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\subsection{Behavioural \subsection{Behaviour  and Safety Measures} The parameters of interest for this particular study are the most notable surrogate safety measures: speed, time-to-collision (TTC)~\citep{Hayward_1971}, and post-encroachment time (PET)~\citep{allen1978analysis}. The PET is measured specifically at the merging zone yield line, where encroachment is prohibited by way of mandated yielding on the part measures  of safety: two measures obtained directly from  the approaching observed  road user only, trajectories, speed  and is denoted yPET. Speed post-encroachment time (PET)~\citep{allen1978analysis},  and yPET are measured directly time-to-collision (TTC)~\citep{Hayward_1971}, a measure derived  from position and speed based on assumptions of  theobserved  road user trajectories. users' expected motion.  Speed is widely regarded in the literature as a useful predictor of collision severity given the relationship between speed and kinetic energy carried by a road user in motion~\citep{Fildes_1993, elvik2004speed}.Meanwhile, TTC, measured in units of time, is one of the most popular surrogate safety measures intended as a generalised measure of collision probability as it models "near-miss" situations between any types of road users traveling anywhere, at any speed. It is most easily understood as remaining time before a potential collision ensues before a road user initiates evasive action (if at all). In its most basic form, motion prediction at constant velocity \citep{Amundsen_1977}, TTC is the distance between any two road users, at any time, divided by the differential speed between the two.  Like TTC, yPET Provided that road user trajectories overlap, PET  is measured in units of time and describes "near-miss" situations in a similar fashion, though, unlike TTC, without making any assumptions of motion, relying exclusively on observed behaviour. It situations. In this study, the PET  is thus less flexible in modeling as great a variety measured specifically at the merging zone yield line, where encroachment is prohibited by way  of potential outcomes without significantly larger quantities mandated yielding on the part  of observed data. Nevertheless, the approaching road user only, and is denoted yPET.  yPET is of interest as a model of yielding behaviour and merging aggressivity as it is associated with gap time and gap acceptance. Note that yPET values can be of any size, given that the only requirement is that the road user trajectories overlap each other. If size: if  demand is low, someof these  arrivals may be minutes apart and would thus obviously hold no value in interpreting interaction safety. To counter this, a conservative maximum threshold of consideration of 5~s on yPET is used. This value is arbitrarily selected to reject those interactions where it is very clear that road users are not coexisting in time and space (the dwell time across each merging zone rarely surpasses 5 seconds). %$\zeta_{PET} < 5$ In addition to Like PET, TTC is defined for pairs of road users and measured in units of time. It is one of  the most popular  surrogate safety measures outlined above, additional measures of behaviour describing instantaneous collision-course conditions measures. It can be computed only if the road users  are stored alongside each collision-course model (i.e. each TTC measure). These include 15-s exposure, in  a micro-measure situation  of exposure, which counts collision course, where  thenumber of  road users present within the merging zone 7.5~s before and after the uses would collide if their movements remain unchanged. Identifying a  collision course at a given instant thus depends on a method to predict the road users' motion after that instant. TTC  is modeled, as well most easily understood  asintersection angle, which measures the angle of approach of  the remaining time before a potential collision ensues unless a  road users user initiates evasive action (if  at the instant of the collision course in degrees. This angle all). In its most basic form, motion prediction at constant velocity~\citep{Amundsen_1977}, TTC  is$0^{\circ}$ when  the distance between any two  road users are following each other and $180^{\circ}$ when approaching others head on. users, at any time, divided by the differential speed between the two.  In addition to the surrogate safety measures outlined above, additional measures of behaviour describing instantaneous collision-course situations are stored alongside each pair of road user and TTC measure. These include 15-s exposure, a micro-measure of exposure, which counts the number of road users present within the merging zone 7.5~s before and after the collision course is modeled, as well as intersection angle, which measures the angle of approach of the road users at the instant of the collision course in degrees. This angle is $0^{\circ}$ when the road users are following each other and $180^{\circ}$ when approaching others head on.  \subsubsection{Advanced Time-to-collision Modelling and Aggregation} Aggregation TO CONTINUE reorganize}  As stated previously, TTC makes use of collision-course prediction models. Typically, potential collisions are defined as collision-course events using constant velocity motion prediction, i.e. ``with movement remaining unchanged'' \citep{Amundsen_1977}. Given the non-linear driving required to navigate the deflection induced by roundabout central islands and approaches, a more sophisticated collision-course prediction model is used in this work instead: the discretized motion pattern motion prediction model developed specifically to address the issues of modeling movement in complex environments \citep{St_Aubin_2014} i.e. $TTC_{cmp}$. It should be noted, however, that $TTC_{cmp}$ is by no means specific to roundabouts.