# Determining Factors Influencing Operating Speeds on Road Tangents
## 1 Introduction  
Road traffic safety is a global challenge for many countries around the world. According to the World Health Organization report [1], approximately 1.19 million people die in road traffic accidents each year worldwide, with an even larger number of them being injured. Therefore, road traffic accidents are the eighth leading cause of death globally and the leading cause of death among young individuals, aged 5 to 29 [2]. Most fatalities occur in low- and middle-income countries, where infrastructure, vehicles, and education on road traffic safety are not at satisfactory levels. Consequently, road traffic accidents have become one of the most critical public health issues facing modern societies, and their significance is expected to grow in the future. The OECD (2018) [3] estimates the social cost of road traffic accidents in the European Union at around EUR 500 billion (or 3% of its GDP). This cost includes medical expenses, productivity losses, repair and reconstruction costs for road infrastructure, as well as other related costs. Internationally, it is estimated that about one-third of fatal road traffic accidents are partially caused by excessive or inappropriate speed [4]. Numerous studies have found a link between speed and the likelihood of road traffic accidents. Furthermore,an increase in average speed is directly associated with a higher probability of accidents and the severity of their consequences. Specifically, a 1% increase in average speed leads to a 4% increase in the risk of fatal accidents and a 3% increase in the risk of accidents results in serious injuries [1]. Straight road segments, or tangents, facilitate higher vehicle speeds due to the absence of geometric constraints, such as horizontal curves or intersections. While this characteristic enhances traffic flow, it also contributes to increased crash risk and severity. Studies have indicated that excessive speed is a primary factor in road traffic accidents, with a significant proportion occurring on road tangents. According to the National Highway Traffic Safety Administration (NHTSA), speeding contributed to 29% of all fatal road traffic accidents in 2022, emphasizing the relationship between high speeds and accident severity [5]. Furthermore, data have suggested that a substantial share of fatal road traffic accidents on rural (68%) and urban (81%) roads occur on road tangents, where drivers are more likely to exceed speed limits due to the absence of speed-reducing elements [6]. The physics of high-speed travel further exacerbates risk, as increased speed reduces driver reaction time and extends stopping distances, increasing the probability of collision [7]. Moreover, speed-related road traffic accidents on road tangents often result in fatalities or severe injuries due to higher impact forces [8]. A critical issue contributing to road safety concerns is the discrepancy between the speed limits and the actual vehicles' speeds, with research indicating that drivers frequently exceed regulatory limits on straight road segments, leading to increased crash risk and severity [9]. These \"actual vehicles' speeds\" are defined as operating speeds. Following the above, operating vehicle speed can be defined in several ways. It may be considered the actual speed at which a vehicle is driving under free-flow conditions [10], or as the 85 th percentile speed distribution used to measure operating speeds related to specific locations or road geometries [11]. Studies have shown that operating speed under free-flow conditions, particularly on roads without speed monitoring/speed enforcement, tends to exceed posted speed limits [12; 13]. The highest operating speeds are typically achieved under free-flow conditions, where road geometry rather than interactions with other vehicles primarily influences speed. Thus, it may be presupposed that vehicle interaction has minimal effect on operating speed under such conditions [14].