# Determining Factors Influencing Operating Speeds on Road Tangents
## 2 Methodology
### Factors Identified--Single-carriageway Road Tangents  
The external roadside environment can influence driver behavior and, consequently, operating speed. Three distinct passenger-side conditions were identified, including cutting sections, guardrails, and flat area. Each of these conditions can affect driver perception and lane positioning, with cut sections and guardrails potentially inducing a more cautious driving approach. However, following that, shoulder presence and composition were analyzed at each location, with five possible classifications, including absence of a shoulder, extremely narrow shoulder, unpaved (earth/grass) shoulder, gravel shoulder, and paved (asphalt/concrete) shoulder. The shoulder type may affect vehicle stability and driver confidence, therefore influencing speed selection. Similarly, the edge line quality on the passenger side was assessed subjectively in the following four categories: absence of an edge line, poor, satisfactory, and excellent. Clearly marked and visible edge lines may contribute to improved lane discipline and potentially higher operating speeds, while faded or missing lines may cause drivers to reduce their speed (or vice versa) [46]. Visibility was assessed in terms of forward sight distance, considering terrain configuration, longitudinal slope, and vegetation. This assessment was conducted subjectively, with three classifications, including poor, good, and excellent. It should be noted that visibility was assessed in terms of horizontal sight distance, as vertical curves were not included in the scope of this analysis. Horizontal visibility was evaluated based on the driver's line of sight along the road alignment, which can vary significantly depending on several contextual factors. These factors include terrain configuration, road width, and the positional characteristics of the road segment, such as whether the segment is located in a cutting, on an embankment, or on flat terrain. Such conditions may substantially influence a driver's ability to perceive the road ahead and to respond to potential hazards, thereby affecting speed selection. Reduced visibility can lead to lower speeds, as drivers may compensate for the increased risk of sudden obstacles. Moreover, the density of lateral accesses, measured as the number of lateral accesses in one previous kilometer may have an impact on the driver--a higher access density may result in lower operating speeds due to increased potential for vehicle interactions and conflict points. Pavement quality was evaluated based on observable distress features, such as rutting, cracking, and potholes. Three classifications were established, including poor, satisfactory, and excellent pavement conditions. Poor pavement conditions can lead to speed reductions as drivers may need to maneuver to avoid road imperfections. The length of the tangent section, both preceding and following the measurement point, could have a role in determining vehicle speeds. Extended tangents may encourage higher speeds due to the absence of horizontal curvature, while shorter tangents preceding sharp curves may necessitate speed reductions. These lengths were precisely measured using QGIS 3.32.3 software. Speed limits serve as a regulatory measure influencing driver behavior. The default speed limit for single-carriageway roads outside urban areas in Croatia is 90 km/h, though variations due to local conditions may alter actual operating speeds. Furthermore, traffic volume and composition were considered through the average annual daily traffic (AADT) and average summer daily traffic (ASDT) values, obtained from Croatian Roads Ltd. [43]. Higher AADT and ASDT values generally correlate with lower operating speeds due to increased vehicle interactions and congestion effects. Lane width was directly measured on site using precise measuring tools. Wider lanes typically facilitate higher speeds, whereas narrow lanes can contribute to cautious driving and speed reduction.