## 4 Application and Results
### Cross-Year Comparison of Indicator Weights Allocation  
After comparing the overall road safety performance of these 21 European countries in their own group between 2008 and 2015, we further investigate the indicator weights assigned for each country group in different years based on the principal component analysis approach. The results are shown in Figure 6.  
From the view of time dimension, two out of seven SPIs' weights increase from 2008 to 2015 for country Group 1, while the remaining weights show an opposite trend. Amongst others, the weight for _Prot_ped_ increases by three times (from 0.065 to 0.195), and the weight for _P_alc_ declines by more than half (from 0.166 to 0.075). The others only change slightly. Regarding country Group 2, there are also two indicators whose weights change dramatically in these two years. One is _Belt_rear_, which sees a large increase in the assigned weight (from 0.058 to 0.220); the other is _P_alc_, whose weight declines from 0.185 to 0.033. If we further compare the weight allocation between these two country groups, it can be seen that the largest difference in 2008 is _Belt_rear_ (0.173 vs. 0.058), followed by _Prot_ped_ (0.065 vs. 0.098). In 2015, the largest difference appears to be _P_alc_ (0.075 vs. 0.033), also followed by _Prot_ped_ (0.195 vs. 0.115). In general, _P_alc_, _Belt_rear_, and _Prot_ped_, which belong to the three different risk domains considered in this study (i.e., alcohol, protective systems, and vehicle, respectively), are the three SPIs that have the highest variation from the view of either time dimension or country grouping. Although it was indicated in the literature that weights derived from the PCA approach cannot be directly used as a measure of the theoretical importance of the associated indicator [31], given the data-driven nature of this approach, it is important evidence to show that road safety development with respect  
Figure 5: The normalized SPI values for (**a**) Belgium and (**b**) Hungary over time. to these three indicators over this period varied considerably among these 21 European countries. Hence, more attention should be paid to these three aspects when formulating road safety priorities for policy action in these countries. Taking Spain as an example, it suffered the biggest decline in ranking within country Group 1. Figure 7 illustrates the change in its normalized SPI values between 2008 and 2015. We can see that the values of its four vehicle-related SPIs declined largely over this period. Given the fact that the indicator weight assigned to _Prot_ped_ for country Group 1 increases dramatically from 2008 to 2015, the low index score obtained by Spain for 2015 can then mainly be attributed to its poor performance on this indicator. Contrastingly, if more efforts were given to the improvement of pedestrian protection in Spain, such as installing infrared automatic brake control system in new cars, its overall index score as well as the corresponding ranking would improve accordingly. It should be highlighted here that the decline in the normalized SPI values of a country does not always mean a retrogression of its actual performance, but a change relative to the other countries under consideration. It can be treated as an added value of using the normalized data for country evaluation and comparison. Looking at the change in the original values of the indicator _Prot_ped_ over time (see Figure 8), all the countries in Group 1 made considerable progress in this respect from 2008 to 2015. However, relative to the other countries within the group, the improvement made by Spain was smaller, thus, resulting in a big drop in ranking with respect to this indicator and, consequently, a declined normalized value for 2015. Hence, Spain should not be satisfied with the progress it made during this period, but continue to prioritize policy in this aspect. Meanwhile, the progress made by the other countries in the same group also implies the applicability of this improvement direction.