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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 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 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 part 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 time measure the time between complete departure of the first arriving vehicle, and first arrival of the last arriving vehicle. If PET < 0, a collision ensues. As such, higher PETs demonstrate safer behaviour.