## 1 Introduction and Background
Regardless of how the traffic detection sensors classify vehicles into different categories, such as based on vehicle type, number of axles, or gross weight, all types of heavy vehicles with a Maximum Authorised Mass (MAM) exceeding 3.5 are classified as heavy vehicles. In some other countries of the world, heterogeneity is complemented by the presence of a share of tourist or recreational vehicles, e.g., vehicles with tourist trailers. Greater heterogeneity results in worse conditions for the movement of vehicles in the traffic flow, which is particularly noticeable on uphill and downhill slopes [17]. Different vehicles have different driving and dynamic capabilities; therefore, they can be treated separately within traffic control. Changing lanes is also influenced by weather or environmental conditions [18; 19]. Effective operation of AHTMSs at the strategic and operational levels requires knowledge of the effects of different weather conditions on traffic flow [20]. Weather phenomena, such as rain and snow, fog, and other adverse environmental factors that affect the visibility and stability of vehicles, alter conditions that affect highway driving dynamics and capacity [17; 21; 22]. Event forecasts, including weather and traffic advisories, can affect traffic demand [16; 17; 23], resulting in reduced traffic flow and improved road capacity utilisation. The influence of weather and meteorological conditions on the roadway on traffic is also a well-researched area in Slovenia, especially in terms of the type and intensity of precipitation (rain or snow), temperature, and visibility conditions [24]. Somewhat less well-researched are wind and combinations of weather phenomena, as well as other phenomena that are very difficult to measure or monitor regionally such as hail and extreme winds. The impact of forecasts or other traffic and travel information is even less researched [25; 26]. We do not know for sure whether the impacts are the result of a lower traffic flow because vehicle speeds are reduced and drivers travel with a greater safety distance, whether they affect traffic demand (e.g., drivers do not make the trip), or whether it is a coincidence due to an extraordinary event, perhaps a road closure for a particular vehicle type at a different location. We know from experience that meteorological conditions on the road and weather phenomena affect the speed of traffic flow. Regardless of traffic conditions and the mutual influence of vehicles, they affect traffic demand and road capacity. Various studies report that the traffic flow rate on highways is lower due to poor weather conditions than under good conditions, i.e., dry weather and good visibility [22; 24; 27; 28; 29]: in light precipitation (rain, snow) from 5% to 10%; in heavier precipitation up to 15%; in heavy snowfall from 30% to 65%. At free-flow traffic and in light precipitation (light rain or snow), the vehicle speed is reduced by up to 5% compared to up to 10% in medium precipitation intensity (rain), and up to 15% in heavy rain; in limited visibility, speed is reduced between 10% and 15%, and by up to 60% in exceptional cases; in heavy snowfall, it is reduced by up to 30%, and by up to 60% in some exceptional cases. These are general research results, usually related to a heterogeneous traffic flow, where all vehicles are included in the traffic characterisation. In the mass of research conducted, there are not many findings on the impacts on heavy vehicles, the impacts that are local in nature and related to the geometric characteristics of the road, and the impacts on the characteristics of traffic flow along individual traffic lanes. Therefore, as a contribution to the empirical research, this study evaluated the impact of weather conditions on the speed of traffic flow of light and heavy vehicles in the case of Slovenia. Many researchers have attempted to answer the question of how traffic flow structure and ambient illumination affect the distribution of traffic across lanes.