\subsection{Sample Surrogate Safety Analysis}

A sample surrogate safety analysis of three of the sites is demonstrated in Figure~\ref{fig:selected-site}. This shows road user trajectories projected in and with respect to the scene, mean velocity vectors and speed along these trajectories, and spatial distribution of collision points based on motion pattern prediction with instantaneous $probability > 1e5$ and $TTC < 1.5 seconds$. Problematic weaving conflicts are highlighted in the first and third examples, particularly for multi-lane roundabouts. The second example, on the other hand, demonstrates car-following conflicts, particularly at the approach (not surprising, given that this single lane approach frequently experiences queuing as drivers yield to the high conflicting flow) and exit (more surprising).

% paul: the merging zone as a unit of analysis is not well defined. Add a figure if we have enough space
% paul->Nicolas: added a definition at the beginning of the section.
%Figure~\ref{fig:ttc_distro_sample} demonstrates a cross-sectional comparison of TTC distributions based on motion prediction at constant velocity for 20 merging zones for two contributing factors, each using all interaction instants. Kolmogorov-Smirnov tests are performed between the distributions to quantify non-parametric dissimilitude. In the first diagram, a cross-sectional comparison is made for merging zones situated more or less than 300 metres upstream from another intersection. When this distance exceeds 300 metres, the distribution mass appears to shift left except for a sharp increase in small TTC below 0.5 seconds. It is so far unknown whether this small concentration of low-TTC conflicts offsets all other increases in TTC, therefore this comparison remains inconclusive. In the second diagram, a cross-sectional comparison is made between merging zones with high approach traffic volume ratios and low approach traffic volume ratios, where $R$ is the flow ratio between approach volumes and total volumes at the merging zone. In this comparison, a clear and consistent mass shift is observed, suggesting that high approach traffic volume ratios contribute to safer merging behaviour in a roundabout. 
% paul: When this distance exceeds 300 metres, the distribution mass appears to shift left -> right ?
% paul->Nicolas: Not entirely. There is a sharp increase in the 0-0.5 range, small number of possibly "severe" conflicts. Conclusion is therefore inconclusive.