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Review

A Review of the Evolution and Trends in Research on the Emergency Evacuation of Urban Underground Spaces

by
Caiyun Cui
1,
Qianwen Shao
1,
Yong Liu
2,
Guobo Han
1,*,
Feng Liu
1 and
Xiaowei Han
1
1
Architectural Engineering College, North China Institute of Science and Technology, Langfang 065201, China
2
School of Civil Engineering and Architecture, Zhejiang Sci-Tech University, Hangzhou 310018, China
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(5), 1325; https://doi.org/10.3390/buildings13051325
Submission received: 7 April 2023 / Revised: 4 May 2023 / Accepted: 10 May 2023 / Published: 19 May 2023
(This article belongs to the Section Architectural Design, Urban Science, and Real Estate)
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Abstract

:
In recent decades, there has been a significant proliferation of literature pertaining to the evacuation of urban underground spaces. However, few studies have focused on scientometric reviews that identify research topics, evolution patterns, and trends in this field. To bridge this gap, the present study adopted a bibliometric approach by searching the literature related to underground space evacuations in the Web of Science (WOS) core database for the period 1992–2022, from which we visualized and analyzed the issuance status of the country, author, and institution, as well as the cooperation status, using Citespace software. The findings of this paper are as follows: First, there has been a consistent upward trend in the quantity of literature within this field over time. Second, Swedish authors and institutions have made important contributions to the advancement of research in this area. Third, the current research topics are concentrated on themes such as modeling and simulation, smoke control methods, integrative research on factors affecting pedestrian evacuation behavior, pedestrian walking speed under smoke-filled conditions, and the influence of wayfinding devices on exit selection/wayfinding. Finally, to advance the safety in underground spaces, future research trends should include improving basic data, refining simulation model parameters, assessing the evacuation capabilities of special populations, examining the impact of risk perception on evacuation behavior, and conducting specific research on the different risks in underground spaces. The findings may contribute to the managerial development of more comprehensive and effective emergency plans for underground spaces, thereby enhancing the survival rates and safety of pedestrians during disasters or other emergencies.

1. Introduction

As urban populations continually grow, and available land resources become scarce, the development and utilization of urban underground spaces has gradually accelerated, entering a new stage of large-scale, networked, three-dimensional comprehensive development and utilization [1]. However, this also causes problems concerning the intensive and mixed flow of people and goods, spatial form differentiation and superposition, the increased risk of disaster, and the complex issue of disaster reduction in underground spaces [1]. These problems not only pose a challenge to the safety in underground space environments, they also pose a potential threat to pedestrian safety. In recent years, the occurrence of some major accidents has highlighted the urgency of addressing this issue. For example, a fire broke out in the underground space of the Shanghai Hongqiao No. 1 Terminal Building in 2016, which caused casualties and property losses. Similar incidents continue to occur frequently, such as the floods in the Zhengzhou Metro Line 5 and the Beijing–Guangzhou Road tunnel in Henan Province in 2021, which resulted in 20 passengers failing to evacuate in time, and their tragic deaths. These accidents illustrate that the issue of safety and evacuation in underground spaces is extremely complex and urgent, thus requiring extensive research and discussion by scholars.
In order to address the issue of safety and evacuation in underground spaces, scholars have conducted research from multiple perspectives. These include summarizing the optimal smoke control mode in underground spaces to facilitate a safe evacuation [2,3,4] and simulating pedestrian evacuation behavior in underground spaces using various pedestrian dynamics models (such as the cellular automata model (CA), social force model (SFM), and agent-based model (ABM), etc.) [5,6,7]. Additionally, researchers have developed various improved evacuation simulation models and calculation methods based on the basic pedestrian dynamics model in order to study pedestrian evacuation; these include a risk assessment model [8] and a guidance evacuation model [9]. Other studies have focused on analyzing the influencing factors concerning exit/route choice in environments where smoke diffusion occurs [10], evaluating the effectiveness of underground space guidance devices [11], studying the factors that influence evacuation behavior [12], and proposing optimization measures for bottlenecks in underground spaces [13,14]. These achievements have not only significantly promoted the development of the field of underground space evacuation research, they have also helped researchers to explore the status of research and trends in this field. A thorough analysis of the pertinent research outcomes published in academic journals can provide a comprehensive understanding of the progress in the selected topics for this field, and also outline potential directions, offering valuable references and guidance for future research.
There are comprehensive analyses that focus on “safe evacuation” and “personnel evacuation”. For example, Yang and Tan [15] provide a comprehensive review of the historical development, and current status, of research by Chinese and foreign fire research scholars on the safe evacuations of personnel from buildings that had caught fire. Chen et al. [16] focused on “personnel evacuation”, using bibliometric methods to examine the current status and the development trends of research, both in China and abroad, through 5944 articles related to personnel evacuation in the WOS core database and the CNKI literature database. Their analysis shows that research on evacuations is still in the exploration stage, and there is significant room for expansion. Although the aforementioned reviews comprehensively analyzed certain results from research on evacuations, they did not conduct a review and analysis of the literature pertaining to underground spaces and their evacuation. Due to the particularity of the field of evacuation in underground spaces, its research faces some unique challenges and difficulties; therefore, it is necessary to focus on research specific to evacuations in underground spaces in order to gain a deeper understanding of its status and trends. Wang and Luo [17] used bibliometric methods to investigate “emergency evacuation in subway” in the China National Knowledge Infrastructure (CNKI) database, from 1990 to 2020, reviewing the existing achievements, the evolving trends and popular research topics, and providing targeted suggestions, based on the analyses. Xu et al. [18] reviewed articles on the safe evacuation of subway station personnel over the past 20 years. The study examined both the research methods and the research objectives, extracting five research themes: smoke characteristics during a subway fire evacuation, pedestrian emergency evacuation behavior, pedestrian psychological reactions, evacuation bottlenecks, and evacuation paths. Zhang and Huang [19] reviewed the latest progress and research on safe evacuations in tunnel fires in China, introducing the unique problems that China faces with regard to this issue, and discussing recent evacuation strategies and research. They emphasized the research progress and challenges in tunnel fire evacuation, as well as the needs and directions for future research. However, most of the above reviews were written from specific perspectives, lacking a systematic and quantitative method to explore the current research status and topic evolution trends in this field, and failing to conduct a systematic review and bibliometric analysis. Therefore, it is necessary to conduct in-depth research, and explore and establish a knowledge map in this field, in order to better grasp the research status and future development trends, and inject new vitality into the development of this field.
Consequently, the objectives of this paper are as follows: (1) to map the country, author, institution, keyword, and literature co-citation data using Citespace software and to analyze the issuance status of countries, authors, and institutions as well as their cooperation status; (2) to explore research topics and their evolution in this field; and (3) to analyze existing research development trends.
The present study comprehensively reviews the literature on the evacuation of underground spaces over the past 30 years, according to the following stages: Firstly, the process of literature collection is described, and the research method, using Citespace software to analyze data, is outlined. Secondly, the collected data were subjected to a visual analysis and the research hotspots, evolution patterns, and development trends of this field of research are identified. Subsequently, the research gaps identified in previous relevant review studies are addressed, providing important reference value for relevant scholars and departments. Finally, the overall review is summarized.

2. Data Source and Research Methodology

2.1. Data Collection for Visualization Analysis

This paper reviews the current state of research on evacuation in underground spaces by both Chinese and international scholars, using the literature data from the core collection database of the WOS. The literature databases for retrieval were SCI, SSCI, AHCI, CPCI-S, ESCI, CCR, and IC, and the data were updated until 31 December 2022. The topic for literature retrieval was [TS = ((subway AND evacuat*) OR (tunnel AND evacuat*) OR (underground stor* AND evacuat*) OR (underground small AND evacuat*) OR (underground parking AND evacuat*) OR (underground space AND evacuat*) OR (underground building AND evacuat*) OR (metro AND evacuat*) OR (light rail AND evacuat*) OR (underground construction AND evacuat*)) OR ((subway AND escap*) OR (tunnel AND escap*) OR (underground stor* AND escap*) OR (underground small AND escap*) OR (underground parking AND escap*) OR (underground space AND escap* ) OR (underground building AND escap*) OR (underground construction AND escap*) OR (metro AND escap*) OR (light rail AND escap*))]. A total of 2285 publications were retrieved, spanning from 1992 to 2022, with literature types limited to “article” and “review”, and the language of the literature being exclusively “English”. The individual stages of the entered data (keywords) are presented as shown in Table 1.
To ensure content accuracy and a precise focus on the theme of “underground space evacuation”, we screened the retrieved publications. Initially, we eliminated any repetitive articles and obtained a total of 2188. Subsequently, we meticulously read the title, abstract, and keywords of each article to identify those that were pertinent to underground space evacuation. Publications that did not align with the research content and theme were excluded, resulting in 420 remaining. Finally, in order to further focus on this topic, we scanned the full texts of 420 articles and eliminated any that had vague or unclear subject matter—ultimately obtaining 349 articles.

2.2. Research Methodologies

2.2.1. Visual Analytics

In this study, CiteSpace 6.1R3 software was used to conduct a visual analysis of the authors, countries, institutions, keywords, and co-cited literature in the field of underground space evacuation over the last three decades. This study aimed to explore the research topics and the evolution and research development trends by analyzing cooperative relationships among authors, countries, and institutions, as well as the keyword co-occurrence in this field [20].

2.2.2. Parameter Settings

The relevant literature retrieved from WOS was filtered and stored in the data folder in plain text format, and the file was named “download_XX”. The data were then imported into the software. In the software, the time (Time) was set to 1992–2022, and the time slice (Years Per Slice) was set to 1, which indicates that 1992–2022 was divided into 30 time segments, and the data analysis was conducted on the literature from each year in the 30 time segments. Finally, the node types (Node Type) were chosen as author, country, institution, keyword, and co-cited reference, and the software was executed by clicking “GO” to generate a visual graph.

3. Data Visualization and Analysis

3.1. Publication Statistics

The number of published articles and the citation frequency of research on underground space evacuation were statistically examined, and the variation trends of both indicators were derived, as shown in Figure 1. During the period of 1992–2022, the overall published number of relevant articles exhibited an upward trend. Between 1992 and 2005, the number of published articles was relatively low. However, the continuous increase in the number of underground buildings and the increasing integration of underground spaces into the more diverse demands of business, social, recreational, and travel utilization, etc., pose a potential risk to pedestrian safety in terms of the threat of fire, flood, and other disasters. As a result, ensuring safe evacuation from such environments has emerged as a significant challenge. From 2005 to 2016, there was a notable increase in the number of published articles, compared with the previous period. This trend can be attributed to the heightened awareness among scholars regarding the importance of addressing safety concerns related to evacuating people from underground spaces during emergencies or disasters. Consequently, an increasing number of scholars conducted research on various aspects related to underground space safety and evacuation. From 2017 to 2022, the number of published articles in this field surged, indicating that the underground space evacuation issue has garnered more attention from scholars, and also suggesting that research in this field has become a focal point in the academic community.
As shown in Figure 1, the citation frequency of the literature on underground space evacuation in the WOS database was 16.62, which was high, and the citation frequency followed an increasing trend from this point. The citation frequency increased significantly after 2011, indicating that this topic was gaining more attention from scholars both in China and internationally. The citing papers were 2790, the total citation frequency of all papers was 4954, and the h-index was 39. These metrics demonstrate that there is huge potential for exploration in future research on underground space evacuation with the development of underground spaces [21].

3.2. Author Collaboration Network

The author’s collaboration network (Figure 2) and the author publication statistics (Table 1) were obtained by importing the literature into Citespace software. The author collaboration relationship was analyzed to identify some high-yield authors who have made significant contributions to underground space evacuation.
Figure 2 depicts a network graph comprising 352 nodes and 502 links, showing the number of published articles by main authors and their collaboration relationships. A large scientific research collaboration community was formed around Ronchi E, Nilsson D, Frantzich H, and Fridolf K as the core, who not only had close and frequent cooperation with each other, but also collaborated strongly with other authors. Similarly, as a core member of another community, Wang Z collaborated extensively with multiple authors in the community. Notably, Kawabata N and Hasegawa M were also among those who formed robust collaboration bonds with others in this community. This indicates that while these communities formed by core authors displayed close cooperation within themselves, there was limited interaction between different communities.
Table 2 lists the top 10 scholars with the most prolific publication record, among whom Ronchi E and Nilsson D had the most articles published, each with a total of 13. It indicates that they have made great contributions to the field of underground space evacuation. Additionally, other scholars listed in Table 1 also played an indispensable role in advancing this area of research through their valuable publications, which significantly contributed to the development and progress of this field.

3.3. Country and Institution Collaboration Network

To achieve a more complete comprehension of the distribution of publications in this field, an analysis was conducted by integrating the country cooperation (Figure 3), the institution cooperation (Figure 4), and the country and institution publication statistics (Table 3). Figure 3 illustrates the cooperation relationship among countries. Clearly, China has the largest node, indicating that it holds significant research power in the research efforts within this field. Moreover, the periphery nodes surrounding China, Sweden, Scotland, and the United Kingdom are noticeably purple. This suggests that they possess higher betweenness centrality [22], and have played an important role in advancing research and development in underground space evacuation. As shown in Figure 4, a significant portion of institution cooperation occurs within the same country. For example, Beijing Jiaotong University and Chang’an University are both located in China, while Hiroshima University and Kanazawa University are situated in Japan. This suggests that cross-boundary cooperation among institutions is relatively weak, with most institutions tending to collaborate with others within their own country, possibly due to factors such as language barriers, cultural differences, and so on. Therefore, effective measures should be taken in the future to promote international cooperation and exchange among institutions, and to facilitate the sharing of knowledge and resources, in order to advance the development and progress in this field.
Table 3 summarizes the number of publications and betweenness centrality of countries and institutions. In terms of the number of published articles, Chinese researchers contributed the majority of the publications, and three of the top five research institutions are from China. Among them, Beijing Jiaotong University (17 papers) and Tongji University (13 papers) ranked second and third in publications, respectively. The first place was Lund University in Sweden, with 18 papers. In terms of betweenness centrality, the centrality of institutions in this field is generally lower. Lund University has a centrality of 0.2, Hong Kong Polytechnic University has a centrality of 0.1, and the centrality of other institutions is 0. In contrast, the centrality of countries in this field is relatively high. China (0.32), Sweden (0.21), and other countries have higher betweenness centrality.
Based on the above analysis, it can be seen that Sweden has a prominent position in the field of underground space evacuation, with a higher number of publications and centrality. Lund University, a Swedish institution, ranks first in the world in terms of the number of published articles, and its researchers Ronchi E and Nilsson D are the most prolific authors in this field. Furthermore, Lund University has established collaborations with other research institutions, both within and outside Sweden.

3.4. Document Co-Citation Network

When two articles are simultaneously referenced by one or more later articles, they form a co-cited relationship, which can be used to explore the development and evolution dynamics of the field [23]. Firstly, the co-cited literature was analyzed, and the top five articles with the highest co-citation frequency are listed in Table 4. These articles mainly focused on two topics: pedestrian evacuation behavior under smoke diffusion in tunnels (including pedestrian speed and exit choice, etc.) and subway evacuation using simulation methods. Notably, all of these articles were published after 2010, suggesting that evacuation problems in underground space environments (such as tunnels and subways) have become significant trends for research directions in the field of underground space evacuation in the past decade.
Figure 5 shows the co-cited clustering, displaying two important metrics: Modularity Q (Q value) and Mean Silhouette (S value). These two metrics can serve as the basis for evaluating the effectiveness of the network visualization. The Q value represents the degree to which a complex network can be divided into independent blocks and recombined, and is used to assess the significance of a clustering structure. The S value is a measure of network homogeneity and is used to evaluate the credibility of clustering results. Generally, when Q > 0.3, the clustering structure is considered significant. When S > 0.5, the clustering result is considered reasonable, and when S value > 0.7, the clustering result is very convincing. Given that the Q value in Figure 5 is equal to 0.8539 and the S value is equal to 0.9367, the clustering structure is deemed significant and highly convincing [24]. The two largest clusters, #0 fire and #1 building information modeling (BIM), were analyzed. Among them, #0 fire is the largest cluster, with 53 members. Based on the literature review, it was found that there has been a growing incidence of underground space fires in recent years. This has prompted scholars to focus on fire evacuation strategies for underground spaces. Their main concerns are the evacuation behavior of pedestrians after a fire, the design of wayfinding devices, and the control of smoke diffusion, which can facilitate successful evacuation and reduce casualties. The second largest cluster is #1 building information modeling (BIM), with 50 members. The focus of this cluster is modeling and simulation. Scholars argued that using computers for modeling and simulation is easier than conducting large-scale evacuation research on site. Therefore, they proposed or improved some simulation models to emulate the evacuation process and provide effective evacuation strategies for underground spaces.

4. Analysis of Research Topics and Evolution

4.1. Keywords Analysis

The keywords of an article can help us quickly understand the core content and theme, and are an important component of expressing thematic concepts in scientific papers [25]. We employed Citespace6.1R3 software to conduct a keyword analysis, and trimmed the network graph by selecting the three functions of “Pathfinder”, “Pruning sliced networks”, and “Pruning the merged networks” for the clarity network diagram. Other settings were the same as previously stated. The resulting co-occurrence map of keywords is shown in Figure 6.
As shown in Figure 6, the network consists of 341 nodes and 762 links, with a network density of 0.0131. The node size indicates the frequency of keywords, and the font size reflects the betweenness centrality. Each keyword node is formed by rings, and the color of each ring and the link matches the year color of the keywords at the top. Among all the words displayed in the figure, the keyword nodes such as “simulation”, “model”, “behavior”, “tunnel fire”, and “evacuation” are relatively large and appear with high frequency; the keywords such as “flow”, “behavior”, “model”, “emergency exit”, and “fire” exhibit higher betweenness centrality. Table 4 presents the frequency and centrality of the top fifteen keywords.

4.2. Research Topics

Synthesizing Figure 6 and Table 5, the research topics in this field mainly focused on developing models for simulating evacuation scenarios, smoke control methods research, research on factors influencing pedestrian evacuation behavior, research on pedestrian walking speed in a smoke-filled environment, and research on the impact of different wayfinding devices on exit selection and wayfinding.

4.2.1. Developing Models for Simulating Evacuation Scenarios

Simulation models aim to represent the real evacuation scenarios in underground spaces, including the process of pedestrian evacuation. Currently, scholars have established simulation models including the CA model, SMF, and ABM. Due to the different scenarios in underground spaces and the complexity of pedestrian evacuation behavior, researchers have proposed various new models based on the above three models to adapt to different evacuation scenarios in underground spaces. For example, Zheng et al. [26] developed an improved floor field CA model to simulate pedestrian evacuations under flooding conditions in underground spaces. Shi et al. [27] used ABM to simulate pedestrian evacuations under different fire scenarios and proposed safety strategies for effective evacuations. Evacuation, as a dynamic crowd behavior, must consider the influence of the environment and the movement of a large number of pedestrians. SMF, compared with the other two models, can more vividly simulate the complex pedestrian movement in underground spaces [28], and therefore has been widely used. Wan et al. [29] proposed a comprehensive SFM that effectively simulated the entire evacuation process of pedestrians in subway stations, both prior to and following exposure to toxic gas attacks, reproducing pedestrian behaviors such as competition, grouping, and flocking. Furthermore, Wang et al. [28] extended the initial SFM by considering the interaction between companions and explored the impact of various factors on evacuation efficacy, investigating how pedestrian density, panic level, and friend group size affect evacuation efficiency in order to provide insights for improving safety measures in underground spaces.
Due to the complexity of underground spaces, the difficulty of pedestrian evacuation increases in the event of an emergency. Therefore, conducting pedestrian evacuation risk assessment is crucial for developing reasonable emergency paths, reducing casualties, and minimizing property losses. Therefore, many scholars have developed new evacuation risk assessment models that build upon existing frameworks to assess pedestrian evacuation risk in different scenarios. Wang et al. [30] proposed a multi-exit fire-source-location choice model that can clearly show the distribution of evacuation risk in each area of the building, combined it with the ABM method to simulate the pedestrian evacuation process when a fire occurs in an underground mall, and explored the effects of population size, pre-movement time, and safe exit width on pedestrian evacuation risk. Similarly, Cheng et al. [8] developed a pedestrian evacuation risk quantification assessment model that takes into account the probability of a pedestrian stampede and pedestrian casualties, and based on SFM simulated several different scenarios to assess the pedestrian evacuation risk of the “Olympic Park Station”.
The evacuation of pedestrians can be challenging without proper guidance in an emergency [9]. To address this issue, some scholars have established evacuation guidance models based on basic models to assist pedestrians in quickly escaping dangerous environments. In order to improve the evacuation efficiency in underground slender infrastructures (such as pedestrian tunnels and underground shopping malls), Han et al. [31] established a digital twin simulation system based on the CA model, which can guide pedestrians toward the exit to evacuate in a shorter time. Yang et al. [32] proposed a pedestrian dynamics correction method based on SFM and applied it to the modeling of guided pedestrian groups. The research proved that in large venues, such as subway stations, guidance can effectively enhance the speed of pedestrian evacuation. Moreover, Long et al. [9] considered various factors influencing pedestrian characteristics, established a guidance evacuation model combining SFM with guidance factors, and studied the impact of evacuation guidance on pedestrian behavior.
Using model simulation to study the evacuation of underground spaces has become an increasingly popular method among researchers, as it not only avoids the inconvenience of recruiting pedestrians for on-site evacuation studies, but also saves costs. Additionally, simulation models can aid in identifying potential bottlenecks or obstacles that may impede the evacuation process, enabling pedestrians to take proactive measures beforehand, and thereby minimizing risks. Consequently, establishing simulation models for studying evacuations has become an increasingly popular research hotspot among the academic community.

4.2.2. Smoke Control Methods Research

Underground spaces are prone to fire and smoke due to their high degree of enclosure. The smoke contains toxic gases that could threaten the lives of evacuating pedestrians. Moreover, the smoke spread also may reduce the visibility of the escape route, resulting in a slower evacuation speed of pedestrians and casualties. In addition, the direction of smoke flow after a fire is usually consistent with the direction of personnel evacuation, which makes it difficult for pedestrians to evacuate. Therefore, the study of smoke control methods in underground spaces, to create a smoke-free corridor and promote safe personnel evacuation, has attracted the attention of scholars.
In the context of fires in underground spaces, early research efforts focused on estimating the critical ventilation velocity required to prevent smoke from spreading from various fire locations, and designed forced ventilation systems to control the smoke flow during fires in underground spaces, creating a safe evacuation route for pedestrians [33,34]. A ventilation system is an effective way to suppress smoke spread in underground spaces. Therefore, some researchers have conducted several related studies on smoke control in underground spaces with ventilation systems and established computer simulation models, which aimed at optimizing smoke control strategies through the study of ventilation systems [35]. Meng et al. [2] utilized the Fire Dynamics Simulator (FDS) to investigate different ventilation modes in subway stations with fully enclosed platform screen doors (PSD) and half-height safety doors, aiming to identify the optimal mode for smoke control. Liu et al. [35] studied the ventilation modes of ventilation fans, jet fans, ceiling ducts, and their coordination in subway transfer tunnels when a train catches fire, by using numerical simulation and full-scale experimental methods. The results demonstrated that proper coordination of these modes can effectively suppress smoke spread. Xu et al. [36] focused on ultra-long railway tunnels with rescue stations and the numerically simulated spread of smoke and scenarios for its control. They found that sufficient longitudinal airflow could limit the thickness of the smoke layer, while reducing air temperature and wind pressure in cross passages, therefore facilitating emergency evacuation. Additionally, researchers have also investigated the smoke exhaust efficiency of single-point and multi-point smoke exhaust ports in the upper tunnel, as well as the smoke control effect of point smoke exhaust systems in long-distance and large cross-section shield tunnels using numerical simulation methods [37,38].
Overall, in cases of fires in underground spaces, effective smoke control measures are crucial for ensuring the safety of evacuating personnel and creating a safe evacuation environment. Therefore, the further exploration of smoke control methods will continue to be a research hotspot.

4.2.3. Integrative Research on Factors Affecting Pedestrian Evacuation Behavior

When an emergency occurs in an underground space, people’s perceptions, information processing, decision making, and behaviors are all subject to multiple factors. Moreover, information sources may contain misleading or conflicting information that makes it difficult for pedestrians to take appropriate actions [10]. In this uncertain situation, the success or failure of pedestrian evacuation largely depends on the pedestrian evacuation behavior, which is influenced by various factors.
Social influence is considered to be an important factor affecting pedestrian evacuation behavior. This was demonstrated by an experimental fire evacuation study that measured car drivers’ evacuation behaviors in a tunnel in Gothenburg, Sweden [39]. The study showed that social influence is crucial when car drivers evacuate in situations with limited or ambiguous information [39]. Another experimental study conducted in a virtual reality tunnel fire also showed that participants were influenced by virtual agents, and followed the agents to perform similar evacuation behaviors [40]. In addition, gender, age, occupation, education level, and safety knowledge have certain effects on crowd evacuation behavior [40,41]. When facing dangerous situations, emotions such as fear and anxiety may affect pedestrians’ evacuation behaviors, thus there is a certain correlation between pedestrian psychology and evacuation behavior [41]. In recent years, some scholars have also found that pedestrian spatial knowledge completeness has a significant impact on five performance indicators of pedestrian evacuation behavior (route/direction selection, time, speed, and distance) [12].
Therefore, integrating these factors and conducting research is essential to improve pedestrian evacuation behavior and enhance safety in underground spaces. In order to develop effective evacuation strategies, it is necessary to understand the interactions between these factors and design more efficient evacuation plans that consider various influencing factors, minimizing potential risks in underground spaces. Thus, the further promotion of research on this topic remains a research hotspot.

4.2.4. Research on Pedestrian Walking Speed in a Smoke-Filled Environment

The presence of smoke in underground spaces has a serious impact on pedestrian walking speed, and this has been examined mainly in tunnels. Fridolf et al. [42] conducted an evacuation experiment in a tunnel filled with artificial smoke and acetic acid, with an average extinction coefficient of 2.2 m−1, to investigate the pedestrian walking speed, while also examining the potential influence of the ground material and the inclination of the tunnel on the pedestrian walking speed. The results indicated that the average pedestrian walking speed was 0.9 m/s, which remained consistent regardless of variations in tunnel ground material or inclination. Furthermore, their additional study also confirmed this finding, and reported that the visibility of smoke, that is, the extinction coefficient, had a negative impact on pedestrian walking speed [43]. Ronchi et al. [44] measured the walking speed of pedestrians in the smoke-filled and smoke-free parts of a road tunnel in Stockholm, with the walking speed in the smoke-filled part being 1.2 m/s, and the walking speed in the smoke-free part being 1.6 m/s. The results revealed that individuals walked significantly slower when exposed to smoky conditions compared to non-smoky conditions. Seike et al. [45,46,47] investigated the walking speed in a full-size tunnel filled with smoke and elucidated the relationship between the extinction coefficient and walking speed, which showed a negative correlation. Specifically, as the extinction coefficient increased, the walking speed decreased accordingly.
Investigating pedestrian walking speeds in smoke-filled environments not only provides fundamental data for evacuation simulation models, but also assists the managers of underground spaces to design more effective emergency evacuation plans and evaluate/improve fire safety measures. Therefore, it is a research hotspot to examine further.

4.2.5. Research on the Impact of Wayfinding Devices on Pedestrian Exit Selection

The diffusion of smoke generated by a fire outbreak in an underground space can significantly impair the visibility for pedestrians, thereby making it difficult for them to make correct route and exit choices. Consequently, it is imperative that effective wayfinding devices be designed and implemented in such scenarios to facilitate safe egress for evacuees.
Researchers have conducted extensive studies on the effectiveness of different wayfinding devices to guide people during evacuations. Among these devices, flashing lights are thought to be an effective tool for guiding pedestrians to choose safe exits and correct routes. Nilsson’s [48] study has shown that flashing lights in road tunnels can significantly reduce uncertainty in exit selection and guide pedestrians to choose appropriate routes. According to relevant studies, green is considered a suitable color for pedestrians to recognize flashing lights [39]. A virtual reality experiment on the design of emergency exit flashing lights in tunnels found that green flashing lights were more effective in pedestrian wayfinding compared to lights of other colors [49,50]. For smoke-filled railway tunnels, continuous or alternating green flashing lights were found to have a positive impact on wayfinding [51]. In terms of frequency selection, frequencies of 1 Hz and 4 Hz are more effective than a frequency of 0.25 Hz [49]. Furthermore, some scholars have suggested that installing loudspeakers at emergency exits can be more effective in attracting pedestrians to use these exits during an evacuation. These researchers have also investigated what kind of voice information is most likely to encourage pedestrians to choose safe exits [42,44]. In addition, other studies have demonstrated the importance of phosphorescent guidance signs for pedestrian wayfinding during an evacuation. For example, one evacuation experiment was conducted in a smoke-filled subway station where participants were able to find their way out following these signs [52].
To summarize, there has been growing interest in investigating the effectiveness of various wayfinding devices for enhancing exit selection and improving the overall safety level within underground spaces. By using wayfinding devices, pedestrian evacuation can be promoted more efficiently, while reducing potential hazards associated with low-visibility conditions. Therefore, the exploration of the impact of different wayfinding devices on exit selection/wayfinding is a major research hotspot.

4.3. Research on Evolution and Development

Figure 7 lists the 15 words with high burst intensity in the keyword analysis of this study. By analyzing the burstiness of keywords in a certain time period, the evolution and development of underground space evacuation can be further explored [53]. From Figure 7, it can be seen that the dynamic development process of research on underground space evacuation from 1992 to 2022 can be divided into three stages.
The first stage Is from 2005 to 2012; “computer simulation”, “smoke flow”, and “life safety” are the keywords with high burst intensity and the research foci of this stage. Since the most common disasters in underground spaces are fire and smoke, the researchers have mainly focused on how to reduce smoke in order to minimize casualties and improve evacuation efficiency. To achieve these objectives, researchers have utilized computer simulation technology to simulate smoke flow and explore its characteristics, in order to design effective smoke control systems for facilitating evacuation and protecting pedestrian safety.
The second stage is from 2013 to 2016; “road tunnel”, “evacuation experience”, “exit choice”, and” virtual reality” are the research foci of this stage. Among them, “road tunnel” has the highest burst intensity, which is 4.43, indicating that the research in this stage mainly revolves around “road tunnel”. After summarizing and organizing the literature in this stage, it was found that researchers have mainly focused on pedestrian walking speed and exit/route selection issues in smoke-filled road tunnels. Through evacuation experiments in real environments or virtual reality evacuation experiments, pedestrian evacuation behavior is studied to propose effective measures to ensure the safe evacuation of pedestrians.
The third stage is from 2017 to 2022; “numerical simulation”, “cfd simulation”, and “crowd evacuation” are the research foci of this stage. In the context of emergency situations in underground spaces, it is often challenging to conduct real experiments that accurately capture the complex dynamics of crowd evacuation processes. Moreover, ethical considerations may preclude researchers from conducting experiments. Consequently, numerical simulation methods have emerged as a viable alternative for studying and analyzing these scenarios, that is, using evacuation software or evacuation models to analyze the crowd evacuation process in underground spaces. These simulations enable researchers to study various aspects of crowd evacuation behavior, in order to propose effective evacuation strategies and maximize the evacuation crowd. In addition, the number of studies using numerical simulation methods, including the CA model, SFM, etc., to study pedestrian evacuation problems in underground space is increasing.

5. Research Trends

As discussed above, the existing research provides a theoretical basis for safe evacuation practices in underground spaces, and the research trends in this field are focused on refining the basic data, elaborating the simulation model parameters to accurately reflect real-world scenarios, evaluating the evacuation ability of certain people, studying the impact of risk perception on evacuation behavior, and conducting specific risk-targeted research on underground spaces.

5.1. Refining Basic Data and Elaborating Simulation Model Parameters

Inadequate basic data may result in discrepancies between the simulation of pedestrian evacuation processes by models and actual situations, leading to the ineffective application of corresponding evacuation plans [54]. In order to establish an accurate simulation model, it is necessary to collect basic data, such as pedestrian reaction time, walking speed, and psychological characteristics, when facing real risks in underground spaces for parameter setting [27]. Detailed and realistic parameters facilitate the simulation of scenarios that more closely resemble real situations, thereby enabling researchers to develop more effective evacuation strategies based on simulated conditions. Researchers have taken further measures to collect the psychological characteristics and evacuation behavior data of pedestrians facing emergency situations by video investigation, fire evacuation drills, and other methods [55]. However, these methods still have limitations. For example, video investigations may not fully capture all pedestrians’ psychological characteristics due to their observational nature. Similarly, fire evacuation drills are often limited by their small sample size and may not accurately reflect how people would behave in larger-scale disasters, which can result in insufficient or incomplete data collection, and so on. Therefore, in order to make the parameters of the simulation model more detailed and generate evacuation strategies that are more effective, it is a future research trend to conduct further studies on how to improve the basic data of pedestrian behavior in real risk situations.

5.2. Evacuation Capacity Assessment for Special Populations

The pedestrians in underground space activities include not only physically healthy young and middle-aged people, but also some special groups such as the elderly and the disabled. Due to the inconvenience of movement, special populations may face challenges in evacuating during emergency situations, which can result in the decreased evacuation efficiency of the whole group, and potential casualties [56]. Therefore, assessing the evacuation capacity of special groups is an important goal to promote the safe evacuation of mixed-capacity groups. As such, this topic has garnered wide attention from researchers, who have conducted experiments to evaluate the evacuation capacity of special groups and identify factors that affect their ability to evacuate, proposing countermeasures to improve their evacuation capacity [57]. However, despite existing research on the evacuation capacity of certain special groups, most of the completed studies have problems, such as the small number of participants and the uneven distribution of gender ratio among the tested special groups. As a result, assessments of the evacuation capabilities of these special populations may not be comprehensive. Therefore, more studies will further evaluate the evacuation capacity and influencing factors of various types of special groups from the perspective of evacuation safety in the later stage.

5.3. The Influence of Risk Perception on Evacuation Behavior

Researchers have extensively investigated various factors that affect pedestrian behavior during emergency evacuations, in order to improve pedestrian evacuation efficiency. However, current research has not adequately explored pedestrian risk perception when encountering hazards and how this perception affects the evacuation behavior. Risk perception is the subjective judgment that each individual makes based on intuitive judgment, subjective feeling, life experience, etc., when faced with potential risks, and behavioral decisions are based on these. In underground spaces, pedestrian risk perception plays a crucial role in determining judgments and subsequent behaviors during emergencies. Pedestrians may perform different evacuation behaviors depending on their perceived level of risk. Factors that affect pedestrian risk perception include common sense and experience-based judgments, psychological states under environmental stimuli, differences in information filtering abilities among individuals, and so on. As each pedestrian makes further evaluations based on the individually perceived level of risk during an emergency evacuation scenario, it is likely that different evacuation behaviors will be exhibited by different pedestrians. A study has shown that an increase in pedestrian risk perception will prompt pedestrian evacuation, which will lead to a shorter evacuation delay and improve evacuation efficiency [30]. Additionally, another study found that in some cases, the difference in pedestrian risk perception can impact behavior during evacuations [58]. It is evident that more studies will consider adding pedestrian risk perception to the list of factors affecting pedestrian evacuation behavior in the future, in order to reduce evacuation delay and improve evacuation efficiency.

5.4. Targeted Study of Specific Types of Risks in Underground Spaces

The risks associated with underground spaces are complex and varied. Some scholars have used simulation models, controlled experiments, virtual experiments, and other methods to study the safety evacuation issues in the event of risk occurrence, and proposed some targeted evacuation suggestions. However, these evacuation recommendations are mainly for fire incidents in subways and tunnels, with limited research on safe evacuation measures for other types of underground spaces such as underground shopping malls and parking lots. Additionally, there is also inadequate research on evacuation strategies for other risks that may occur in underground spaces, such as terrorist attacks or floods, and so on. The behavior and psychological responses of pedestrians facing different risks during evacuation in underground spaces also have some uncertainty, which means that the evacuation strategy proposed by researchers for one type of risk may not be universally applicable across all risks. Therefore, more researchers may conduct comprehensive research based on specific risk situations in the future to provide more detailed analysis and recommendations for safety evacuation, to improve the safety in underground spaces, and ensure the protection of pedestrian lives.

6. Discussion

Despite the significant achievements in the field of underground space evacuation research over the past few decades, there is a relative scarcity of review articles. The reason for this situation may be that underground space evacuation involves multiple aspects, such as building structure, personnel evacuation, safety assessment, etc., and the research content is relatively complex. Therefore, preparing a comprehensive review article on this topic requires a significant amount of time and effort. Moreover, the uniqueness and complexity of underground space evacuation scenarios lead to a dispersion of research findings, necessitating the integration and synthesis of research outcomes across multi-disciplinary domains to produce in-depth and comprehensive review articles. Although some scholars have already conducted reviews in this field, their research perspectives and scope remain limited. Therefore, when composing review articles on underground space evacuation, it is essential to consider various factors and research findings from different domains to accurately reflect the latest advancements and future research directions in the relevant fields.
In analyzing the collaboration status of scholars publishing articles in this field, some scholars have pointed out that the cooperation intensity among authors in this field and between institutions is insufficient, and the level of collaboration in the research field needs to be improved [17]. Another study found similar results, noting that domestic and international exchanges and cooperation are infrequent, and a research main force and core author group have not yet been formed [16]. These results corroborate the analysis of collaboration relationships among Chinese and international scholars, institutions, and countries in this study, indicating that the cooperation situation in this field remains relatively weak and requires improvement and promotion from various aspects. Specifically, efforts should be made to enhance collaboration and communication among research institutions, emphasize international cooperation and exchanges, and establish efficient academic exchange platforms to promote the development of research on the evacuation of underground spaces and achieve a win–win situation for academic progress and social benefits.
The identification of the main research themes in a field is a crucial component of a review. In this regard, this study points out that the establishment of simulation models, smoke control methods, and factors affecting evacuation are the research themes in this field. This is similar to some of the research themes identified by previous researchers [17,18]. However, the current review overlooks the theme of accident analysis and countermeasures. It is worth noting that the aforementioned reviews on accident analysis and countermeasures are mostly limited to the subway aspect. This study expands the research scope by extending the focus to the entire underground space, thus the omission of this topic has a weak impact on the integrity of this study. Nevertheless, in-depth research on accident analysis and countermeasures remains highly significant, as it can help us to gain a more comprehensive understanding of accident occurrence and causes, provide scientific evidence for effectively preventing and responding to accidents, and promote the sustainable development of research on the evacuation of underground spaces.

7. Conclusions

This paper analyzed a number of publications, authors, countries, institutions, co-cited references, keywords, and other aspects in the field of research on the evacuation of underground spaces using Citespace software, revealing the research topics, evolution, and trends within this field.
From the perspective of the number of publications, in recent years, there has been a notable increase in the number of articles published on the safe evacuation of underground spaces. This trend suggests that the theme is becoming a topic of lively discussion among researchers. The network graph of authors, countries, and institutions shows that Swedish authors have outstanding performance, ranking higher in the number of published articles and being frequently cited. They have made significant contributions to the rapid development of this field. From the perspective of cooperative relationships, there remains room for improvement regarding collaboration among authors, institutions, and countries. The network diagram reveals that the overall levels of cooperation are relatively dispersed within this field and there is weak transnational communication, which could potentially impact further advancements in this field. Therefore, it is crucial for researchers to engage with one another across borders and disciplines to foster more collaboration towards advancing the development of this field.
The analysis of keywords identified that the research in this field is focused on developing models for simulating evacuation scenarios, smoke control methods, integrative research on factors affecting pedestrian evacuation behavior, the walking speed of pedestrians in smoke-filled environments, and the influence of different wayfinding devices on exit choice/wayfinding. Additionally, according to the burstiness of keywords, the evolution of this field can be classified into three stages. The first stage focuses on exploring methods to suppress smoke diffusion. The second stage involves conducting real or virtual reality experiments to study pedestrian evacuation behavior in tunnel settings. Finally, the third stage utilizes numerical simulation methods to analyze pedestrian evacuation processes that may arise in underground spaces, in order to develop more reliable, effective, and targeted evacuation plans. This can also promote the further improvement and development of safety management systems related to urban underground spaces.
Future research trends in this field are oriented towards improving the fundamental data pertaining to pedestrian behavior during emergency situations, and elaborating simulation model parameters with a view to devising effective evacuation strategies. Furthermore, future studies should encompass an assessment of the evacuation capabilities of special populations, as well as an exploration of the factors that influence evacuation behavior from diverse perspectives, with the aim of improving pedestrian evacuation efficiency. Additionally, there are many different types of risks that may occur in urban underground spaces, yet research on these risks is currently limited to only a few types. Consequently, future studies should focus on comprehensively and systematically exploring the different types of risks that may arise in underground spaces, in order to develop more reliable, effective, and targeted evacuation plans. This could also promote the further improvement and development of safety management systems related to urban underground spaces.

Author Contributions

Conceptualization, C.C.; data curation, Q.S. and X.H.; funding acquisition, C.C. and Y.L.; investigation, F.L. and X.H.; methodology, Q.S.; project administration, C.C. and G.H.; software, Y.L.; supervision, G.H. and Y.L.; writing—original draft, Q.S.; writing—review and editing, C.C. and F.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC) (Grant No. 72001079, 72072165) and the Fundamental Research Funds for the Central Universities (3142021010).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The annual distribution of publications and citation frequency of underground space evacuation research from 1992 to 2022.
Figure 1. The annual distribution of publications and citation frequency of underground space evacuation research from 1992 to 2022.
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Figure 2. The cooperative network of authors of papers related to underground space evacuation.
Figure 2. The cooperative network of authors of papers related to underground space evacuation.
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Figure 3. The national cooperation network of papers related to underground space evacuation.
Figure 3. The national cooperation network of papers related to underground space evacuation.
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Figure 4. The institutional cooperation network of papers related to underground space evacuation.
Figure 4. The institutional cooperation network of papers related to underground space evacuation.
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Figure 5. A cluster view of cited papers related to underground space evacuation.
Figure 5. A cluster view of cited papers related to underground space evacuation.
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Figure 6. The co-occurrence of keywords in papers related to underground space evacuation.
Figure 6. The co-occurrence of keywords in papers related to underground space evacuation.
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Figure 7. The emergence of keywords in papers related to underground space evacuation.
Figure 7. The emergence of keywords in papers related to underground space evacuation.
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Table
Table 1. The statistical results of the quantity of literature collected for each stage of subject terms.
Table 1. The statistical results of the quantity of literature collected for each stage of subject terms.
TSNumbers of
Documents Collected		TSNumbers of
Documents Collected
subway AND evacuat*214subway AND escap*38
tunnel AND evacuat*505tunnel AND escap*954
underground
stor* AND evacuat*		21underground
stor* AND escap*		56
underground
small AND evacuat*		14underground
small AND escap*		49
underground
parking AND evacuat*		8underground
parking AND escap*		15
underground
space AND evacuat*		62underground
space AND escap*		37
underground
building AND evacuat*		58underground
building AND escap*		32
metro AND evacuat*152metro AND escap*20
light rail AND evacuat*10light rail AND escap*2
underground construction AND evacuat*22underground construction AND escap*16
* represents any character group, including empty characters.
Table
Table 2. The top 10 authors of papers related to underground space evacuation.
Table 2. The top 10 authors of papers related to underground space evacuation.
AuthorYear of First PublicationAmounts of Papers
Ronchi E201213
Nilsson D200913
Wang Z20179
Zhang Y20188
Frantzich H20098
Kawabata N20158
Fridolf K20137
Chow W20097
Chen Y20147
Hasegawa M20167
Table
Table 3. The top five countries and institutions for papers related to underground space evacuation.
Table 3. The top five countries and institutions for papers related to underground space evacuation.
RankCountryAmounts of PapersCentralityRankResearch InstituteAmounts of PapersCentrality
1China1610.321Lund Univ180.2
2Sweden240.212Beijing Jiaotong Univ170.0
3U.S210.033Tongji Univ130.0
4Japan190.004Hong Kong Polytech Univ90.1
5Italy190.085Kanazawa Univ70.0
Table
Table 4. Information on cited papers related to underground space evacuation (first five).
Table 4. Information on cited papers related to underground space evacuation (first five).
AuthorCited FrequencyCentralityYear of PublicationCo-Cited Documents
Fridolf K170.132013Movement speed and exit choice in smoke-filled rail tunnels.
Seike M170.072016Experiment of evacuation speed in smoke-filled tunnel.
Ronchi E170.022018A tunnel evacuation experiment on movement speed and exit choice in smoke.
Fridolf K160.162013Fire evacuation in underground transportation systems: a review of accidents and research.
Zhang LM150.112016Simulation-based route planning for pedestrian evacuation in metro station: a case study.
Table
Table 5. The top 15 keywords and their frequency and centrality of papers related to underground space evacuation.
Table 5. The top 15 keywords and their frequency and centrality of papers related to underground space evacuation.
RankKeywordsFrequencyRankKeywordsCentrality
1simulation471flow0.41
2model442behavior0.35
3behavior413model0.33
4tunnel fire314critical velocity0.24
5road tunnel285emergency exit0.18
6flow276dynamics0.15
7emergency evacuation267fire0.15
8evacuation248road tunnel0.15
9subway station239cfd simulation0.14
10fire2210design0.14
11movement2111escape0.12
12dynamics1912smoke flow0.12
13system1913human behavior0.12
14smoke1914evacuation0.12
15social force model1615choice0.11
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Cui, C.; Shao, Q.; Liu, Y.; Han, G.; Liu, F.; Han, X. A Review of the Evolution and Trends in Research on the Emergency Evacuation of Urban Underground Spaces. Buildings 2023, 13, 1325. https://doi.org/10.3390/buildings13051325

AMA Style

Cui C, Shao Q, Liu Y, Han G, Liu F, Han X. A Review of the Evolution and Trends in Research on the Emergency Evacuation of Urban Underground Spaces. Buildings. 2023; 13(5):1325. https://doi.org/10.3390/buildings13051325

Chicago/Turabian Style

Cui, Caiyun, Qianwen Shao, Yong Liu, Guobo Han, Feng Liu, and Xiaowei Han. 2023. "A Review of the Evolution and Trends in Research on the Emergency Evacuation of Urban Underground Spaces" Buildings 13, no. 5: 1325. https://doi.org/10.3390/buildings13051325

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