\section{Methodology}

\subsection{Overview}

Figure~\ref{fig:1} outlines the general data collection and analysis methodology. Video data is collected for the study: for a cross-sectional study, at a number of sites with adequate representation of contributing factors and controlled external factors; for a before-after study, at one or more sites before and after a change in contributing factors.  With scene data and camera calibration parameters, feature tracking is performed to extract road user trajectories. The trajectories are in the form of series of positions of moving objects over time within the scene. This positional data is then processed to obtain derived measures such as speed, heading, and acceleration. Finally, data is analysed and interpreted in a variety of ways: i) simple summaries such as average speed and volume counts; ii) generalized spatial relationship analysis, such as surrogate safety analysis; or iii) high-level interpretation of behaviour relative to elements of the scene (typically specific to the study) such as gap times and motor vehicle infractions, etc. With a large number of potential contributing factors, it may be beneficial to apply site clustering techniques before initiating behavioural measure correlation.