## 2 Indicator Selection and Data Collection  
In this study, two types of road safety related indicators are considered. One is background indicators, which are used to divide countries into comparable groups, and the other is safety performance indicators, which are used to construct the road safety performance index.  
The background indicators give an essential background condition of a country or its policy context. Progress in road safety may not be fully understood or even be misinterpreted by not knowing or ignoring this background information [9]. The background indicators should influence the road safety level of countries and hardly be directly influenced by the road safety policy of countries [12]. According to the above characteristics and taking data availability and reliability into account, four background indicators describing the level of a country's motorization, economic development, population density, and urbanization are chosen and shown in Table 1. They are (in parentheses are abbreviations): the number of passenger cars per 1000 inhabitants (_N_p_cars_); Gross Domestic Product (GDP) per head (_GDP_); population per 1 km\({}^{2}\) of a country's territory (_Pop_den_); the percentage of population living in urban areas (_Pop_urb_).  
The safety performance indicators are the measures reflecting those operational conditions of the road traffic system that influence the system's safety performance [20]. They can show in more detail the state of risk factors and their trends, as well as the potential to reduce these types of crashes [21]. Ideally, SPIs should encompass the most important risk areas related to humans, vehicles, and roads. Based on the literature review, seven SPIs with currently available data for two different years are selected, which represent three risk domains, i.e., alcohol, protective systems, and vehicles (see Table 2). Specifically, the percentage of drivers complying with the alcohol limit in road-side checks is the selected alcohol indicator (_P_alc_); the daytime seat belt wearing rates on front seats and rear seats of passenger cars are chosen for the domain of protective systems (_Belt_front_ and _Belt_rear_); and the vehicle domain is represented by the average score of occupant protection for new cars (_Prot_occ_), the average score of pedestrian protection for new cars (_Prot_ped_), the renewal rate of passenger cars (_Renewal_), and the percentage of new passenger cars of a maximum of five years old (_Car_age_). The first two indicators in this domain are generated from the European New Car Assessment Programme, i.e., EuroNCAP, where vehicle crash performance is evaluated by rating the vehicle models according to their safety level for occupant protection (1 to 5 stars), pedestrian protection (1 to 3 stars), etc. [22].  
From several international databases and publications [22; 23; 24; 25; 26], indicator values related to both 2008 and 2015 are collected for 21 European countries. They are Austria (AT), Belgium (BE), Switzerland (CH), Czech Republic (CZ), Germany (DE), Denmark (DK), Estonia (EE), Spain (ES), Finland (FI), France (FR), Hungary (HU), Ireland (IE), Italy (IT), Lithuania (LT), the Netherlands (NL), Poland (PL), Portugal (PT), Romania (RO), Sweden (SE), Slovakia (SK), and the United Kingdom (UK). The missing values are imputed by using the Expected Maximization method in SPSS 26.0 [27]. The indicator values are presented in Appendix A.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Background Characteristics** & **Selected Background Indicators** & **Abbreviation** \\ \hline Motorization & The number of passenger cars per 1000 inhabitants & _N_p_cars_ \\ Economic development & Gross Domestic Product (GDP) per head, EU27 = 100 & _GDP_ \\ Population density & Population per 1 km\({}^{2}\) of country’s territory & _Pop_den_ \\ Urbanization & The percentage of population living in urban areas & _Pop_urb_ \\ \hline \hline \end{tabular}
\end{table}
Table 1: Selected background indicators for country grouping.