## 1 Introduction  
A commercial motor vehicle (CMV) is any vehicle used to transport goods or passengers for an individual or business [1]. The Korean Standard Classification of Occupations (KSCO) [2] classifies CMV drivers into taxi, bus, truck, and other drivers. Truck drivers are the basis of logistics and transportation in the country, while taxi and bus drivers play an essential role in providing public mobility [3; 4; 5; 6]. To drive a certain CMV, drivers require a commercial driver's license and must pass both skills and knowledge tests [1]. CMV drivers are also required to comply with traffic regulations to ensure the safety of passengers or customers' goods [1; 5]. The work environments of CMV drivers are associated with noise and vibrations, as well as sitting for a long time [5; 6; 7; 8]. CMV drivers are also exposed to intense sunlight and glare during the day, and poor lighting, reflections, and neon lights at night [3; 4; 5]. On the other hand, bus and taxi drivers may be exposed to wind, heat, cold, noise, odor, and moisture as they open doors [3; 4]. Therefore, the work situations of CMV drivers are known to be poor [7; 9; 10]. CMV drivers' schedules may vary, and some work weekends, evenings, or early mornings [1; 5; 6]. Drivers have tiring and stressful work traits, such as prolonged concentration and vigilance, long work hours, tight schedules, shift work, and insufficient rest time [11; 12; 13]. Thus, CMV drivers may spend less time with family members, leading to conflict between work and family [14]. CMV drivers play a critical role in the uninterrupted flow of goods and passengers throughout the country [1]. However, in the USA and Canada, the truck industry is under serious threat due to the aging of drivers [3; 15]. In Korea, the percentage of older CMV drivers (\(\geq\)65 years old) has risen sharply from 5.9% in 2010 to 19.3% in 2020. In particular, the proportion of taxi drivers aged \(\geq\)65 years among Korean taxi drivers was 36.0% in 2020 [16; 17]. In Korea, the phenomenon of aging drivers is common in the CMV industry [16]. Many studies explain the relationship between fatigue, sleep problems, and accidents [7; 9; 10; 13; 18; 19]. Fatigue in drivers is highly associated with sleep-related problems, causes drowsy driving, and can lead to accidents [9; 10; 13; 18; 19]. In order to prevent CMV drivers having accidents, it is important to investigate not only drivers' fatigue and sleep-related problems, but also the relationships between psychological factors. Depression is an essential indicator of the wellbeing of older people [20]. Depression is a mental state characterized by a pessimistic sense of inadequacy [20]. The loneliness of professional drivers may cause depression [20]. The WHO-5 index is an adequate measure in screening for mental wellbeing and depression [21; 22]. From 6.8% [23] to 70.3% [24] of truck drivers are reported to have depression. However, there is a lack of studies on depression among taxi and bus drivers. Thus, this study is concerned with the characteristics of depressive symptoms in taxi, bus, and truck drivers. Factors influencing the depression of older drivers are meaningful for preventing CMV drivers having traffic collisions. This study starts from the hypothesis that the driver's working situation and work-life conflict can affect depression and work engagement. Work situations are social relationships that workers interact with within their work or workplace [7; 9; 10]. In this study, poor work situations mean negative relationships experienced in their working environment and employment conditions [7]. Poor work situations are common among drivers and can intrude into workers' private lives and lead to depression [9; 10; 25]. This research created the following hypotheses. First, poor work situations affect depression (Hypothesis 1). Work-family conflict is the extent of a worker's dissatisfaction with his or her work and family roles in aspects such as time-sharing, participation, and satisfaction [26].