The improvement of road construction caused an increase in the number of motor vehicles and their speed. The safety of road traffic becomes increasingly important (Forman et al., 2003). Notions about traffic safety have mostly been associated with the knowledge in the field of traffic construction, without delving into the influence of the surrounding landscape (Pérez de la Cruz et al., 1995). Also, there was never a discussion about the not so visible, but very sensitive effects of climate elements on traffic safety, except in case of extreme events (Theofilatos and Yannis, 2014).
The importance of the impact of weather conditions on traffic safety (the number of road accidents that they cause), can be found in scientific papers by several European authors (Perry and Symons, 1994; Edwards, 1996; Hassan and Barker, 1999; Musk, 2003; Hermans et al, 2006; Bergel-Hayat et al., 2013; Tsapakis et al., 2013). Interest in this topic began to develop in the 1960s, while a considerable rise was noticed in the early 21st century. As authors Hassan and Barker (1999) underline, traffic specialists are well-aware of the seasonal variation in traffic activity, but it has been noted that the knowledge regarding the impact of all climate elements on traffic flow is far from complete. Climate conditions may not be the most common reason for traffic accidents, but they should not be neglected, as they can create very dangerous driving conditions (Edwards, 1996; Hassan and Barker, 1999; Chapman, 2007; Kim, 2018; Wang et al., 2020).
Most of the information in the literature regarding the impact of climate elements on the traffic flow was given at a macro level (mean values of annual or monthly data), while at a micro level (surveys based on daily values for specially selected locations), the data are scarce (Edwards, 1996; Hermans et al., 2006; Chen et al., 2018). According to the authors (Hassan and Barker, 1999; Hermans et al., 2006), traffic activities on a macro level can be used in the process of passing the laws in transport planning. Evaluation of daily values of traffic activity takes a deeper approach to the issue of influence of climate elements on the traffic safety (Eisenberg, 2004). This can be used to improve the future new roads planning and construction process (Musk, 2003).
Climate elements that have a major impact on traffic safety include: rain, snow, ice, fog, temperature and wind (Fridstrøm et al., 1995; Hassan and Barker, 1999; Chapman, 2007; Theofilatos and Yannis, 2014). Their effects are even stronger when they occur in combination with one another (Musk, 2003). Moreover, the global climate change scenarios indicate the possibility of more frequent occurrences of climate extremes (storms, extreme heat, extreme wind gusts, etc.) and their influence on the environment and human activities (Koetse and Rietveld, 2009; IPCC, 2014; Lakatos et al., 2016; Deng and Zhu, 2021; Oscar Júnior and de Assis Mendonça, 2021). All this supports the need for more detailed study of the relation between the climate elements and the traffic safety.
Many authors wrote about the importance of wind from the standpoint of generating electric power (Frandsen et al., 2006), the impact on vegetation (Sellier and Fourcaud, 2009; Mitchell, 2013) and the impact on people’s lives (Knopper and Christopher, 2011), but very few have discussed the impact on traffic safety (Hermans et al, 2006; Theofilatos and Yannis, 2014). There is no significant database from which to draw lessons on how to design safe windy highway landscapes. Therefore, any research regarding this issue represents a major contribution to the scientific world. The attention of researchers (Baker and Reynolds, 1992; Perry and Symons, 1994; Edwards, 1996) was attracted by a severe storm caused by the wind in the British Isles in October 1987 and then again in January 1990, when a large number of fatal traffic accidents was recorded, and these are, at the same time, some of the few studies conducted on the effect of the wind on traffic safety.
According to Perry and Symons (1994) and Forman et al. (2003), wind can significantly upset the drivers directly, by turning or pushing their vehicles off the roadway or indirectly, by drifting and accumulating soil particles, snow, rain, etc. on the road. If trees are planted just near the road, wind can tear them or break branches, which can be a significant obstacle for drivers on the road (Koetse and Rietveld, 2009). Baker and Reynolds (1992) and Hermans et al. (2006) underlined that when the wind reaches an intensity of approximately 13–15 m/s, it obstructs the movement of the vehicles. On the other hand, when it reaches intensity of 20 m/s, it can turn over a vehicle on the road which then poses another kind of danger for the drivers. The authors (Baker and Reynolds, 1992; Perry and Symons, 1994; Hermans et al, 2006) further emphasized that it is sufficient for winds of these intensities to occur only for a few days during the year, or several hours during the day, in order to have devastating effects with fatal outcomes. Other authors (Hassan and Barker, 1999; Eisenberg, 2004; Dupont and Martensen, 2007; Bergel-Hayat et al., 2013) confirmed that large oscillations in meteorological conditions can occur, but extremes on a daily basis are caused by atypical weather in the season, and their impact on traffic safety is not negligible.
The greatest danger for drivers on the road is crosswind (Perry and Symons, 1994). In order to increase the traffic safety, more recent researches focus on identifying strong crosswinds by using mobile mapping technologies (Chen et al., 2010; Pu et al., 2018). These modern technologies are used for collecting data on crosswind speed in specific locations for moving vehicles (Chen et al., 2010). Considering that its distribution varies in both spatial and time domains, it is necessary to identify specific locations in which there is a high possibility of strong crosswind and where safety measures should be implemented. In the last few years there have been some studies focusing on this issue (Chen et al., 2018; Pu et al., 2018). Researchers Shin and Lee (2014) also showed that traffic safety is a worldwide issue that affects human life by taking it one step further and doing a reliability-based analysis of road vehicle accidents and the optimization of roadway radius and speed limit design based on vehicle dynamics, mainly focusing on windy environments. Windy traffic environments were studied by Maruyama and Yamazaki (2006) as well. The authors have investigated the moving stability of a vehicle under seismic motion based on both numerical simulation and virtual experiments using a driving simulator.
All of this shows that traffic safety is broad and multidisciplinary issue and that it arouses attention of scientists working in various disciplines. The mentioned issue needs to be seen from different aspects of view so that future highway projects and traffic safety assessments can be improved.
Therefore, afforestation of the windy roadside is more than necessary. In this way, protection from strong steady winds is provided, as well as protection from individual crosswinds (Alemu, 2016). Depending on the density, the vegetation belt (windbreak) can be used to protect the road from excessive snowdrifts, dust, etc. (Lorenz, 1980). A properly formed windbreak provides protection in the area at a distance of 20 times the height of the largest tree in the assembly, in the area on the leeward side, as well as 5–10 times the height of the largest tree on the windward side (Alemu, 2016). The role of windbreaks in traffic planning has been addressed by other authors as well (Wight and Straight, 2015).