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\subsubsection{Time-to-collision}  Time-to-collision is the surrogate measure of safety of choice for its relative maturity, simplicity, and transferability properties. Time-to-collision measures the time remaining, at any instant in time before two road users on a potential collision course collide: higher values are better for safety.  It doesn't have the same level of validation as speed does in the literature, but while speed is a good predictor of collision severity, TTC promises to be a good predictor of collision probability, a property which is arguably lacking with speed \cite{Hauer_2009}. Therefore, modelling the two together should give a good overall coverage of collision risk. The constant velocity motion prediction model is deemed inadequate for TTC measurement in roundabouts however, as road users in roundabouts rarely follow straight trajectories. Fortunately, some more sophisticated naturalistic motion prediction models have been developped to overcome this shortcoming: motion patterns are used for their ability to learn normal movement within a traffic scene. A discretised motion pattern matrix method developed specifically for roundabouts \cite{St_Aubin_2014} is used for this study. We also elect to model all traffic events, use no minimum detection probability, and use 15th percentile unique user pair indicator aggregation as is described in \cite{St_Aubin_2015}. \subsection{Site Selection}