Mapping knowledge structure and research trends of emergency evacuation studies

Safety Science 121 (2020) 348–361

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Safety Science journal homepage: www.elsevier.com/locate/safety

Mapping knowledge structure and research trends of emergency evacuation studies

T

⁎

Hui Liua, , Hailun Chena, Rui Honga, Huiguang Liub, Wenjiao Youa a b

College of Quality & Safety Engineering, China Jiliang University, China Construction Quality & Safety Supervision Station, HangZhou Economic & Technological Development Area, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Emergency evacuation Bibliometrics Visualization Mapping knowledge domain Evolutionary trend

To understand the current development and status of research on emergency evacuation, the published literature in the Web of Science core database from 2008 to 2018 were taken as the research objects, and the knowledge structure and research trends in the field of emergency evacuation were visualized and analyzed by using the scientific mapping knowledge domain. It is found that research on emergency evacuations throughout the world has a basic theoretical framework, but the various research branches have not yet formed a perfect knowledge network. The results show that China's publishing volume ranks first among countries/regions, and Tsinghua University has the largest number of publications among related institutions; traffic evacuation modeling, group behavior and crowd evacuation are the main knowledge groups for emergency evacuation research; the research fields mainly include disaster emergency management, personnel evacuation simulation and emergency traffic evacuation; and the research topic of emergency evacuation mainly evolves into three core paths, namely, “disaster - management - preparation - response – information”, “evacuation - behavior - model - cellular automaton - time – design” and “evacuation - behavior - simulation - social force model - exit choice”. Crowd evacuation and exit choice have become the frontiers of current emergency evacuation research.

1. Introduction In recent years, emergencies have occurred frequently around the world. In addition to traditional emergencies caused by natural disasters such as hurricanes, earthquakes and floods, new types of events such as terrorist attacks, stampedes and fires are also emerging; these catastrophic events have caused great harm to mankind, human life and property, and other losses are incalculable (Galindo and Batta, 2013). Examples include the “9/11″ terrorist attacks in the United States in 2001 (Liu and Lo, 2011), the Indian Ocean tsunami in 2004 (Mas et al., 2015), Hurricane Katrina in the United States in 2005 (Hasan et al., 2011), the severe stampede in Mecca, Arabia, in 2006 (Shiwakoti et al., 2011), the Wenchuan earthquake in China in 2008 (Yang et al., 2011), the Fukushima nuclear power plant leakage caused by the earthquake in Japan in 2010 (Lv et al., 2013), and the Saudi pilgrims’ stampede in 2015 (Shiwakoti et al., 2011). With the increasing losses caused by various disaster events to human society, emergency management, as a new research field, is attracting increasing attention of researchers, for whom emergency evacuation has been among the important contents of emergency management (Brachman and Dragicevic, 2014). In the event

of an emergency, emergency evacuation is a process of rapidly evacuating people from danger zones to safe ones (Joo et al., 2013). Although the impact of a sudden event may be small at the beginning of the event, if the situation spreads, it is likely to cause greater impact and casualties. Therefore, it is extremely necessary to evacuate people in the area immediately after an emergency (Dulebenets et al., 2018). Emergency evacuation research is a complex and systematic problem involving personnel behavior and organization, traffic management and control, rescue response and logistics support, etc. (Murray-Tuite and Wolshon, 2013; Liu et al., 2014; Lu et al., 2017). Effective emergency evacuation is of great significance for mitigating disaster or accident losses and ensuring the safety of people's lives and property (Wijermans et al., 2016). Undoubtedly, emergency evacuation is an interdisciplinary scientific problem in engineering, operational research, transportation and social sciences (Tanimoto et al., 2010; Pel et al., 2012). At present, scholars have contributed abundant research in the field of emergency evacuation. The existing research mainly involved pedestrian evacuation models, group simulation technology, intelligent evacuation management systems and emergency evacuation planning. In recent

⁎ Corresponding author at: College of Quality & Safety Engineering, China Jiliang University, No. 258, Xueyuan Street, Xiasha, Hangzhou City, Zhejiang Province, China. E-mail address: [email protected] (H. Liu).

https://doi.org/10.1016/j.ssci.2019.09.020 Received 25 March 2019; Received in revised form 1 September 2019; Accepted 18 September 2019 0925-7535/ © 2019 Elsevier Ltd. All rights reserved.

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years, most scholars have proposed pedestrian evacuation models that including pedestrian dynamics, paying more attention to the calibration and implementation of models (Vermuyten et al., 2016). Group simulation technology is a research hotspot in computer science and traffic science, mainly focusing on evacuation simulation and traffic simulation (Xu et al., 2014). The research on personnel evacuation management is mainly focused on developing intelligent evacuation management systems, including crowd monitoring, disaster prediction, and evacuation path optimization (Ibrahim et al., 2016). Orderly emergency evacuation planning and effective emergency rescue play an important role in disaster response. The development of the integrated optimal evacuation planning model is the main research goal (Abdelgawad and Abdulhai, 2009). In view of the fact that the existing research reviews often focus on a single aspect of the status quo, although rarely from the perspective of the bibliometrics to study the current emergency evacuation development process and trends, the work of this paper helps to fill the gap in this area. Bibliometrics is a quantitative analysis and statistical method used to evaluate the quality and quantity of published scientific literature and to explore the development status, research frontiers and trends of a particular field (Cobo and Gutiérrez-Salcedo, 2015; Li et al., 2017a, 2017b; van Nunen et al., 2018; Liu et al., 2018; Hong et al., 2019; Ivanović and Ho, 2019). An increasing number of researchers are using this method in their respective fields of research. For example, Liao et al. explored the status quo of medical big data (MBD) using the bibliometrics analysis method, so that the medical community could better grasp the cutting-edge technology of MBD (Liao et al., 2018). Sweileh et al. used this method to study the literature distribution and research trend of mobile health in the past decade (Sweileh et al., 2017). Based on nearly 20 years of research, Li and Zhao (2015) revealed the emphases of environmental assessment in the future by means of bibliometrics and keyword analysis. To fully grasp the current research situation in the field of emergency evacuation throughout the world, based on the Web of Science (WOS) core database, in this paper, we comprehensively use the bibliometrics analysis method and scientific knowledge mapping software and deeply examine the literature distribution characteristics, knowledge basis, research fields, research hot spots and frontier aspects in order to provide a useful reference for the subsequent study of emergency evacuation. The structure of this paper is organized as follows: In Section 2, the data sources and research methods of the paper will be elaborated. In Section 3, the research results are analyzed and discussed in detail, including literature distribution, literature co-citation analysis and keyword co-occurrence analysis. The relevant conclusions of emergency evacuation research are drawn in Section 4.

Table 1 Types of retrieved documents in emergency evacuation studies. Rank

Type of document

Frequency

Proportion

1 2 3 4 5 6 7 8 9 Total

Article Proceedings paper Review Editorial material Correction Letter Meeting abstract Retracted publication News item

1409 948 52 10 4 4 4 3 2 2436

0.59426 0.39983 0.02193 0.00422 0.00169 0.00169 0.00169 0.00127 0.00084 >1

publication, date (volume) and references, etc. These 2,330 document records cover 9 types of documents, mainly based on the three types of Articles (59%), Proceedings Paper (40%) and Review (2%) (see Table 1). Because the literature may be classified into two different types of document, the total frequency of document types used for the statistics is exceeds the number of documents retrieved. We mainly focused on the first 3 items (Article, Proceedings Paper, Review) throughout the whole study. 2.2. Methods and tools First, this article carries out a bibliometric analysis on the emergency evacuation domain literature; the analysis objects include the document quantity, the research country, the research institute, and the research discipline, and, from the whole, grasps the general developmental situation and trends of the world emergency evacuation. Second, citation analysis and cooccurrence analysis are carried out using bibliometric analysis methods. Citation analysis and cooccurrence analysis are the use of mathematical algorithms and measurement methods for data statistics and cluster analysis of literature keywords and subject words to obtain hot topics and frontier trends in a certain field (Cobo et al., 2011). The concrete measure of this paper is its use of the scientific mapping software CiteSpace V and VOSviewer to make a more detailed analysis of the literature data by the mapping of scientific knowledge and using visualization software. CiteSpace is a software written by American scholar Chaomei Chen based on java program (Chen, 2014) and Vosviewer is a software based on vos visualization technology developed by Dutch scholar N.J. van Eck and his team (van Eck and Waltman, 2010). Both scientific mapping tools are available free of charge on the website. CitespaceV can be downloaded from http://cluster.cis.drexel.edu/~cchen/citespace/. Vosviewer software can be downloaded from www.vosviewer.com. Scientific mapping clearly embodies the connections between disciplines, fields, professions and papers (Zupic and Čater, 2014). Citation analysis includes high citation analysis and co-citation analysis and then explores the knowledge base in the field of emergency evacuation research. Cooccurrence analysis uses keywords to determine the research field, research hotspot and evolution process of emergency evacuation research.

2. Data and methods 2.1. Data source The data was retrieved from the core collection database in the WOS comprehensive bibliographic database, including SCIE, SSCI and other citation index databases. The topic “emergency evacuation” was entered in the search bar of the WOS, and the category of medical surgery was removed from the search, so that the search is designed to collect more accurate data. It has been found that the publication time of the literature was recorded after the 1990s in a preliminary search. In particular, the past decade has been the fastest period of emergency evacuation development. Haghani and Sarvi (2018) used existing data retrieval techniques to analyze the literature distribution of pedestrian crowds and emergency evacuation, and the study show that the volume of literature on crowd model is on the rise after 2008. This is consistent with the results retrieved in this paper. So the time span was set from 2008 to 2018. The retrieval date was September 23, 2018, and a total of 2,330 documents had been recorded up to the date of retrieval. Each record includes the author, institution, abstract, keywords, year of

3. Results and discussion 3.1. Literature distribution characteristics in emergency evacuation 3.1.1. The time distribution of literature A statistical analysis of the published years of literature in a certain field can be used to understand the development of the research in the field of research from the time distribution. Preliminary search found that records in the core collection database of the Web of Science had only records after 1990s. To more comprehensively understand the emergency evacuation development process, we selected data from all 349

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Fig. 1. The annual trends of published articles in emergency evacuation studies.

published papers, with the development of new technologies, the number of publications on emergency evacuation presents an increasing trend year by year, indicating that the attention level of this field shows an increasing trend year by year.

the databases in the Web of Science for analysis. As we can see from Fig. 1, the entire course of development can be divided into the following four stages: the initial germination stage, the initial growth stage, the rapid development stage and the steady growth stage. Initial germination stage (1995–2004): In all the databases in the Web of Science, the documentation on emergency evacuation studies began as early as 1995. Prior to 2008, there were very few articles on relevant studies, especially before 2004, when the annual publication of researches was basically below 10 articles. In this initial stage, there was no lack of high-quality literature in this field. For example, Schultz et al. studied the evacuation methods and emergency management strategies of hospitals after disasters (Schultz et al., 2003). Heath et al. discussed the impact of keeping pets among family residents on evacuation in the event of an emergency (Heath et al., 2001). These explorations laid the research foundation for the subsequent development. Initial growth stage (2004–2008): Although there were few papers on relevant research before 2008, the number of publications has been stabilizing year by year, and there are approximately 20 papers per year. With the rapid development of computers, the research methods in this stage range from statistical investigation to simulation. In 2004, Isobe et al. studied the evacuation process of students in a room filled with smoke by combining experiments and a lattice gas model simulation, which is of great significance for the reference of emergency evacuations of buildings (Isobe et al., 2004). In 2008, Johansson et al. used video to study crowd behaviors under a large number of basic conditions, thus improving the maximum capacity of pedestrian evacuation facilities to ensure crowd safety (Johansson et al., 2008). Rapid development stage (2009–2014): After 2009, the number of papers exceeded 100 for the first time, and since then, the volume of publications on emergency evacuation research has basically shown a multiple growth trend; this stage belongs to the rapid development stage. At this stage, the increasing trend of literature quantity is most obvious, and the research contents begin to be more abundant and include the crowd evacuation model of buildings, optimization of evacuation path, postdisaster operation management and emergency rescue and evacuation (Zheng et al., 2009; Stepanov and Smith, 2009; Galindo and Batta, 2013; Gelenbe and Wu, 2012). Steady growth stage (2015 until now): The steady growth period in the field of emergency evacuation research (the number of articles in 2018 has not been counted yet) spans from 2015 until now, and the accumulation of articles continues to increase, although at a smaller growth rate. As seen from the variation trend of the number of

3.1.2. The country and institution distribution of research In the core database of the Web of Science, according to the number of publications, there were 46 countries (regions) involved in emergency evacuation studies between 2008 and 2018. The research in the database is primarily from China and the United States, which account for 34% and 29%, respectively, indicating that China and the United States have been at the forefront of emergency evacuation research. The United States is among the first countries to have carried out research, and as early as the 1950s began to study emergency evacuation behavior. After the September 11 terrorist attacks and natural disasters such as hurricanes, emergency evacuation research has intensified. Although China started late, with the increase of domestic emergencies, the government gradually attached importance to the construction of emergency management. The other major research countries (regions) were the UK, Japan, Italy, Australia, Canada, Taiwan area, South Korea and Germany, with the publication volume ranging from 67 to 135 (see Fig. 2). We imported the retrieved literature data (2330 articles in total) into the VOSviewer software. In the VOSviewer software, the network node selects the “country” where the paper is published, the size of the node selects the “central connectivity” and then selects the appropriate threshold, and the VOSviewer software is run to obtain the cooperative network of countries mainly studied in the field of emergency evacuation research, as shown in Fig. 3. The centrality of the country increases with more nodes. We can see that the United States, China, the UK and Australia have a relatively large influence and are in the core position. The color of the node shows the average time of the country's published volume. Countries such as France, Poland, New Zealand, and Israel are relatively active research countries in recent years. In addition, it can be seen from the figure that concerning the cooperation among the research countries (regions), thicker lines imply closer connections. The United States and China have the closest research relationship in the field of emergency evacuation and cooperate with Britain, Japan, Australia and other countries. There were 147 major research institutions in the field of emergency evacuation in the past decade, and the top 10 institutions cited in the total citation volume are shown in Table 2. It can be seen from the table that the research institutions in the field of emergency evacuation 350

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Fig. 2. The top 10 countries/areas of published articles in emergency evacuation studies.

(as shown in Fig. 4). The larger the size of node is, the more the number of published papers is. The links between the nodes represent cooperation between countries. The figure is drawn based on setting the connection strength greater than or equal to “2” in the software, in order to clearly display the cooperation relationship between the main institutions. We can see find close cooperation between several domestic research universities. At the same time, it is not hard to find that Tsinghua University is in the middle of the cooperation network, which indicates that the institution has the most frequent cooperation with other countries and has some cooperation with foreign institutions. However, in addition, the cooperation among various research institutions throughout the world is relatively small, which is basically

mainly focus on several universities in China, such as University of Science and Technology of China, Beijing University of Chemical Technology, Tsinghua University, Beijing Jiaotong University, Beijing Normal University and City University of Hong Kong. Among them, the highest number of cited documents in the field of emergency evacuation are from the University of Science and Technology of China, with 493 papers cited. Tsinghua University is the world's most published institution, with a total of 64 papers published. Texas A&M University in the United States and Monash University in Australia also ranked in the top five for citation volume. We use the VOSviewer software to obtain the cooperative network map of major research institutions in the field of emergency evacuation

Fig. 3. Cooperation countries around the world in emergency evacuation studies from 2008 to 2018. 351

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3.2. Literature cocitation analysis in emergency evacuation

Table 2 The top 10 research institutions in emergency evacuation studies. Rank

Institute

Country

Frequency

Citations

1 2 3 4 5 6 7 8 9 10

Univ sci & technol china Beijing univ chem technol Monash univ Texas a&m univ Tsinghua univ Delft univ technol Beijing jiaotong univ Rmit univ Beijing normal univ City univ hong kong

China China Australia USA China Netherlands China Australia China China

46 14 18 20 64 25 45 15 23 21

493 355 263 245 230 198 156 147 143 132

Highly cited references are an important source of knowledge in a research field, reflecting the research level and development direction of a certain discipline, and they are an important basis for exploring hot topics and research frontiers. The frequency of citations after publication is the basic index used to evaluate the influence of academic achievements. Table 3 shows the top 10 cited papers in the field of emergency evacuation from 2008 to 2018, which are mainly from the disciplines of engineering technology, operations research and management, as well as the disciplines of mathematics, physics and communication, etc., and mainly involve topics such as pedestrian evacuation models, disaster operation management, networks, traffic dynamic distributions and emergency rescue. The key literature that lay the foundation for this field can be found by analyzing the cocited documents. The methods and ideas proposed in these documents are the core ideas in the field of emergency evacuation research. The literature cocitation analysis is a cocitation analysis of the literature cited in the article, which constitutes a cocitation relationship when two or more literature are referenced in the same paper. Cocitation analysis is one of the core functions of CiteSpace software. Therefore, this paper uses CiteSpace to conduct a cocitation analysis on the documents of emergency evacuation studies. When setting the parameters, we choose the node type for the cited conference and obtain the literature cited network (Fig. 7). As a result, a total of 331 references were cited, with a total of 1012 cocitation relationships among them, scattered in 7 clusters (as shown in Fig. 8). The clustering tags are traffic network management, pedestrian-vehicle mixed evacuation, tsunami preparedness, crowd evacuation, neurological activity analysis, crowd behavior and agent-based modeling. In Fig. 7, the size of the grape node is determined by the cooccurrence frequency, the node position is controlled by centrality or density, and the frequency and centrality are not strictly proportional. A node with a high centrality or density is a core node, which represents the degree and importance of the research object being paid attention to and is the hub connecting and communicating with other nodes. As shown in Fig. 7, these documents are hub nodes with high frequency and high centrality (Cocitation frequency > 30) (Zheng et al., 2009; Murray-Tuite and Wolshon, 2013; Kobes et al., 2010a, 2010b; Pel et al.,

based on the cooperation within each country, and the global cooperation network of emergency evacuation research institutions has not yet been formed.

3.1.3. The category distribution of research The high output of the subject in a certain field means that the subject has many academic achievements, strong research strength and high discipline status. These subjects are more powerful than other subjects, and we call them leading disciplines. The field of emergency evacuation has penetrated into a large number of disciplines, as shown in Fig. 5. The size of the circle is proportional to the output of discipline, the purple outer circle represents high centrality and the width of links represents the strength of collaboration. From the Fig. 5, we can see that different subjects are closely connected, among which the strong subject in this field is engineering, which is in the core position. Furthermore, Fig. 6 shows the annual output of each major discipline in recent 10 years. There are 1,043 published papers, accounting for 44.2% of the total literature. It is followed by computer science, transportation, operations research management science; for these three disciplines, in addition, a number of published papers are also distributed in the fields of geology, water resources, environmental sciences ecology, meteorology and atmospheric science.

Fig. 4. Collaboration network map of main research organizations in emergency evacuation studies. 352

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Fig. 5. The main research categories in emergency evacuation studies.

large number of cited references with high centrality, the three clusters were analyzed to further understand the knowledge basis of emergency evacuation. The #0 cluster contains the largest number of publications, having a total of 52 cited literature. The cluster label is traffic evacuation model (pedestrian and vehicle mixed evacuation model). The most cited article is by the American author P. Murray-Tuite and her coauthors, who reviewed the evacuation modeling and simulation of highways and their development (Murray-Tuite and Wolshon, 2013). This paper emphasizes the interdisciplinary modeling work in evacuation to help bridge the gap between behavioral science and engineering technology and to validate and calibrate evacuation models by applying emerging technologies. In evacuation planning, AJ Pel, a writer from the Netherlands, along with his coauthors, established the travel behavior model in the dynamic traffic simulation model of evacuation by considering

2012; Pelechano and Malkawi, 2008; Gladwin, 2007; Mehdi et al., 2011). The main function of CiteSpace is to detect the changing trend and evolution of a research field or discipline over time. Betweenness centrality, one of the measurement indexes of each node in the CiteSpace map, refers to the influence of shortest path between a node and other nodes. The closer to the center of the network, the greater the impact. In the clustering process of this paper, Cosine algorithm was used to calculate the connection strength between keywords, and each cluster was named by LLR algorithm to obtain the cluster label. In the co-cited clustering network, key references of each cluster are determined by the co-cited count and centrality, and the results are shown in Table 4. These works often represent the knowledge basis of emergency evacuation research. We have mainly analyzed the three main clusters of the cocited clustering network. As the three clusters of #0, #1 and #2 contain a

Fig. 6. The number of published articles in the main category. 353

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& Management Science

Engineering Operations Research Telecommunications Engineering Operations Research Mathematics Operations Research Engineering Physics Operations Research

& Management Science

such factors as evacuation selection, departure time selection, destination selection and route selection (Pel et al., 2012). Authors Chiu et al. proposed a network transformation and demand specification method for evacuation modeling without warning to optimize the evacuation model of groups without warning (Chiu et al., 2007). A. Stepanov et al. carried out an optimal design and analysis of evacuation routes in traffic networks and proposed an optimal exit path allocation method (Stepanov and Smith, 2009). M.K. Lindell et al. calculated the estimated evacuation time of communities threatened by hurricanes and analyzed the main behavioral variables affecting evacuation time in the model (Lindell and Prater, 2007). The #1 cluster contains 49 cited references, and the cluster label is group behavior. The American author M. Mehdi and coauthors proposed a simple, behavioral, scientific and cognitive approach to better understand group behavior (Mehdi et al., 2011). The authors Kobes et al. conducted 83 evacuation experiments on a hotel building at night and obtained that smoke, low-placed exit signs and familiarity with the environment all have an impact on evacuation behavior (Kobes et al., 2010a, 2010b). L Hou et al. studied the influence of the number and position of evacuation leaders on evacuation dynamics in rooms with limited visibility (Hou et al., 2014). Guo et al. established a micropedestrian model with discrete space representation through a group of classroom experiment results and studied the path selection of pedestrians under evacuation with good and zero visibility conditions, which is helpful for the design of building interior layouts and exit layouts similar to the classroom (Guo et al., 2012a, 2012b). The #2 cluster contains 39 cited references, and the cluster label is crowd evacuation. A. Varas et al. used a two-dimensional cellular automata model to simulate the pedestrian evacuation process in a room with fixed obstacles (Varas et al., 2007). W.F. Yuan et al. proposed a two-dimensional basic cellular automata model based on human behavior, revealing the potential application of CA model in fire safety assessment (Yuan and Tan, 2007).

2009 2013 2014 2011 2009 2008 2009 2010 2008 2011 197 162 139 110 103 101 96 90 82 80 Modeling crowd evacuation of a building based on seven methodological approaches Review of recent developments in OR/MS research in disaster operations management Relay-by-Smartphone: Realizing Multihop Device-to-Device Communications Robust optimization for emergency logistics planning: Risk mitigation in humanitarian relief supply chains Evacuation planning using multiobjective evolutionary optimization approach From crowd dynamics to crowd safety: a video based analysis Multi-objective evacuation routing in transportation networks A dynamic evacuation network optimization problem with lane reversal and crossing elimination strategies A mobile lattice gas model for simulating pedestrian evacuation A multi-objective robust stochastic programming model for disaster relief logistics under uncertainty Zheng, XP Galindo, G Nishiyama, H Ben-Tal, A Saadatseresht, M Johansson, A Stepanov, A Xie, C Guo, RY Bozorgi-Amiri, A 1 2 3 4 5 6 7 8 9 10

Citations Cited references Author Rank

Table 3 The top 10 papers with the most citations in emergency evacuation studies.

Year

Category

& Management Science

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3.3. Keywords cooccurrence analysis of emergency evacuation research 3.3.1. Research field analysis of emergency evacuation In this paper, VOSviewer's mapping and clustering techniques are used to explore hot words in the paper titles and abstracts to determine the hot areas of emergency evacuation research. The VOSviewer has two ways to import data: text corpus and network. For co-occurrence analysis of keywords in the title and abstract, the data is imported in the form of text corpus, the analysis scope is selected and the threshold is set, then keywords are screened, finally map is formed. The main principle of map formation is the visualization algorithm of node similarity, and the correlation strength of each node is displayed by the distance of each node. In VOSviewer software, the key words of similar research topics are gathered in the same color. Fig. 9 shows three core clusters of emergency evacuation research; the blue cluster is the field of emergency traffic evacuation, and the red cluster is the disaster emergency management field. The green cluster is the field of personnel evacuation simulation. The keyword cooccurrence density map (Fig. 10) provided by VOSviewer can visually reflect the frequency density that occurs together between high-frequency words. The higher the frequency of cooccurrence of two keywords, the closer and stronger the correlation. These highly correlated objects are clustered together to form a cluster. The color represents frequency density. The warmer(redder) the color is, the higher the density is. While the cooler(bluer) the color is, the lower the density is. From the map, we can see that the density of red and green clusters is relatively high, indicating that the studies in these two fields (disaster emergency response and personnel evacuation simulation) are relatively popular. Cluster #1: disaster emergency management field. The cluster contains 146 keywords in total, and the high-frequency keywords are as follows: disaster, case, response, plan, risk, population, hazard, life, 354

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Fig. 7. Map of co-cited literature network in emergency evacuation studies.

agent, parameter, evacuation process, flow, crowd, individual, speed, movement and space. In recent years, crowd evacuation simulation has been a hot research area. When riots, fires and other emergencies occur in large public places or high-rise buildings, the psychological factors of crowd overcrowding and panic conformity are the main causes of casualties, so it is of practical significance to model the evacuation behavior of personnel. Cellular automata, lattice gas and social force models are the models primarily used in pedestrian flow microsimulations (Alizadeh, 2011; Guo et al., 2012a, 2012b; Wan et al., 2014). Cluster #3: emergency traffic evacuation field. The cluster contains 57 keywords in total, and the high-frequency keywords are as follows: route, algorithm, network, solution, efficiency, path, congestion, optimization, cost, shelter, demand, device, service, traffic, and uncertainty. In the case of emergencies, the increasing traffic demand in a short time will break the previous equilibrium state, and how to meet the traffic demand in time is an urgent problem to be solved. The selection of an evacuation path is one of the key parts of emergency evacuation, and it is the criterion for whether an emergency management plan is feasible (Li et al., 2015).

Fig. 8. Map of cocited literature cluster label network in emergency evacuation studies.

3.3.2. Research hotspots and evolutionary process of emergency evacuation In bibliometrics, keyword cooccurrence analysis is often used to study the hot spots or subject evolution of a certain field or subject. Keywords with high frequency are often used to identify hot issues in a research field. In CiteSpace, the node is set as the keyword, and the keyword cooccurrence relation chart is obtained. As seen from Fig. 11, the purple outer circle represents high centrality; the width of links represent the strength of collaboration. Keywords are closely connected, indicating that research hotspots in the field of emergency evacuation are concentrated. Table 5 shows the high-frequency keywords. The words with high centrality include evacuation, model, simulation, emergency evacuation, behavior, system, dynamics, emergency, disaster and management. The degree of keyword centrality reflects the core research topic of

need, city, earthquake, loss, action and damage. Destructive natural disasters have occurred frequently around the world in recent years. The Chinese government attaches great importance to disaster prevention and mitigation and accordingly established the Ministry of Emergency Management to carry out comprehensive disaster risk management. Natural disaster emergency decisions play an important role in disaster response (Zhou et al., 2018). At the same time, in the process of emergency response, information acquisition is also among the important means of emergency management (Simon et al., 2015; Thompson et al., 2017). Cluster #2: personnel evacuation simulation field. The cluster contains a total of 110 keywords, and the high-frequency keywords are as follows: simulation, behavior, fire, experiment, evacuation time, exit, 355

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Table 4 The key literature of co-cited cluster label network in emergency evacuation studies. Cluster

Co-citation count

Centrality

Author

Cited references

#0 #0 #0 #0 #0

60 37 29 29 27

0.11 0.03 0.09 0.03 0.06

Murray-tuite P Pel AJ Stepanov A Chiu YC Lindell MK

#1 #1 #1 #1 #2 #2 #2

31 29 27 27 23 13 16

0.05 0.02 0.11 0.06 0.06 0.03 0.03

Moussaid M Kobes M Guo RY Hou L Varas A Song WG Yuan WF

#3 #3 #3 #4 #5

75 39 26 21 30

0.16 0.05 0.15 0.09 0.12

Zheng XP Kobes M Alizadeh R Sbayti H Pelechano N

#6

38

0.04

Gladwin H

Evacuation transportation modeling: An overview of research, development, and practice A review on travel behaviour modelling in dynamic traffic simulation models for evacuations Multi-objective evacuation routing in transportation networks Modeling no-notice mass evacuation using a dynamic traffic flow optimization model Critical behavioral assumptions in evacuation time estimate analysis for private vehicles: Examples from hurricane research and planning How simple rules determine pedestrian behavior and crowd disasters Way finding during fire evacuation; an analysis of unannounced fire drills in a hotel at night Route choice in pedestrian evacuation under conditions of good and zero visibility: Experimental and simulation results A social force evacuation model with the leadership effect Cellular automaton model for evacuation process with obstacles Simulation of evacuation processes using a multi-grid model for pedestrian dynamics A novel algorithm of simulating multi-velocity evacuation based on cellular automata modeling and tenability condition Modeling crowd evacuation of a building based on seven methodological approaches Building safety and human behaviour in fire: A literature review A dynamic cellular automaton model for evacuation process with obstacles Optimal scheduling of evacuation operations Evacuation simulation models: Challenges in modeling high rise building evacuation with cellular automata approaches Evacuation decision making and behavioral responses: Individual and household

this field to some extent. Using CiteSpace software, the node is still a keyword, and the node size is selected as the centrality. The keyword cooccurrence view of each stage of the emergency evacuation research in the past ten years is obtained, as shown in Fig. 12. From 2008 to 2010, with the introduction of the big concept of emergency evacuation, the basic concepts of disaster, emergency response, management and dynamics evolved. During this period, the research field gradually formed two fields, disaster emergency management and personnel evacuation. The key word disaster has the highest centrality, indicating that disaster is the core research topic at this stage. The model is a hot research topic, and then the cellular

automata model evolved. The research of these models is mainly applied to building evacuation. Between 2011 and 2013, the research field remained disaster emergency management and personnel evacuation, but the research topic has changed. At this stage, the centrality of behavior is the highest, and it is the core research topic in the field of personnel evacuation. Under the large framework of personnel evacuation, the theme of simulation has attracted more and more people's attention and evolved the derivative themes such as time and egress. In terms of models, cellular automata models remain a research hotspot. In addition, disaster emergency management includes the basic concept of

Fig. 9. Keyword cooccurrence network of publications in emergency evacuation studies. 356

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Fig. 10. Keyword density visualization map of publications in emergency evacuation studies.

movement, flow, and human behavior. The emergency response needs the feedback of information, so the research on the key word of information is intensified. The distinction between research areas is no longer obvious, and each keyword is found in all areas. Between 2017 and 2018, emergency evacuation research belongs to the stable development stage, and there is no special breakthrough. Model and simulation are two constant research topics, and the

emergency response, thus deriving a series of key words, such as information, system, support and logistics. Between 2014 and 2016, emergency evacuation research reached a period of prosperity. There are more keywords in the cooccurrence network, and the centrality is generally balanced. Under the framework of emergency evacuation, behavior and model remain important research topics and evolved a series of keywords such as optimization,

Fig. 11. Keyword cooccurrence map of publications in emergency evacuation studies. 357

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Table 5 The top 10 keywords of the publications in emergency evacuation studies. Number

Keywords

Frequency

Centrality

1 2 3 4 5 6 7 8 9 10

Evacuation Model Simulation Emergency evacuation Behavior System Dynamics Emergency Disaster Management

492 367 338 261 217 143 125 124 112 112

0.31 0.18 0.1 0.26 0.16 0.17 0.09 0.22 0.44 0.23

behavior, optimization, time and exit of previous studies remain research hotspots. At this stage, the basic concept of disaster mainly focuses on the theme of natural disaster, involving key words such as environment and disaster preparation. In particular, in terms of models, studies of cellular automata models are no longer a research hotspot, but social force models become new research topics. Based on the analysis of the CiteSpace keyword cooccurrence map, this paper selects key topics, sorts out the evolutionary path of emergency evacuation research, and clearly shows the evolutionary process of topics (as shown in Fig. 13). In this picture, the lighter the color, the later the year. The size of the cross in the picture represents the number of studies of the keyword. In the process of emergency evacuation development, with the introduction of the basic concept of evacuation,

Fig. 13. Evolutionary path of the research topic in emergency evacuation studies.

two important research topics were discovered: disasters and behavior. Two methods are employed to study behavior: models and simulations. Therefore, based on these three themes, this paper sorts out three important research paths for emergency evacuation research, as follows. “Disaster Emergency Management” Research path: In the various stages of emergency evacuation research, we found that in the context of the basic concept of evacuation, the emergency management of disasters runs through the entire emergency evacuation research.

Fig. 12. Keyword cooccurrence map of publications in emergency evacuation studies for different time periods. (a) The data has been extracted from 2008 to 2010. (b) The data has been extracted from 2011 to 2013. (c) The data has been extracted from 2014 to 2016. (d) The data has been extracted from 2017 to 2018. 358

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force model and information, showed strong citation bursts. According to the centrality and the strong citation bursts, considering the influence of the keyword, it can be predicted that information will still be a hot word in the field of emergency evacuation in the future. The emergence of the two key words, crowd evacuation and exit choice, represents that the two themes of crowd evacuation and exit choice have become the forefront of current emergency evacuation research. Exit choice is an indispensable decision for pedestrians in the evacuation process. Shuaib and Zainuddin (2017) incorporated the individual's intellectual ability into the exit choice model to avoid the factors of individual panic behavior. Li et al. (2017a, 2017b) proposed an exit choice model that considers pedestrian psychology, and combined with cellular automata to simulate the evacuation behavior of pedestrians. The improvement of exit choice model will remain the research hotspot in the future.

Combined with the keyword cooccurrence network, we can clearly see that under the framework of evacuation, the theme of management gradually evolves to preparedness and response, and the current theme is information, forming the evolutionary path of “disaster - management - preparation - response – information”. “Model” Research path: During the development process of the research on behavior, the topic of models has always been the main research focus. In the framework of this model, the cellular automata model is the most popular. Combined with the keyword map, it can be seen that evacuation time gradually evolved in the model, followed by the theme of design, forming the evolutionary path of “evacuation behavior - model - cellular automaton - time – design”. “Simulation” Research path: In the development process of studying the topic of behavior, the simulation method was not as important as the model at the beginning, but with the passage of time, the simulation research was intensified. Combined with the keyword map, we can see that the simulation gradually evolved into the social force model, followed by the theme of exit selection, forming the evolutionary path of “evacuation - behavior - simulation - social force model - exit choice”.

3.3.4. Keywords classification of emergency evacuation According to the meanings expressed by the keywords based on Citespace software, further classification was carried out, the results can be seen in Table 7. In terms of emergency types, major disasters are earthquake, tsunami, hurricane, flood, terrorism, volcanic eruption and fire. From the research object, emergency evacuation research mainly involve building evacuation, road tunnel, subway, station and civil airplane and other places. In terms of the solution approach, cellular automata, social force model, agent-based modeling and network flow and other modeling and simulation methods can be applied to various emergency environments such as buildings and open spaces. Considering social factors, it is mainly the psychological and behavioral characteristics of researchers during evacuation in emergency scenarios, which involve environmental factors, individual age, ability, panic level and other influencing factors. In addition, collective behavior is also an important aspect. According to the characteristics of evacuation route, the shortest evacuation distance and the lowest probability of traffic bottleneck are the best targets of evacuation route selection. The evacuation method is divided into pedestrian evacuation, vehicle evacuation and pedestrian-vehicle mixed evacuation.

3.3.3. Research frontier identification of emergency evacuation The timeline view of CiteSpace (see Fig. 14) is a unique feature of the software that distinguishes it from other bibliometric visualization software. The timeline view based on the cooccurrence relationship of keywords divides all keywords into 6 categories, and each category comprises several keywords. The nodes in the purple outer circle have high centrality and strong ability to combine with other words. Words are marked in the year when they first appear, and the connecting line indicates the cooccurrence relationship between keywords. As the time span studied in this paper is from 2008 to 2018, it can be clearly seen from the visualized the timeline view that many keywords were already available in 2008, and they are basically high-frequency keywords such as evacuation, model, simulation, emergency evacuation, behavior, system, dynamics, emergency, disaster and management. The top 10 high-frequency keywords have been studied since 2008. By 2010, export keywords began to appear. In 2011, cellular automata, emergency response, emergency management, wireless sensor networks and other key words appeared. In 2013, two key words appeared: safety and design. In 2016, social force model and evacuation time appeared. Table 6 is a list of keywords with the strongest citation bursts. It can be seen that the social force model is a keyword with the greatest strongest citation bursts. Between 2016 and 2018, only two key words, social

4. Conclusions In this paper, emergency evacuation is taken as the research object, and visualization tools such as CiteSpace and VOSviewer are used to conduct bibliometric analysis of 2330 related documents in the core

Fig. 14. The keyword timeline view of publications in emergency evacuation studies from 2008 to 2018. 359

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Table 6 Top 15 keywords with the strongest citation bursts in emergency evacuation studies. Keywords

Year

Centrality

Strength

Begin

End

Social force model Emergency management Evacuation simulation Information Earthquake Design Safety Emergency response Building evacuation Wireless sensor network Crowd Cellular automata Egress Hurricane Evacuation Preparedness

2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008

0.01 0.11 0.01 0.03 0.04 0.02 0.01 0.04 0.04 0.01 0.02 0.04 0.04 0.01 0.05

13.4996 8.5329 8.5155 7.4741 7.3801 7.2578 6.8648 6.4409 6.2929 5.511 4.6162 4.6095 4.5442 4.239 4.0601

2016 2011 2012 2015 2015 2013 2013 2011 2008 2011 2014 2010 2011 2008 2009

2018 2013 2013 2018 2016 2014 2014 2013 2011 2012 2015 2014 2013 2013 2012

2008–2018

Table 7 Classification of keywords in emergency evacuation. Classification

Keyword

Emergency type Object of study

Earthquake, tsunami, hurricane, flood, hurricane, terrorism, inundation, volcanic eruption, fire Building, road tunnel, subway station, tunnel fire, trade center, high-rise building, indoor, civil airplane, pitch, passenger station, super-long railway tunnel Model, simulation, algorithm, cellular automaton, social force model, experiment, agent-based modeling, agent-based simulation, network flow, lattice gas model, tabu search, genetic algorithm, GIS, game theory Environment, social influence, human factor, children, social force, capacity, climate change impact, emergency planning, emergency condition, crowd behavior, pedestrians panic, psychological experiment, collective behavior Route, uncertainty, shortest path, traffic bottleneck, congestion, intelligent transportation system, dynamic traffic assignment, genetic algorithm, navigation, transportation, wireless sensor network, route choice, vehicle routing problem, minimum flow Pedestrian evacuation, occupant evacuation, emergency vehicle, car

Solution approach Social factors Evacuation route characteristics Evacuation method

institutions has not yet been formed. The field of emergency evacuation has penetrated into various subjects, and different subjects are closely connected. The leading discipline in this field is engineering. It is followed by computer science, transportation, and operations research management science, and these three disciplines, in addition, are also distributed in the fields of geology, water resources, environmental sciences ecology, meteorology and atmospheric science. Emergency evacuation research has formed a basic theoretical framework, with evacuation model, group behavior and crowd evacuation theory as the main knowledge base, applied in the three fields of emergency traffic evacuation, disaster emergency management and personnel evacuation simulation, respectively. The research topic of emergency evacuation mainly evolves into three core paths: “disaster management - preparation - response – information”, “evacuation behavior - model - cellular automaton - time – design”, and “evacuation - behavior - simulation - social force model - exit choice”, laying a solid foundation for the continuation and development of emergency evacuation knowledge in the future. Emergency evacuation research has gradually shifted from the establishment of the evacuation process model to the study of group behavior. The computer simulation of evacuation process considering the influence of group behavior has gradually become the research hotspot and highlight in this field. Information-based emergency management is the guarantee for emergency evacuation research. Crowd evacuation and path planning have become the research frontiers of emergency evacuation. Although some interesting results have been obtained through the bibliometric analysis and visualization on emergency evacuation studies related documents by using the scientific mapping knowledge domain in the study, there are also some limitations. Firstly, the incomplete data in 2018 may lead to some shortcomings in the current analysis. Then, the co-occurrence analysis of keywords can reflect the overall structure of the research field, but there is a lack of quantitative

database of Web of Science. The main contents include the distribution of publications, countries and institutions studied, subject categories, literature co-citation analysis and keyword co-occurrence analysis, and mapping of scientific knowledge. This paper reveals the research hotspots and hotspots in the field of emergency evacuation as well as the development trend, which provides a reference for relevant researchers to further understand this field. The main research results show the following. The whole development course in the field of emergency evacuation can be divided into the following four stages: initial germination stage, initial growth stage, rapid development stage and steady growth stage. In all the databases of the Web of Science, the earliest literature records began in 1995. Before 2004, the number of publications was basically below 10. After 2009, the number of published papers basically showed a multiple growth trend. The country with the largest number of publications in the field was China, which was followed by the United States, and they accounted for 34% and 29%, respectively. China and the United States are at the forefront of emergency evacuation research. The United States and China have the closest research and cooperation in the field of emergency evacuation and have more links with the United Kingdom, Japan, Australia and other countries. The research institutions in the field of emergency evacuation are mainly concentrated in several universities in China, such as the University of Science and Technology of China, Beijing University of Chemical Engineering, Tsinghua University. Among them, the highest ranking concerning the total cited reference in the field of global emergency evacuation is the University of Science and Technology of China. Tsinghua University has the largest number of publications in the world and has the closest cooperation with other national research institutions. However, in addition, the cooperation among various research institutions in the world is relatively small, which is basically based on the cooperation within each country, and a global cooperation network of emergency evacuation research 360

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research methods in the aspect of trend prediction, which is important for the identification of research frontiers. Finally, as the number of papers increases, the selection of keywords may not be comprehensive enough. For future work, the weights of keywords, journals and books published during the period will be to make the research results more convincing. The topics in the research field of emergency evacuation will also be studied to further investigate the overall trends and the development of a certain research branch or topic.

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