## 5 Concluding Remarks  
Over the past decade, a great number of safety performance indicators have been developed describing the complex character of the road safety phenomenon, and the construction of a safety performance index has been widely accepted as a supportive instrument to reduce the dimension of selected SPIs and to compare and prioritize road safety actions among various countries. However, due partly to the data unavailability, most of the current applications concentrated on the index development for only one year. In other words, there is a lack of cross-year comparison based on the constructed safety performance index, so as to assess the progress of road safety performance in different countries over time. In this study, by collecting the data on four background indicators and seven safety performance indicators of 21 European countries for both 2008 and 2015, a safety performance index was developed and a cross-year comparative analysis on country grouping, ranking, and weight allocation was conducted. The main results are summarized as follows:  
1. By employing the hierarchical clustering analysis on the four background indicators describing the level of a country's motorization, economic development, population density, and urbanization, respectively, the 21 European countries were classified into two homogeneous groups, and the members in each country group remained the same in these two years, implying that there was no dramatic change with respect to these countries' road safety policy context in 2008 and 2015. In general, most of the western and northern European countries were within one group and could be treated as \"Highly Developed Countries\", while most eastern European countries belonged to the other group and could be treated as \"Less Developed Countries\". However, the overall socioeconomic development gap between these two country groups enlarged during these seven years. Figure 8: The change in the original values of the “Average score of pedestrian protection for new cars” indicator (_Prot_ped_) for different countries over time. Figure 7: The normalized SPI values for Spain over time. 2. By applying the principal component analysis approach to combine the seven SPIs into an index, the overall safety performance index scores and the corresponding rankings related to the two country groups were derived for 2008 and 2015, respectively. Belgium and Austria achieved a large improvement in ranking within country Group 1, and the Czech Republic and Poland in country Group 2 over this period. In contrast, Spain and France in Group 1 and Hungary in Group 2 suffered a large retrogression from 2008 to 2015. On the whole, the average index score increased for country Group 1 but declined for country Group 2, implying that the gap between these two country groups with respect to the road safety performance also expanded. 3. Based on the indicator weights assigned for each country group in different years via the principal component analysis, _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), were found to be the three SPIs that had the highest variation from the view of either time dimension or country grouping, implying that the road safety development of these 21 European countries with respect to these three indicators differed considerably over this period, and thus, more attention should be paid to these three aspects when formulating road safety priorities for policy action in these countries. Different from evaluating the road safety performance of a set of countries at one specific point of time by developing a safety performance index, a cross-year comparative analysis based on such an index was proven to be valuable in gaining a clear understanding of the road safety development of these countries over time.