# Determining Factors Influencing Operating Speeds on Road Tangents
## 2 Methodology
### Data Collection
#### 2.2.1 Locations Selection  
To ensure a representative and methodologically valid dataset, the locations for speed measurements were selected following a structured approach. According to the previously described metrics, and to ensure relevant results, this study focuses on road tangents on single-carriageway roads (state roads; county roads) and dual-carriageway roads (motorways). The following parameters were considered for selecting the representative locations:  
* Efforts were made to select locations as randomly as possible within the constraints of the road network, ensuring national representativeness and covering all major geographic regions (Central Croatia, Slavonia and Baranja, Lika, Istria, Dalmatia);
* Existing speed limit (70, 80, 90 km/h for single-carriageway roads; 100, 110, 120, 130 km/h for dual-carriageway roads). It should be noted that, in Croatia, the default speed limit outside of urban areas is 90 km/h, unless otherwise indicated by a posted speed limit sign;
* Other relevant road design, vehicle structure, road capacity, and other contributing elements;
* Speeds were measured during the week (from Monday to Friday) at off-peak hours so that the established patterns of driver behavior could be disregarded or excluded from consideration;
* To maintain data integrity and to avoid behavioral bias, all selected locations were situated away from fixed or mobile speed enforcement zones;
* Only extra-urban areas were considered to eliminate the influence of urban traffic patterns and to minimize the impact of pedestrian activity;
* Measurements were conducted exclusively on tangent segments, with a minimum tangent length of 100 m;
* Within each tangent, measurements were performed at various positions (beginning, middle, and end) to account for potential intra-segment speed variations;
* Each location also met the following geometric and environmental criteria:
* A straight and uniform road section, free from nearby horizontal curves that could affect vehicle speed;
* This study did not consider the horizontal tangents in vertical curves;
* A section where it is technically possible to exceed the posted speed limit;
* Sections exhibiting a range of longitudinal slopes (from \(-5.20\%\) to \(+5.20\%\) (single-carriageway roads); from \(-4.00\%\) to \(4.00\%\) (dual-carriageway roads));
* Locations chosen across different terrain types (flat, hilly, mountainous);
* Locations exhibiting a variety of AADT and ASDT values.  
To ensure adequate traffic flow for statistical relevance, locations were only included if a minimum hourly traffic volume of 10 vehicles was observed during the measurement window. Additionally, favorable weather conditions were a prerequisite for conducting measurements--locations experiencing rain, snow, strong winds, or extreme temperatures during the survey period were excluded to ensure consistency under free-flow and safe driving conditions.  
In total, 44 measurement locations (22 cross-sections) were selected and distributed across the Croatian road network, as shown in Figure 4 and detailed in Appendix A.  
As shown in Appendix A, the locations encompass various speed limits as well as different values for AADT and ASDT. It should however be noted that, for dual-carriageway roads, AADT and ASDT data were only available for whole cross-sections (both carriageways) [43]. Therefore, the AADT and ASDT values for each carriageway were assumed to be half of the AADT and ASDT values of the dual-carriageway cross-sections. Similarly, for single-carriageway roads, AADT and ASDT data were only available for the entire cross-section (both directions of travel) [43]. Therefore, the AADT and ASDT values for each direction of travel were assumed to be half of the AADT and ASDT values of the single-carriageway cross-sections.