## 4 Application and Results
### Cross-Year Comparison of Country Ranking  
By combining the seven SPIs via the principal component analysis approach, the overall safety performance index scores and corresponding rankings related to the two country groups are computed for the years 2008 and 2015, respectively. The results are shown in Table 4.  
As can be seen, Ireland obtains the highest index score of 0.865 and ranks the first among all the countries in Group 1 for the year 2008, but it drops to fourth place in 2015. Sweden moves from second place to the top over these seven years, with an increasing index score from 0.747 to 0.870. On the other hand, Italy keeps last place in this group, although its index score improves from 0.159 in 2008 to 0.221 in 2015. Regarding country Group 2, Estonia is the best-performing country for 2008 with an index score of 0.854, but such a ranking is replaced by the Czech Republic in 2015, with a somewhat lower index score of 0.804. Romania always ranks last within this group, and its index score also declines from 0.242 to 0.196. In general, from 2008 to 2015, nine out of 13 countries within Group 1 see their index score increase, and the overall road safety performance of the countries in this group improves with an average index score increased from 0.623 to 0.653. However, the situation is different for the countries in Group 2. Six out of eight countries obtain a lower index score, and the average index score of all these eight countries declines from 0.610 to 0.546, implying that the gap between these two country groups with respect to the road safety performance expanded during these seven years. Regarding the ranking of these 21 European countries based on their index scores, we further use Figure 4 to illustrate its changes between these two years. As we can see, a relatively large positive change in ranking occurs for Belgium and Austria in country Group 1 and to the Czech Republic and Poland in country Group 2. In contrast, Spain and France in Group 1 and Hungary in Group 2 see a dramatically negative shift in their rankings from 2008 to 2015. Taking Belgium as an example, it achieved the largest improvement in ranking among all the countries in Group 1, moving from 10 th place in 2008 to third place in 2015. Such a result can mainly be attributed to the uptrend change in most of its normalized SPI values (see Figure 5a). On the contrary, Hungary suffered the biggest retrogression within Group 2, moving from the second best-performing country in 2008 to \begin{table}
\begin{tabular}{l l c c c} \hline \hline  & & _N\_p\_cars_ & _GDP_ & _Pop\_den_ & _Pop\_urb_ \\ \hline Year 2008 & & & & & \\ Country Group 1 & 486 & 119 & 166 & 0.778 \\ Country Group 2 & 369 & 66 & 95 & 0.647 \\ _p_-value of _t_-test & 0.0067 & \(4.69\times 10^{-9}\) & 0.0320 & 0.0012 \\ Year 2015 & & & & \\ Country Group 1 & 504 & 131 & 179 & 0.792 \\ Country Group 2 & 424 & 46 & 94 & 0.655 \\ _p_-value of _t_-test & 0.0320 & \(8.99\times 10^{-10}\) & 0.0284 & 0.0009 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The average values of four background indicators related to different country groups and different years, as well as the _p_-value from the hypothesis testing. the last but one in 2015. As can be seen from Figure 5b, all safety performance aspects of this country deteriorated over these seven years relative to the other countries within the same group, thus, resulting in this big decline in ranking.