# Determining Factors Influencing Operating Speeds on Road Tangents
## 2 Methodology
### Factors Identified--Single-carriageway Road Tangents
Longitudinal slope could impact the vehicles' acceleration and deceleration patterns, particularly on extended gradients. Indeed, empirical studies have confirmed the impact of longitudinal slope on speed variation [47; 48; 49]. This parameter was measured using field equipment to ensure accuracy. On the other hand, a crash ratio is an indicator that was derived by calculating the ratio of registered vehicles to speed-related crashes within the county where each site is located. This factor was defined as the ratio between the number of recorded speed-related crashes and the number of registered vehicles within a specific county in Croatia, both measured over a one-year period. This annualized metric serves as a normalized indicator of speed-related crash frequency relative to the size of the vehicle fleet in each county. By accounting for the number of registered vehicles, the crash ratio enables a more accurate comparison of road safety conditions across counties with differing vehicle ownership levels. The variable was introduced to capture the potential regional variations in driver behavior, enforcement intensity, and road infrastructure quality, which may influence the prevalence of speed-related incidents. A higher accident ratio may suggest an increased likelihood of speed-related incidents, influencing driver behavior and risk perception. Defined by Croatian regulations [50], the design speed (Vp) represents the maximum safe speed under optimal weather conditions. This factor reflects roadway design intent and provides a benchmark for expected operating speeds. Terrain classification--flat, hilly, or mountainous--was included as a key factor due to its influence on vehicle speeds. The study by [PERSON] et al. [51] indicated that operating speeds tend to be higher in flat areas and lower in hilly and mountainous regions due to challenging road geometry. The presence of guardrails enhances roadside safety and can encourage higher speeds by providing a perceived safety buffer. Conversely, roads without guardrails may prompt cautious driving. Similarly, shoulder lanes provide additional maneuvering space, which could potentially influence speed selection. Also, two road categories were analyzed, namely state roads and county roads. State roads generally exhibit superior design characteristics and maintenance, accommodating higher traffic volumes compared to county roads. The share of heavy goods vehicles and buses could also be an important factor. Large vehicles generally reduce speeds due to their lower acceleration capabilities and extended braking distances. Data on vehicle composition were sourced from Croatian Roads Ltd. [43]. Finally, the disruptive presence factor captures the influence of roadside infrastructure elements that may affect driver behavior and operating speed. This includes features such as bus stops, lay-bys, rest areas, pedestrian crossings, and similar elements located within or in close proximity to the observed road segment. The presence of these features can introduce potential conflict points, requiring drivers to reduce speed preemptively due to the increased likelihood of vehicle deceleration, pedestrian activity, or unexpected maneuvers [52; 53]. As such, these factors represent localized disturbances to the otherwise uniform driving environment of tangent sections, and their inclusion in the analysis helps to account for reductions in speed that are not attributable to road geometry or traffic volume alone, but rather to contextual roadside conditions that influence risk perception and driving decisions. **Factors Identified--Dual-carriageway Road Tangents**  
Tunnels have a potential impact on driving speeds due to narrower lanes, an absence of emergency lanes, and lower speed limits. Additionally, the enclosed environment of tunnels alters driver perception and behavior, contributing to reduced speeds. This parameter was quantified by summing the lengths of tunnels over the designated distance (previous 20 km) using the official road network data.