deletions | additions       diff --git a/Methodology Measures PET.tex b/Methodology Measures PET.tex  index 966ff16..8dd6f27 100644  --- a/Methodology Measures PET.tex  +++ b/Methodology Measures PET.tex 
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Instantaneous surrogate safety indicators such as speed and TTC are observed continuously. Currently, there is a lack of consensus in the literature on how to interpret continuous measures in terms of safety besides the event-based \cite{Hyden_1987,Svensson_2006} or safety continuum \cite{Zheng_2014} paradigms. One qualitative approach used in the literature has been to compare shifts in the probability distributions of these indicators when the magnitude of impact of individual indicators has no bearing on the overall direction of shift of the probability distribution of indicators \cite{Ismail_2010,Autey_2012,St_Aubin_2013}. For example a continuous mass shift of a probability distribution function as demonstrated in Figure~\ref{fig:distro-comparison} a) and b) may be indicative of an overall shift in traffic behaviour to safer levels after implementation of a counter measure, although there may be more complex phenomena related to driver awareness and learning as described at two sites in \cite{Svensson_2006}. Alternatively, changes in frequency of events meeting a given threshold may yield similar results. While traffic event conversion factors have been developed and used in the past \cite{Hyden_1987,Svensson_2006}, transferability of validated conversion factors has been cited as problematic \cite{Mohamed_2013}. Indeed, while the shape of probability distributions of TTC has generally been characterized as Gamma-like in the literature \cite{Ismail_2010,Autey_2012}, variations or compound effects can be found, particularly when traffic streams become mixed \cite{St_Aubin_2013}. One recent approach proposed a shifted gamma-generalized Pareto distribution model \cite{Zheng_2014}.  Indicator interpretation may depend on the precise definition of what constitutes a "traffic interaction". The disaggregated interaction approach treatsall  indicators as finite, with risk a representation  of collision adding incremental risk which add  up over time, but it tends to bias slower moving objects which dwell in the scene longer and complicates conditional probability calculations. The user-pair aggregated approach solves these problems but is sensitive to how the analysis area is defined and which users pairs are joined. Overall, a more detailed interaction exposure framework is still needed in the literature. Some simplifications have been made in the meantime. The traditional TCT methodology frequently represented user pair interactions using a single value of TTC, the minimum TTC ($TTC_{min}$). In practice, due to the imperfect nature of automated video data extraction, minimum values tend to oversample instantaneous tracking errors (e.g. a skipped frame), and so a 15th percentile approach is preferred \cite{St_Aubin_2015_TRBa}.