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\subsection{Merging Zone}  The use of the reasoning for using  merging zones as the unit of study---instead of roundabouts as a whole---is that, while many factors such as land use are shared, many more are not. This includes flows and flow ratios especially, but may also include a host of  geometric factors such as lane configuration, signalisation, presence of a crosswalk, approach angle, etc. which can vary from one merging zone to the next even within the same roundabout \cite{St_Aubin_2013b}. Studying merging zones individually also better encapsulates the microscopic nature of the data being collected and analysed: roundabouts are often large enough for road user interactions on different sides of the roundabout to occur independently \citep[this is especially true if center islands obstruct view][]{Jensen_2014}. While the sites are selected to control for as many factors as possible, inevitably, some variation between sites still exists, especially regarding traffic volumes and patterns; these are identified such that they may be controlled during analysis.  Table~\ref{tab:analysis_zones} lists a summary of the merging zones at each roundabout studied and the any remaining variable  geometric and land use characteristics of each, characteristics,  as well as a summary of historical accident data at each roundabout. Quality of the available historical accident data is relatively poor, with sampling periods ranging between 2 and 15 years (average of 7) and data missing entirely at one roundabout. Furthermore, this historical accident data is representative of the entire roundabout instead of the merging zone exclusively since accident geolocation is often not fine enough to distinguish roundabout merging zones (this is especially true for the Québec data). These problems justify using surrogate safety measures for this study, and more generally. In any case, the pattern in this data is consistent with regional trends in accidents cited earlier: accidents are twice as likely in Québec than in Sweden, even at comparable sites, suggesting that unexplained effects can be found between the sites selected.    \begin{table}  \caption{Analysis Zones Inventory}