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  \subsection{Behavioural Measures}  The parameters of interest for this particular study are the most notable surrogate safety measures: speed (and speed profiles), speed,  time-to-collision (TTC) \citep{Hayward_1971}, (TTC)~\citep{Hayward_1971},  and post-encroachment time (yPET). Note that yPET is an ordinary (PET)~\citep{allen1978analysis}. The  PETmeasure \citep{allen1978analysis} but is designated yPET as it  is measured specifically at the merging zone yield line, where encroachment is prohibited by way of mandated yielding on the part of the approaching road user only. Other than this selection criterion, it only, and  is comparable to any other standard PET measure. denoted yPET.  Speed and yPET are measured directly from the observed road user trajectories as they occur. trajectories.  Speed is widely regarded in the literature as a useful predictor of collision severity \citep[e.g.][]{Fildes_1993, \citep[e.g.\ ][]{Fildes_1993,  elvik2004speed} given the relationship between speed and kinetic energy carried by a road user in motion. Meanwhile, TTC, measured in units of time, is one of the most popular surrogate safety measures intended as a generalised predictor 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 imitates evasive action (if at all). In its most basic form, constant velocity modeling \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 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 is thus less flexible in modeling as great a variety of potential outcomes without significantly larger quantities of observed data. Nevertheless, yPET is of interest as a model of yielding behaviour and merging aggressivity as it is greatly 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 users forming the crossing paths be successive in their arrival. If demand is low, some of these arrivals may be minutes apart and would thus obviously hold no value in interpreting interaction safety. To counter this a conservative minimum criteria of consideration $\zeta_{PET} < 5$~seconds 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).