## 2 Materials and Methods  
### Data Sources  
The data collection phase of this study integrated three distinct yet complementary datasets from authoritative national sources involving traffic accident records, meteorological data, and socioeconomic data.  
The primary source for traffic accident records was the Accident and Collision Recording System (SEWIK), a national database supervised by the Traffic Bureau of the National Police Headquarters [34]. SEWIK provides comprehensive information on each reported incident, including the date, time, and geographic location, as well as details on accident type, severity, casualties, and property damage. Part of this database is publicly available through the Polish Road Safety Observatory (POBRD), ensuring transparency and ease of data retrieval [35]. In addition, SEWIK's data were also gathered from a non-commercial service sewik.pl [36]. While this source contains supplementary information not readily available in POBRD, its lack of official status raises concerns regarding data accuracy and completeness. Therefore, both sources were analyzed to identify potential discrepancies, ensuring the highest possible data reliability.  
The meteorological data utilized in this study were sourced from the Institute of Meteorology and Water Management (IMGW) [37], Poland's principal source of weather observations, operating under the Ministry of Infrastructure [38]. IMGW maintains an extensive network of meteorological and hydrological stations across the country, as shown in Figure 1, systematically collecting high-resolution data [39]. The publicly available historical records include key atmospheric parameters such as temperature, precipitation, and relative humidity. Although IMGW's dataset provides sufficient temporal resolution, the uneven spatial distribution of its stations introduces challenges that might affect the precision of weather interpolation between monitoring locations.  
Socioeconomic data were obtained from the Local Data Bank of Statistics Poland (GUS), the nation's primary source of official statistics [40]. GUS provides detailed insights into key indicators such as population density, vehicle registrations, and road network coverage across Poland's hierarchical administrative divisions (municipalities, counties, vovodeships) and at the national aggregate level. These variables serve as proxies for regional traffic exposure and infrastructural development. By integrating these factors, the study aims to assess the role of regional disparities in shaping road accident dynamics, ensuring a more holistic perspective on the complex interplay between environmental, infrastructural, and human-related determinants.  
To enhance the accuracy of meteorological data interpolation, Poland's elevation information was sourced from the Digital Elevation Model (DEM) acquired from the Geographic Information System of the Commission (GISCO) [41]. These data played a meaningful role in refining the spatial distribution of weather parameters, allowing for a more precise representation of local conditions.  
Figure 1: Geographic distribution of IMGW’s weather stations across Poland active during the study period. Stations are categorized by their measurement capabilities: those measuring precipitation, temperature, and humidity, and those measuring precipitation only. The network shows broader spatial coverage for precipitation measurements, with a particularly dense concentration of both station types in the south, mountainous regions. Northern and central regions display a more dispersed pattern with relatively fewer stations, while maintaining a fairly balanced distribution between full-parameter and precipitation-only stations throughout the country.  
Additionally, information on the administrative boundaries of Poland's vovodeships and counties was obtained from the National Register of Boundaries (PRG) [42]. These boundary datasets were subsequently employed to help estimate average weather conditions within each administrative unit and to facilitate data visualization.