deletions | additions       diff --git a/Methodology Measures PET.tex b/Methodology Measures PET.tex  index 84a6af5..661761d 100644  --- a/Methodology Measures PET.tex  +++ b/Methodology Measures PET.tex 
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\subsubsection{Post-encroachment time}  While prediction models and TTCs motion collision potential, other surrogate safety measures aim to measure collision proximity from crossing, but not necesarily necessarily  colliding movements. Trajectory data is detailed enough to provide gap acceptance time (GT) and post-encroachment times (PET). These are measures that broadly characterise how aggresively aggressively  merging and crossing tasks, respectively, are performed. As such, there is generally only one of these measures for the entire common time interval of a pair of road users. Gap acceptance time and PET fall under the category of high-level interpretation measures as the calculation of these measures cannot be generalised for all traffic studies, in part because the behaviour does not apply to all types of traffic interactions, and, in the case of gap acceptance time, because the measuring method may vary from one type of geometry to another. For the crossing zone defined by the intersection of the two trajectories of a pair of road users, the post-ecnrochment post-encroachment  time measures the time between complete departure of the first arriving vehicle, and first arrival of the next arriving vehicle. If $PET = 0$, a collision ensues. As such, higher PETs should demonstrate safer behaviour, although not necesarily necessarily  linearly. An alternative to PET is predicted PET (pPET) which is measured from motion prediction instead of direct observation \cite{Mohamed_2013}. PET is, along with divergence, the complement to situations involving a TTC for any type of interaction. Gap acceptance time similarly measures arrival and departure of a road user at a common crossing zone, but in this case, the crossing zone occurs inline in-line  during a merging task, usually followed by following behaviour.