## 2 Theoretical Framework: Definitions and Classifications
### Characteristics of Global Crises  
Before reviewing the research, it is important to establish a theoretical framework that outlines the characteristics of undesirable phenomena, such as global-scale crises. In the literature, the term 'crisis' is defined in various ways. [PERSON] [50] provided a basic definition, describing a crisis as an undesirable event resulting from natural, technological, or man-made causes. A crisis is also characterized by widespread negative consequences that can affect entire communities or regions [3; 4]. [PERSON] et al. [51] further explained  
Figure 1: Adopted classification of modern transport solutions.  
that a crisis occurs when fundamental values, such as safety, security, health, or wealth, are threatened.  
Given these definitions, a global crisis can be understood as a situation where problems or threats have an international reach, impacting multiple countries and societies at once. Such a crisis has the potential to destabilize the economy, public health, politics, the environment, or other critical areas on a global scale. It is also emphasized that the recovery period following a crisis can be lengthy and may require coordinated efforts and international cooperation [52, 53]. Global crises can be classified in different ways, depending on their causes and the areas they affect. For the purposes of this study, the classification was based on [PERSON] et al. [54], [PERSON] [55], and [PERSON] [56], with additional consideration given to military crises, reflecting the geopolitical tensions witnessed in recent years across many regions [57, 58] (Table 1).  
Additionally, in order to deepen the characterization of the types of crises considered, a comparative analysis was conducted, which identified the symptoms that distinguish a specific crisis from others and showed the possible disruptions that these events may cause in the transport sector (Table 2).  
It is difficult to classify crises in a clear and objective manner. Each crisis may have unique characteristics and varying levels of impact. Additionally, crises often have multiple interconnected causes and effects. For instance, the health crisis triggered by the COVID-19 pandemic had severe global economic consequences [83], while the 2008 financial crisis led to significant social challenges [84]. Military threats can also contribute to crises, as demonstrated by the influx of immigrants to Europe [7]. As shown in Table 1, the early 21 st century has been marked by a variety of global crises with diverse origins. This suggests that a range of threats will likely continue to arise in the future, highlighting the need to explore sustainable transport solutions that could prove effective in mitigating potential global crises.  
\begin{table}
\begin{tabular}{c c c} \hline
**Type of Crisis** & **Description** & **Example** \\ \hline Economic crisis & International economic problems that impact the stability of markets, nations, and societies. & \(\bullet\) (2008–2009) [6, 59] \\ \hline Environmental (ecological) crisis & Threats to ecosystems and human health caused by environmental pollution, climate change, and overexploitation of natural resources. & \(\bullet\) (2008–2009) [6, 59] \\ \hline Health crisis & Public health threats from widespread diseases, limited access to healthcare, or events, such as pandemics, epidemics, and natural disasters. & \(\bullet\) (2020) [62, 63] \\ \hline Social crisis & Disruptions in societal functions, such as mass protests, social tensions, growing inequality, or challenges to social integration, leading to destabilization and conflict. & \(\bullet\) (2015) [7, 64] \\ \hline Military crisis & Armed conflicts or the threat of military force destabilizing regions and having global consequences. & \(\bullet\) (2022) [9] \\ \hline \end{tabular}
\end{table}
Table 1: Adopted classification of global-scale crises.