## 3 Results and Discussion
### Spatial Distribution of Rainfall and Accident Locations  
To analyze the spatial correspondence between rainfall distribution and accident location, we plotted data for localized rain events and stratiform rain events [24; 25]. A localized rain event is defined as when the area of the rainfall-field is less than 30% of the total area of Seoul. The criterion was determined to be less than 30% by reflecting the heavy rainfall area of Seoul at the time, defined as locally heavy rain in the report [26]. The total number of observed rainfall events related to the 24,797 traffic accidents during rainy conditions was 8288, and localized rain accounted for 2546 events, which corresponded to 30.7% of all rainfall events. The spatial scale of the localized rain, such as the convective band, was set at 10 km based on the research report by the Meteorological Administration, which noted that the convective region is formed while maintaining a width of 2-20 km and a length of several hundred km in radar images. Figure 5 shows the locations of accidents (red stars), and interpolated rainfall from AWS (left) and radar rainfall (right) for localized rain events. Localized rainfall distributions were not observed by AWS and were estimated as non-rainfall in many cases, resulting in considerable differences in rainfall intensity and spatial distribution of rainfall. In particular, the rainfall of the line-shape was concentrated in southeastern Seoul on 27 July 2011 and it is strongly suspected that this caused traffic accidents; AWSs could not measure the rainfall in the region (Figure 5e,f) [27]. The distribution of interpolated AWS rainfall was far less consistent with accident locations than that of radar-derived rainfall, which indicated that localized heavy rainfall likely affected accident occurrence. A quantitative analysis of the spatiotemporal correlation between rain strength and accident occurrence was performed by actively utilizing the spatial distribution information of the radar-derived rainfall. Figure 6 shows the images for stratiform rainfall events. In this study, a rainfall event was classified as stratiform when the area of rainfall exceeded 50%. Compared with Figures 5 and 6, there is a difference in rainfall amount based on the type of rainfall data, but the area covered by rainfall is generally similar between interpolated AWS and radar-derived rainfall distribution with traffic accident locations in the case of stratiform. However, in the case of very light rainfall intensity, as that seen on 9 May 2007, the area observed by AWS did not cover the entire area of rainfall and regarded the rainfall as local (Figure 6a,b). This occurred due to a measuring error of AWS because a 0.5 mm tipping bucket is used which cannot measure light rainfall below 0.5 mm. Figure 5: Comparison of interpolated AWS (left column) and radar-derived (right column) rainfall distribution with traffic accident locations (red circles) for localized rain events. Figure 6: Comparison of interpolated AWS (left column) and radar-derived (right column) rainfall distribution with traffic accident locations (red circles) for stratiform rain events. Figure 9 shows the mapping result of RAR for the administrative districts of Seoul. The RAR of all accidents during rainy conditions differs according to the number of accidents and rainfall occurrence patterns in each administrative district. In particular, there are many districts with a RAR of 6 or higher  
Figure 8: RAR for all 467 administrative districts in Seoul (fatal and severe accidents during rainfall). Figure 7: Relative accident rate (RAR) for all 467 administrative districts in Seoul (all accidents during rainfall). estimated according to radar data. Considering the recent increase in localized rainfall, the analysis using radar rainfall should also be considered for future safety planning applications in Seoul. Because traffic accidents occur more frequently during rainfall below 5 mm, the spatial detection of rain affects RAR calculations more than the quantity of rainfall.