A Systematic Review of Analytic Hierarchy Process Applications to Solve Transportation Problems: From 2003 to 2022

Transportation improvements affect technological and socio-economic development, and several scholars have researched various transportation problems. The current study aims to illustrate a thorough review of those transport problems, where the Analytic Hierarchic Process (AHP) is used for enumerating the related criteria and alternatives. A systematic review methodology, the PRISMA protocol, is applied in the review process. The contribution of this work is highlighted along with the extensions of the AHP improving decision-making support. To this end, current research demonstrates the relevant results of 58 papers published from 2003 to 2019. The results indicated that most researchers applied the conventional AHP method to deal with transportation issues, while the most critical issue was public transport, followed by logistics problems. On top of that, TOPSIS was integrated with AHP more than other MCDM methods when dealing with multi-criteria transportation problems. Moreover, the “Transportation Research Part A: Policy and Practice” journal achieved first place by publishing ten papers on the topic, and the highest number of articles was published in 2018. The results are discussed adequately, and in the conclusion, policy implications are presented.


I. INTRODUCTION
Transportation science comprises the fundamental theories of transportation processes and observational and experimental research [1]. Furthermore, transportation science emphasizes novel methodologies, mathematical models, and applications for various purposes, including design, operation, planning, and construction. At the same time, there is a focus on maintaining different transportation modes [2], [3]. For instance, selecting a decent transportation enterprise may The associate editor coordinating the review of this manuscript and approving it for publication was Ugur Guvenc . be biased due to potential interconnected challenges like delivering on time but cost-effectiveness, making decisionmaking more challenging [4]. Also, sustainability in transportation has made decision-makers evaluate and redesign their processes considering sustainable development indicators. In other words, developers need to assess their progress in adopting sustainability, motivating the decision-makers to imply advanced frameworks to increase the accuracy and reliability of the evaluation [5]. On top of that, transportation efficiency is critical for companies, urging them to develop assessment frameworks for evaluating the efficiency considering various criteria, barriers, or challenges, while evaluation VOLUME 11, 2023 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ usually happens under uncertainties, complexities, indeterminacy, and vagueness [6]. Performance measurement is also a requirement in designing, producing, and applying new facilities in the transportation sector, such as evaluating the performance of battery electric vehicles (BEVs). To this end, decision-makers need to take a wide variety of indicators into account in the decision-making process, motivating them to propose a personalized evaluation framework to deal with specific issues, like BEV selection [7] or, in general, selecting the best way of transportation among all available options [8].
As mentioned above, the MCDM technique can be applied in various fields, including transportation. For this reason, some scholars conduct systematic reviews to determine the contribution of the MCDM approach to solving transportation issues. Pérez et al. [96] examine the connection between the MCDM approach and the urban passenger transport system. However, transportation issues have various aspects, Pérez et al. [96] studied only one of the issues. There is a shortage of studies investigating the MCDM approach's application in the case of different transportation issues. Rezaei also conducted review research to assess the MCDM approach to logistics. Although the study is not a comprehensive review, the results indicate that AHP is the most popular method for dealing with logistics-related cases. Moreover, Mardani et al. [97] systematically review the literature to investigate the MCDM approach's implementation in transportation. The results indicate that the AHP and the fuzzy AHP methods are primarily applied in transportation. Furthermore, Yannis et al. [1] examine the literature systematically to find the contribution of the MCDM approach to various transportation problems. The results show that AHP is the most common method applied in the field.
According to the previous reviews, AHP is the most practical MCDM approach in transportation; however, a complete overview of recently published papers in this field is currently missing. To be more specific, the present study aims to fill the following research gaps: (1) many AHP extensions were developed over the years, while some were applied in transportation, but some were not. Therefore, the present study aims to analyze the status quo of AHP applications in transportation so that not only the efficiency of the obtained results of the AHP applications can be investigated but also it could be figured out which extensions of AHP have not been applied so far, motivating future studies to take advantage of them. (2) As mentioned, AHP has been frequently used in transportation; however, there might be further transportation problems that could be solved using AHP. While highlighting the applications of AHP in dealing with various transportation problems could boost the knowledge of scholars regarding the applicability of AHP and motivate them to apply AHP when a decision support system is needed. Therefore, the present study addresses the following research questions to fill the mentioned gaps: RQ1. What is the contribution of AHP to transportation problems?
RQ2. How is AHP used to deal with transportation problems?
RQ3. What kind of transportation problems has been solved by AHP so far?
RQ4. How is the distribution of the papers by the year of publication, journals, and authors' nationalities?
The present study is organized as follows. Section 1 covers the introduction. Section 2 presents the recent extensions of AHP. In Section 3, the methodology is described. The results of the research are demonstrated in Section 4. A discussion of the research findings is to be found in Section 5. Finally, Section 6 presents the main conclusion of the study.

II. A REVIEW OF AHP EXTENSIONS A. THE PARETO OPTIMAL SOLUTIONS
The most recent extension of AHP concentrates on gaining more reliable results on the decision maker's knowledge or preference while optimizing the weight distribution of the pairwise comparisons. The eigenvector method is applied to derive the weights from the pairwise comparisons in the AHP approach [85]. However, recently, it has been proven that the eigenvectors and the final scores of the criteria and the alternatives are not necessarily Pareto optimal [98]; thus, the eigenvectors can be improved to get a better solution.

1) THE PARSIMONIOUS AHP (PAHP)
AHP contains several pairwise comparisons, which require strenuous cognitive effort from the decision-makers. Recently, it has become an aim to reduce this effort or to allow them to provide preliminary evaluations in the survey. The a priori reduction of the number of comparisons is called parsimonious AHP, while the ex-post analysis is called incomplete AHP.

2) THE FUZZY AHP (FAHP)
Recently, several authors have extended the AHP approach based on fuzzy logic to address the consistency and demonstrate the application with illustrative examples. A type of many-valued logic called fuzzy logic allows for the truth value of variables to be any real number between 0 and 1 [99], [100], [101].

3) THE FUZZY AHP WITH INTERVAL TYPE-2 FUZZY SETS (IT2FS)
In many real situations, the evaluators' judgments might be imprecise, or the evaluators cannot select the exact number for the evaluation process. Kahraman et al. [102] create a new model to solve these significant problems by combining the fuzzy AHP and the IT2FSs. In their paper, the authors extend the fuzzy AHP method utilizing the interval type-2 fuzzy sets. Additionally, linguistic scales are developed to be employed in the fuzzy AHP.

4) THE HESITANT FUZZY LINGUISTIC AHP (HFL-AHP)
The information connected to the decisions might be vague and uncertain; thus, considering a fuzzy set is an appropriate solution to address the problem Zheng et al. [51] develop a super decision matrix including objective and subjective criteria to derive the total scores. Their model is based on the hesitant fuzzy linguistic Analytic Network Process (ANP). Furthermore, the authors illustrate the new model with a reallife problem [103].

5) THE GROUP INTUITIONISTIC FUZZY AHP (GIF-AHP)
Liao, et al. [99] provide theoretical support for the GIF-AHP to express the evaluators' uncertain judgments. The authors propose a novel aggregation technique to integrate personal preference relations. Additionally, if the particular intuitionistic fuzzy preference relations have perfect multiplicative consistency, the aggregated intuitionistic fuzzy preference relation always provides perfect multiplicative consistency.

6) THE INTERVAL-VALUED INTUITIONISTIC FUZZY AHP (IVIF-AHP)
The evaluators' linguistic judgments overcome various AHP problems, such as uncertainty and vagueness. Therefore, Abdullah and Najib [104] developed a novel approach for the preference scale based on the IVIF-AHP. Their research shows that IVIFs with a hesitant degree represent the pairwise comparison matrix.

B. THE PYTHAGOREAN FUZZY AHP (PF-AHP)
The conventional fuzzy AHP approach is unsuitable for assessing some issues properly since the linguistic judgment involves uncertainty, and the summation of the membership and non-membership functions should be less than one [105], [106]. To this end, Mohd and Abdullah [107] extend the AHP method by integrating the Pythagorean Fuzzy Sets. In the PF, the summation of the membership's square and nonmembership's square must be less than one.

1) THE AHP-HESITANT GROUP DECISION-MAKING (AHP-HGDM)
Zhu and Xu [108] develop a new group decision-making AHP method to overcome the evaluators' hesitant judgments. The evaluators might apply several possible values to refer to the original judgments, which can be considered an extension of AHP-group decision-making.

2) THE HESITANT FUZZY AHP
The hesitant fuzzy AHP approach is proposed by Öztaysi et al. [109] to aggregate the experts' evaluations. Furthermore, Oztaysi et al. [110] use the proposed method VOLUME 11, 2023 to select a warehouse location in Turkey under an uncertain fuzzy environment.

3) THE NEUTROSOPHIC AHP
Due to the evaluators' limited knowledge, they might not be able to express their points of view precisely by evaluating the pairwise comparison judgments in group decision-making. Abdel-Basset et al. [111] employ the neuromorphic set theory to handle the problems represented by a triangular neuromorphic number.

4) THE INTERVAL-VALUED NEUTROSOPHIC AHP (IVN-AHP)
Bolturk and Kahraman [112] propose the IVN-AHP model to select the modes of alternative energy, which illustrates the integrated model.

5) THE TYPE-2 FUZZY SET OF LINGUISTIC AHP
Abdullah and Najib [113] developed a novel model for fuzzy AHP characterized by interval type-2 fuzzy sets, which can solve the uncertainty and vagueness problems more optimally to enhance judgment in the decision process.

6) THE Z-NUMBER EXTENSION OF AN INTEGRATED AHP
Azadeh et al. [114] extend the Z-number of the AHP approach to avoid the uncertainty and vagueness problems in the classical AHP method. Moreover, the new model differentiates the criteria priority ranking problem.

III. MATERIALS AND METHODS
Related articles are selected and reviewed based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol. The protocol is primarily used to report a review of randomized trials and can be used to evaluate interventions [115]. PRISMA has some benefits over other reviewing methods motivating the present research to apply the PRISMA: firstly, it includes a thorough, accurate, well-described checklist that enables researchers to achieve more robust systematic review reports and metaanalyses. Secondly, the protocol has been updated many times over the years to overcome its weaknesses, making PRISMA a novel method for reviewing articles [116]. Moreover, Abelha et al. [117] reviewed 69 published articles over the period 2009-2019 using PRISMA on competence development, graduate employability, and intending to develop an international framework. They reviewed. Also, Regona et al. [118] conducted review research in which 72 articles were reviewed to identify the challenges to adopting Artificial intelligence (AI) in the construction industry. Also, How et al. [119] employed the PRSMA protocol to review published articles between 2000 and 2020 on music practice, and a total of 296 out of 3102 articles were reviewed. Therefore, the literature shows that the PRISMA method could be applied in various fields successfully, while the elementary step of PRISMA is scrutinizing the research literature. To this end, Scopus, Google Scholar, and Web of Science (WOS) are selected to find published articles related to the research topic. Subsequently, the key journals are scanned, and the lists of references are scanned based on the experts' ideas. Secondly, the eligibility of the publications is analyzed, which is emphasized in various publications. These aspects are the publication status and language, the study's design, the population of interest, the time duration, and the year of publication. Afterward, the information sources are investigated based on the PRISMA guidelines.

A. SCRUTINIZING THE LITERATURE
For the current research, public databases are available to find related papers. These databases are ''Science Citation Index Expanded'' (SCI-E), ''Emerging Sources Citation Index'' (ESCI), ''Arts and Humanities Citation Index'' (AHCI), ''Social Sciences Citation Index'' (SSCI), and ''Science Citation Index'' (SCI). Different keywords include AHP and transportation, AHP and airline industry, AHP and road industry, AHP and airport industry, AHP and transport, AHP and railways industry, AHP and urban transport, AHP and terminal and container ports, AHP and logistics transportation, AHP and public transport, AHP, and transport operations, AHP and public bus transport, AHP, and transportation planning, AHP and marine transportation, AHP, and freight transport are applied to find the most related publications within selected databases storing papers published between 2003 and 2019.

B. THE ELIGIBILITY OF THE PAPERS
Two hundred fifty-three publications are identified at the first attempt. The full texts of the selected papers are reviewed to check their eligibility of the papers. Ph.D. and master dissertations, book chapters, editorial notes, press papers, and non-English papers are excluded. Finally, 58 papers related to applying AHP in transportation were selected after a multistep refining process from 23 international journals. The selection process is illustrated in Figure 1. The results of reviewing the selected papers are presented in the Annex.

A. THE CLASSIFICATION OF THE APPLICATION AREAS
One of the primary purposes of this paper is to find the various fields of transportation where AHP is used to deal with specific issues. The papers are classified concerning the experts' opinions. As a result, the selected papers are classified into the following 12 application areas: Public transport, Logistic, Marine transportation, Airline industry, Railways industry, Airport industry, Transportation planning, Road transportation, Transport operations, Shipping industry, Pedestrian, and Other transportation. Table 1 illustrates that AHP is significantly used for public transport problems (18.97%), followed by logistics problems with 17.24%, and other transportation issues (15.52%) with nine papers.
The results indicate that public transport is the most popular section of transportation in which AHP is applied to deal with its problems. Traffic congestion is one of the severe problems in urban management, provoking policymakers to improve public transport performance. Public transport performance measurement is the first step in resolving the complex problems outlined since it is important for evaluating performance over time and continuously enhancing the transportation system. Service quality, productivity, effectiveness, and efficiency are crucial to evaluating the performance of transportation systems which could be done using MCDM methods, especially AHP, such as those presented by Duleba et al. [120], Duleba et al. [121], and Lupo [87]. On top of that, AHP could successfully deal with logistics problems, as indicated by the results. Logistics is the planning, applying, and monitoring activities to efficiently flow materials, products, and information, requiring decision-makers to select sites and providers and evaluate their performance frequently. Therefore, AHP could be a valuable method for selecting and evaluating effectively, such as presented by Yazdani-Chamzini and Yakhchali [122], [123], and [124]. The third most relevant field of application is other transportation covering a wide set of options, while other papers deal with more specific fields, such as maritime, airline, railways, and airport industry.  Table 2 provides information about the 23 journals where the selected papers are published. Accordingly, ''Transportation Research Part A: Policy and Practice'' ranks first with ten publications, followed by ''Transportation Research Procedia,'' with seven records. Transport Policy, Transportation Research Procedia, Sustainability, Journal of Air Transport Management, Transportation Research Part D: Transport and Environment have several papers, while the topic occasionally appears in other journals.   to AHP and its application in transportation is noticeable, especially in 2018, with 15 publications. Researchers applied AHP dealing with public transport, airport location, logistics, maritime transportation, railways, road safety, freight transport, service quality issues, and evaluating walkability in pedestrian areas, while logistics issues were the most popular. Table 3 provides information about the different extensions of AHP used by scholars. The results indicate that the conventional AHP method has the most contributions in transportation, with almost 40%, while FAHP is the second approach scholars use in transportation, with 22%. Furthermore, AHP-TOPSIS is used in three papers, Fuzzy Topsis-Fuzzy AHP and AHP-ISM are applied in two papers, and other techniques are found only once. The reason that many researchers applied conventional AHP or F-AHP more than other integration methods could be rooted in the comprehensiveness of AHP since it can not only calculate the weight of the criteria but also evaluate alternatives based on the weighted criteria. Therefore, it is clear that AHP can solve multi-criteria issues without integrating with other methods.

V. DISCUSSION
This section demonstrates some findings which are not trivial and could be concluded based on the current extensive review. First, it can be stated that a distinction must be made between those transport decision-related cases, where the uncertainty of the evaluations can be excluded, and those cases where there is a high risk of uncertainty in the decisions (e.g., layman evaluators participate, or the problem is too complex to evaluate even in a hierarchical decision structure). The high uncertainty is recommended to be handled by a properly selected methodology, most likely by fuzzy AHP or interval AHP techniques.
Another interesting issue is the redundancy of the results due to the applied methods. In many papers, for more reliable final results, the authors applied combined methods, e.g., AHP-TOPSIS, AHP-DEMATEL, AHP-PROMETHEE, and AHP-ELECTRE. In these cases, the AHP method is dominantly applied for determining the weights of the criteria, while the other method gains the ultimate ranking of the decision alternatives. Redundancy is twofold in every application of the AHP method or its integrations. On the one hand, redundancy might occur when the evaluators express their subjective judgments, and the applied technique quantifies these judgments. During the quantification, some evaluators' scoring intentions might be resilient. That is why linguistic or hesitant methods are applied. The problem is the same for the group AHP applications, where the scoring intention of some individual evaluators might disappear during the aggregation in the quantification phase. On the other hand, redundancy might occur when the evaluators' scoring intentions are well approximated, but their real intentions are not adequately presented in their scoring. The hybrid models try to capture the real intention by applying another technique for creating the ultimate decision on the alternatives through the criteria.
As a limitation, the PRISMA statement contains an evidence-based checklist of items many journals have endorsed. However, including articles might be biased toward highlighting significant findings. Also, selecting, screening, and reviewing articles based on the PRISMA checklist is time-consuming. On top of that, selecting the primary keywords for searching articles on data based might affect the comprehensiveness of the review paper.
The current study states that many extensions of the AHP method are available and applied in various transportation areas, and the results demonstrate that the AHP extensions are appropriate for evaluating and developing transportation systems. However, there are some fields where it would be necessary to provide more contributions. For instance, there is a lack of studies in which scholars apply integrated approaches, such as AHP-MABAC [125], [126], AHP-WASPAS [127], [128], and AHP-COPRAS [129], [130] to deal with transportation problems. Furthermore, there are various types of fuzzy extensions, such as Intuitionistic fuzzy sets [131], [132], Pythagorean fuzzy sets [133], [134], Hesitant fuzzy sets [135], and Neutrosophic sets [136], [137], which scholars may wish to apply to deal with uncertainty in transportation problems. Many AHP extensions were presented in section 2, which could be applied to cope with transportation problems.
One of the critical concerns for the local operators and the government is improving the quality of the urban transportation system. A system upgrade can reduce traffic congestion and pollution while increasing user satisfaction and drawing in new users. Efficient approaches are needed to address complicated traffic challenges and ensure sustainable growth. Based on the outcomes, it can be stated that AHP can deal with complexities in public transport management, where various criteria and challenges affect the transportation system's performance, making developing policies and decision-making difficult. Similarly, in the field of logistics, numerous factors, approaches, and methods must be used to select adequate and complementary third-party providers. The decision-making process complexity and the number of involved factors make the MCDM approaches appealing. Therefore, selecting a third-party logistics provider can be seen as a complex MCDM problem because several quantitative, qualitative, and other characteristics must be considered.
The future of the AHP models in transportation is the integration of two different processes. The researchers unburden the participants as much as possible by offering simple evaluation schemes. Simultaneously, the applied techniques become more complicated to mitigate the two types of redundancies. Nevertheless, MCDM is proven to be an excellent support for many transportation-related problems.

VI. CONCLUSION
This systematic review focuses on the problems of the transportation systems regarding the MCDM theory and practice. An extensive literature review is carried out for specific research areas where AHP can be applied to develop practical approaches for providing suitable solutions to complex problems. AHP can be used for various application areas, such as public transport, logistic, marine transportation, airline industry, railways industry, airport industry, transportation planning, road transportation, transport operations, shipping industry, and in case of pedestrians. Over the decades, the popularity of this MCDM approach in transportation has increased, where the AHP extensions provide different results           to assess and redesign problems in the transportation system. Consequently, scholars want to develop efficient and robust decision-making frameworks to deal with complexity, uncertainty, and indeterminacy.
Based on a thorough overview of AHP and transportrelated scientific literature, it can be stated that the AHP method is proven well-applicable in many fields of transportation. Regardless of the type of the decision problem (i.e., public or private transport), the selected transport mode (i.e., road, rail, maritime, air), and the level of the problem (i.e., micro or macro), the methodology is successfully applied several times. The results indicate that public transport issues are the most exciting topics that researchers have studied. Moreover, the highest number of articles on logistics was published in 2018, with 15 publications. In these studies, AHP, either in conventional form or integrated with other MCDM methods, was applied to solve multi-criteria problems. Table 4.

See
ABBAS MARDANI received the Ph.D. degree in philosophy management. He is currently working at the Business School, Worcester Polytechnic Institute, Worcester, USA. He has top 1% of highly cited researchers, in 2021 and 2022.
AHMAD ALKHARABSHEH is currently working at the Department of Transport Technology and Economics, Budapest University of Technology and Economics. His research interests include safety engineering and transportation engineering. Their current project is supporting public transport development decisions by newly emerged MCDM techniques.
SZABOLCS DULEBA received the M.Sc. and Ph.D. degrees in management science, in 2001 and 2008, respectively. He currently holds the position of an Associate Professor at the Department of Transport Technology and Economics, Budapest University of Technology and Economics. His specific research interests include multi-criteria decision-making in logistics and transportation management. Apart from being published in top journals, he participated in several national and EU research projects.
DOMOKOS ESZTERGÁR-KISS has been the International Project Coordinator of the Faculty of Transportation Engineering and Vehicle Engineering, since 2014. He is currently a Senior Lecturer at the Budapest University of Technology and Economics (BME). His research interest includes the optimization of multimodal travel chains for passengers in the cities. In addition, he is dealing with the development of mobility as a service solutions and establishing workplace mobility plans promoting sustainable commuting. He was the Main Organizer of several international conferences (e.g., MT-ITS 2015, EWGT 2017, and hEART 2019). He is also involved in several Horizon 2020 projects, Interreg projects, and COST Actions (e.g., MoveCit, LinkingDanube, MaaS4EU, electric travelling, BE OPEN, RegiaMobil, and OJP4Danube). He was a Fulbright Scholar at the University of California at Davis, Davis, in 2021.