## 1 Introduction  
Vulnerable road user (VRU) safety is affected by many crucial factors such as vehicle design, its frontal aggressiveness, road and pavement layout, legislation (e.g., speed limitations), active and passive safety systems such as a car's camera/LIDAR/RADAR or bicyclist's helmet, to name a few. Vulnerable road users are defined as \"non-motorized road users, such as pedestrians and cyclists as well as motor-cyclists and persons with disabilities or reduced mobility and orientation\" [1]. Thus, the road users--who have a high casualty rate and should be given special attention in road safety policy--are often referred to as 'vulnerable road users'. The lack of external protection or absence of a protective 'cage' around the traffic participant is also an indicator for pedestrians, cyclists and motorcyclists [2; 3]. In the literature, car drivers and occupants are most often excluded from the definition of VRU [1; 4]. Interestingly, some reports indicate that in the future connected transport system, VRU or \"vulnerability\" could be more related to non-connected users and people unable to fully use the potential of the Information and Communications Technologies (ICT) services offered to mobility [5]. It should be noted that due to traffic accidents, about 1.35 million people die each year [6]. The burden of road traffic injuries and deaths is disproportionately borne by vulnerable road users, who contribute to half of all victims. The World Health Organization (WHO) predicts that by the year 2020 road accidents will become the leading cause of premature death [7]. It proves that irrespective of numerous pedestrian/cyclist crossings, sidewalks/roads and effort made to separate VRUs from vehicles, the danger of death as a victim of a road accident is much higher in a city, mainly due to traffic density and complexity. It is reported that among people killed on city streets, on average 73% are pedestrians, cyclists and motorcyclists [6,8]. As Figure 1 depicts, for high-density population cities--such as Paris--of all fatalities on roads, VRUs make up 90%. In Figure 2, the author presented the data reported for 24 European Union (EU) members, with an additional EU average, for the share of pedestrians and cyclists of all who died on EU roads. The percentage of cyclists killed correlates strongly with countries' bicycle traditions and proper infrastructure, which encourages inhabitants to choose a bicycle as a mean of transport. Thus, higher than average shares are reported in the Netherlands and Denmark. On the other hand, the high share of pedestrian fatalities distinguishes relatively new EU member states such as Romania, Latvia, Poland and Lithuania where pedestrians contribute to more than 30% of all road deaths [9,10]. However, an interesting correlation forms as we compare the road safety level in various countries (fatality rate per million inhabitants) with the pedestrian deaths on roads. For the majority of countries, the lower the overall fatality rate, the lower the pedestrians' fatality contribution [11]--namely, the percentage of pedestrians who are killed on the country's roads may provide an approximate idea of the general country road safety. Figure 1: Vulnerable road users’ (VRUs’) road deaths [%] by city in 2013–2015—based on data from Reference [8]. Figure 2: Pedestrians’ and cyclists’ share of all road deaths [%] in contrast to road fatality rates per million inhabitants [] in European Union in 2014—based on data from [9,12]. The problem of unprotected road users first appeared in the literature in the 1950s [13; 14]. In the late 1970s, pedestrian collision statistics investigated the vehicle front as a major problem for pedestrian injuries. However, until the 1980s, it was not taken into account by designers, engineers or governments, which in fact had the power to exert sufficient pressure on vehicle manufacturers.