Due to the large number of potentially contributing factors (at least 20) and the relatively small number of corresponding sites, sites were clustered into 6 groups according to Table~\ref{tab:cluster_ttc_profile} using the k-means algorithm \cite{St_Aubin_2015_TRBb}.

\begin{table}
    \caption{K-means clusters for weaving zone sites}
    \label{tab:cluster_ttc_profile}
    \begin{tabular}{lLll}
    \textbf{Cluster} & \textbf{Description}                                                                                                       & \textbf{Group size} & \textbf{Observations} \\ \hline
    $cl_1$   & Small single and double lane residential collectors                                           & 11     & 4,200        \\
    $cl_2$   & Single-lane regional highways and arterials with speed limits of 70-90 km/h and mostly polarized flow ratios   & 16     & 26,243       \\
    $cl_3$   & 2-lane arterials with highly polarized flow ratios                                                            & 5      & 13,307       \\
    $cl_4$   & Hybrid lane 1->2 2->1 arterials with flow ratios favouring the conflicting flow                                             & 3      & 4,809        \\
    $cl_5$   & Traffic circles converted to roundabouts (2 lanes, extremely large diameters, tangential approach angles) & 4      & 10,295       \\
    $cl_6$   & Single-lane regional highways with large-angle quadrants (140 degrees) and even flow ratios            & 2      & 2,235        \\
    \end{tabular}
\end{table}

The cumulative distribution of 15th percentile motion pattern TTCs is broken down by roundabout cluster as illustrated in Figure~\ref{fig:TTC_CDF}. Cluster $cl_3$ (2-lane arterials with highly polarized flow ratios) offers the best and clearest safety benefits, while cluster $cl_5$ (traffic circles converted to roundabouts with 2 lanes, extremely large diameters, tangential approach angles) offers markedly decreased safety performance. $cl_6$ (single-lane regional highways with large-angle quadrants (140 degrees) and even flow ratios) is ambiguous in its interpretation: while there is a general left-shift in TTC distribution (towards lower values) there is also a decrease in frequency of the lowest (most severe) TTC indicators. The remaining clusters, representing 31 sites, tend towards an average TTC distribution. This is still a large number of sites. Further investigation, regressing for specific factors, is warranted \cite{St_Aubin_2015_TRBb}.


\subsection{Sample High-Level Interpretation Analysis}
\label{sample_HLI_analysis}

Some high-level interpretation measures are also compiled using the data sample. Figure~\ref{fig:speed_profile} shows the mean speed profiles, with the interval at mean $\pm$ one standard deviation, through the roundabout weaving zones. Speed profiles are mapped, not as units of distance, but rather as proportions of curvilinear location relative to the start and end of the merging zone, measured at the yield line of the approach and equivalent line at the corresponding exit. This is done to account for the large variability in diameter of roundabouts and angular size of weaving zones across sites. Mean speeds are generally consistent with those in the literature, but variation does occur with relative location and movement type \cite{Hyd_n_2000}. In addition, cross-sectional analysis uncovers even larger variations in mean speed profiles \cite{St_Aubin_2013}.

Finally, Figure~\ref{fig:hli_histos} presents boxplots of lag gap acceptance of approaching vehicles and conflicting vehicles. In a cross-sectional analysis, this quantifies vehicle insertion aggressivity. Smaller accepted gaps might be explained by more impatient drivers, typically symptomatic of high-volumes of continuous flows inside the roundabout and long wait times at the approach.