance of revenues and costs the transportation service brings. The study is planned to determine the effective train schedules and freight allocation plans. A MILP model has been developed for this problem. Hatzenbühler et al. [113] investigate the potential of modular vehicle notions and consolidation to raise the efficiency of urban goods and people transportation. The study shows a 48% cost savings due to modularity and an additional 9% cost savings due to consolidation. While the reduction is mainly due to reduced operating costs and travel time, demand can be met in all cases. It is stated that empty vehicle mileage has decreased by more than 60% thanks to consolidation and modularity.

Wang et al. [114] consider an urban freight transportation strategy that uses the excess capacity resources of the airport rail line to solve the problem of intercity freight transportation by adding special freight trains. The model is developed to integrate and optimize passenger and freight train schedules to maximize the system’s profits.

Uncertainty at ODL

This title presents studies addressing uncertainty (such as dynamic structures, stochasticity, and real-time activities) within the scope of ODL.

Dahle et al. [115] consider the uncertainty of the ODs and the dynamic appearance of the vehicles as a stochastic VRP problem. Behiri et al. [100] operationally examine 2D/3D box packaging problems, train schedules, goods delivery, and shipping problems. Ozturk and Patrick [21] consider the shared railway network between people and freight trains and present a decision support framework for the problem of urban freight movement and mathematical methods that address certain and uncertain situations for the optimal distribution of freights.

Archetti et al. [116] include OD in VRP. In the study, each OD indicates the time window during which it is available. Penalties apply if time windows are violated. This approach allows dynamic requests to be met and adjusts the real-time distribution plan. Mourad et al. [117] address the problem of transferring freight demand using a fleet of autonomous pickup and delivery robots in the area where public transportation service is located. Passengers are given priority in capacity sharing, and transportation demands are handled stochastically. Yıldız [35] addresses the problem of package routing in registered ODs and develops a CS model in which couriers record their trips in advance. Express shipment requests go through a stochastic process, and to carry out deliveries most efficiently, network management must decide dynamically on package-courier assignments.

Sahli et al. [118] address the scheduling problem in train transportation to minimize waiting time and maximize total turnover in a dynamic environment. Santini et al. [119] consider the crowdsourcing problem as a probabilistic TSP. This study states that an exponential number of TSPs must be solved to evaluate the objective function of the stochastic problem for a single solution. In the study, Monte Carlo simulation and machine learning methods are recommended to approximate the objective function, and both heuristic methods and branch-and-bound algorithms are recommended to reduce the number of evaluations. Torres et al. [120] address a problem in CS where heterogeneous delivery demands can be met from a central warehouse with certain ODs. In the study, ODs were modeled stochastically, taking into account their uncertainty.

5. Applications

In this section, practical applications for people and freight transport are examined through examples from different countries and cities. Although there are theoretical and case studies in operational, tactical, and strategic studies within the scope covered in Section 4, this section details studies supported by municipalities, cities, or countries. The aim is to reveal how this integrated transport type is managed, planned, and developed globally. The global application examples in this section support the theoretical framework of our study. Thus, a valuable reference is provided for future projects that can be developed in people and freight transport. Looking at the applications, several cities and countries can be cited as examples, including Paris (France) [121,122], Amsterdam [123] and Groningen [124] (The Netherlands), Sweden [125], Dresden [126,127] and Frankfurt [128,129] (Germany), London (England) [124], Zurich (Switzerland) [21], Iasi (Romania) [130], Tokyo [131] and Hokkaido [132] (Japan), Mumbai (India) [133], and New York (USA) [134], Changzhou [135] and Binzhou [136] (China). In addition, European Commission Expert Groups [137] also shared various current applications. Despite these examples, the scarcity of studies in this field remains notable. A detailed review of the studies that were conducted is presented.

In Paris [121], the project aims to transport products (such as textile products, beauty products, and beverages) stored in certain areas via the rail line connecting to the region and integrating them with the river line. This project aims to reduce noise while supporting sustainable development. Another study involving Paris originated from a real-world problem of line planning and scheduling of trains of existing metro networks, proposed by Metrolab^®, a French R&D company. A pilot experience is presented to automate the tactical and operational level decision-making process of a small section of the Paris metro network. Two lines and nine stations are considered. The results of extensive computational experiments demonstrate its applicability and effectiveness for addressing real-world metro networks [122]. Amsterdam [123] implemented a system where people and goods coexisted within the same track network but were conveyed via distinct freight tram services. When the cargo trams arrived, the goods were unloaded into small electrically powered vehicles for final delivery. This application was projected to potentially halve the total count of commercial vehicles, mitigate noise, and alleviate air pollution. Still, it was closed because it could not provide sufficient investment financing, and stakeholders had conflicting goals.

The integrated system in Groningen [124] consists of pilot activities involving integrated urban services and small packages (medicine, magazines, books) for passengers. In Sweden [125], an environmentally friendly and punctual system is implemented where buses transport passengers and packages in the same vehicle. This application enables package delivery to many points of the country every day, from early morning to late evening. The CarGoTram project in Dresden [126,127] is a partnership between automobile company Volkswagen and the city’s transport operator DVB to transport product parts from the freight depot to the factory along the city’s tram passenger route.

In Frankfurt [128,129], the project aims to reduce traffic and emissions by integrating logistics processes with the railway infrastructure and performing cost analysis using fewer delivery vehicles. The test results show that it is technically possible to integrate the tram into delivery. Analysis of the tram network in Frankfurt shows that the existing public transport infrastructure is suitable for transporting goods in many places. London [124] has combined freight-passenger services, which are being developed due to the attractive service quality of passenger-freight vehicles measured in terms of passengers’ travel times. The project envisages loading/unloading models in passenger transportation and an average operational time of three minutes, aiming to reduce the number of vehicles in circulation.

In Zurich [21], a cargo tram is used to collect garbage. Recycling is implemented in Iasi [130], which allows e-waste to be delivered by tram to collection centers in the city. Tokyo [131] plans to transport parcels on the part of the metro network in cooperation with freight forwarders. The Hokuetsu Express train line in Hokkaido [132], the first operation certified under the Act for Promoting Integration and Efficiency of Distribution Operations, aims to use part of the space of its operating passenger train to transport packages daily. Hokuetsu Express is partnering with a leading home delivery company to plan mixed freight and passenger transportation, benefiting from the utilization of transport capacity, reducing driver usage, and reducing carbon dioxide emissions.

The findings of the application study in Mumbai [133] of a lunch box delivery system for the city showed that it was able to develop a reliable and affordable suburban logistics system for the middle-class society. This app uses walking, bikes, and suburban trains to deliver lunch boxes to customers, demonstrating that the system can manage a complex urban logistics network as efficiently and effectively as any other logistics firm operating within the organized sector. Systems are integrated with the New York subway network [134], generally operated in the evening hours, collecting 14,000 tons of waste annually with 11 adapted metro trains. When we look at the practices in China, we see that in Changzhou city, a co-modality network is established between bus and cargo companies, and integrated transportation service is provided [135], and in Binzhou city, a bus line serves both passengers and freight to improve rural delivery service [136].

As Vienna’s public transport operator, Wiener Linien began planning mobility hubs under the name “WienMobil Station” in 2018. The plan, approved by the city in 2021, aimed to build more than 100 hubs. By the beginning of 2023, 50 mobility hubs were completed, offering various services according to local needs. These hubs provide services such as bike and car sharing, e-scooters, taxis, and charging points for electric vehicles. The hubs aim to create a city-wide mobility network, which is being carried out in cooperation with various public authorities and mobility service providers. In addition, these hubs are integrated with a multimodal application called WienMobil. The SmartHubs Project in Warsaw is examining the implementation of mobility hubs. These hubs promote sustainable mobility by combining shared and environmentally friendly transport options. In the project, each mobility center has a parking area for shared vehicles and special areas for micromobility (e-scooters, mopeds). The main objectives are increasing access to shared mobility, revitalizing urban areas, and promoting sustainable transport. In 2023, the Canton of Geneva, Transports Publics Genevois (TPG), and Donkey Republic presented a multimodal transport solution to reduce traffic congestion and strengthen the public transport network. This cooperation was carried out to reduce the congestion on public transport and provide sustainable transport options. In 2023, 48% of the trips made with Donkey Republic bicycles replaced public transport, relieving the pressure on public transport. In addition, 13% of the trips stopped using cars and preferred bicycles. This cooperation brought significant benefits by reducing traffic congestion and using private vehicles. The project plans to increase the number of bike-sharing stations and integrate with public transport applications. SHOW (shared automation operating models for worldwide adoption) Project aims to advance sustainable urban mobility by supporting the integration of shared, connected, and electric automation into urban transportation within the scope of Horizon2020 between 1 January 2020 and 30 September 2024. The project includes the integration of automated vehicle fleets into systems such as public transportation and demand-driven transportation with real-life applications to be carried out in 20 European cities [137].

6. Opportunities and Challenges

In the new economy, many innovative logistics initiatives are emerging based on resource sharing with mass participation [19]. Sustainable IPFT (integrated people and freight transportation) constitutes a primary goal for cities [97], but integrating people and freight transportation involves complex and multifaceted aspects [8]. Therefore, this complexity needs to be managed effectively with the right strategies.

Integrating people and freight flows is a viable strategy to promote efficient, sustainable, and socially beneficial transportation [8]. This approach enables seamless movement of people and goods, particularly through collaborative urban public transport services. It is expected to improve transportation services and mitigate the negative impacts of current urban public transportation systems [9].

Considering the benefits of integrating people and freight transportation from economic, social, and environmental perspectives, economically, transportation costs can be reduced [4,113,138]. Enhancing transportation system efficiency by decreasing vehicle idle time per kilometer [113], reducing fleet size [113], and implementing demand-oriented services to increase flexibility and reliability of operational decisions [138]. It also contributes to achieving Sustainable Development Goals (SDGs), promotes economic development by enhancing transport system efficiency, and increases public policy financial benefits [89].

Socially, transportation network coverage can be expanded [138], traffic density reduced due to fewer required vehicles [7,10,89,99], noise minimized [99], and travel times decreased, saving time [4].

Environmentally, sustainability gains can be increased by reducing energy waste and extending service life [99], lowering greenhouse gas emissions [7,10,89,99,113], and decreasing air pollution [89]. Integrated transportation can also reduce the negative effects of urban logistics activities, such as noise and accidents, by minimizing the number of fossil fuel vehicles used for transportation in cities [10]. It enhances accessibility in peripheral areas and reduces competition for space [89].

Despite the increasing number of studies on IPFT in recent years, there remains a notable lack of quantitative research. Existing studies are often very general and lack concrete transportation solution analyses [3]. Several issues arise when considering the challenges of integrating people and freight transportation from the PESTEL (Political, Economic, Social, Technological, Environmental, and Legal) perspective.

Politically, there is a scarcity of studies and incentives on this subject [3], with different authorities regulating people and goods transportation with separate rules and policies [8]. The impact of transportation policies remains unclear [3], requiring long-term stakeholder collaboration [97,113].

Economically, challenges include finding the economic sustainability of this approach, the need for initial investment [4], and the high investments required for adapted vehicles and stations [89].

Socially, potential issues involve an increase in passengers’ discomfort due to feelings of unsafety and conflicts at transit stations [4], obstruction of traffic and pedestrian movements in dense urban environments [113], congestion during peak hours, and the possibility of faulty deliveries [89].

Technologically, difficulties arise in data collection [97], determining transportation capacities, routing vehicles, and designing programs for people and goods integration [4]. There is also a reduction in knowledge [89], a need for more storage space compared to traditional methods [113], and the complexity of simultaneous loading and boarding processes from an engineering perspective [5].

Environmentally, there are structural unsuitabilities of roads, particularly in cities with old historical structures, especially in European cities [97].

Legally, the absence of defined strategies regarding health and safety and the lack of organizations to protect passengers and cargo within this context pose significant challenges.

7. Future Research

IPFT started to utilize the unused capacity of public vehicles for freight and package transportation. This approach has grown significantly as numerous new companies enter the market, leveraging freight brokerage platforms to match shippers and carriers, thereby maximizing transport capacity sharing, reducing empty distances, and speeding up shipping times [19]. This section presents suggestions for future work that will contribute to the development of this field and help mitigate its challenges.

To advance operational IPFT services, implementation is required at corporate and business levels [110]. Effective planning processes and policies are needed to address these challenges [113]. Challenges could be mitigated through the regulation of goods types, planning integrated transportation services, and implementing regulations regarding privacy and data security [4].

Competitive pricing is essential for attracting demand for freight in public rail transportation [14]. While most studies have traditionally considered customer pricing within various pricing frameworks, due to the intricate influence of pricing on both supply and demand, it may be more appropriate to view pricing as a decision variable and determine it through an optimization model. Integrating different modes of public transportation can optimize resources, providing a time-efficient and cost-effective service for all stakeholders involved. The existing infrastructure of transfer hubs, which facilitate passenger transitions between different modes of public transport, can also serve as transfer points for goods [4]. Additionally, sources of uncertainty, such as the capacity of storage facilities for freight in public transportation, non-time-sensitive operating scenarios, and unexpected operational emergencies, must be managed cost-effectively and time-efficiently [9]. Increasing encouragement in this context will also contribute to resolving real-life problems.

If passengers feel uncomfortable with this integration, research can be conducted to evaluate [14] and understand [4] this service from the passengers’ perspective. Carefully planned freight center locations, routes, and schedules for freight vehicles can address certain challenges, minimizing conflicts between people and cargo and reducing passengers’ inconvenience [4].

In practice, each public transportation mode has its advantages and disadvantages. For example, subways and trains offer greater speed and punctuality than buses, providing more comprehensive service coverage [4]. However, real-life scenarios must be developed to address potential program misses [9,14] and load uncertainties [14]. Researchers can attempt to address sub-problems related to all three types of integrated systems within a dynamic and stochastic environment to reflect greater realism, given the limited scope of research in this domain [4]. Future research may also focus on extending studies with other optimization objectives, such as accounting for vehicle noise [138], randomness in size and weight, and incorporating storage capacity constraints to improve the feasibility of the service plan [9].

In future studies, additional demand types (e.g., waste, recycling) could be added to scenario definitions to examine demand patterns and other future urban transport systems, including return flows [113], thereby contributing to sustainability and the UN SDGs. Determining strategies regarding health and safety and ensuring a structure that protects passengers and cargo will also contribute to developing passenger and cargo transportation integration.

8. Conclusions

IPFT, a logistics innovation developed in recent years, has yet to be fully examined. Therefore, this study is carried out to guide users and researchers.

The study aims to present a roadmap to this developing concept by outlining the general structure of IPFT. It also presents a comprehensive terminology structure for IPFT, detailing the terminologies discussed within its scope and their connections with concepts such as the sharing economy and co-modality. This concept has been introduced and is used in various studies.

The study details research conducted at strategic, tactical, and operational decision levels. At the strategic decision level, it covers network design and long-term decision studies; at the tactical level, it includes medium-term decision planning and pricing studies; and at the operational level, it discusses scheduling and routing studies within the scope of short-term activities. Additionally, the study addresses research conducted under conditions of uncertainty.

Opportunities and challenges for IPFT are also presented. From an economic perspective, IPFT can reduce transportation costs and support economic development. Socially, it can expand the transportation network and usable space and reduce transportation time, traffic density, and travel time. Environmentally, it contributes to sustainability and the SDGs. The study identifies the most significant challenges for IPFT as incomplete policies, high initial investment costs, passenger dissatisfaction, and planning difficulties. It discusses these challenges and offers suggestions for future work and strategies to overcome them.

The applications highlighted in the study are also discussed in detail. The general structure of this study is expected to provide many benefits to the newly developing field of IPFT and contribute to future studies and applications.