How is an interaction defined? In the simplest definition, it is a pair of two road users existing simultaneously and closely in space. To illustrate the constraints, a time snapshot of a series of road users interacting at a roundabout merging zone (the scene) is presented in Figure \ref{fig:timehorizon_interaction}. In this example, road user \(B\) is physically separated from and cannot reach or be reached by all others, except for \(A\) who may or may not exit the roundabout at this point in time. In addition, road user \(B\) does not even cross the merging zone and is therefore not analyzed at all. \(A\) might cross the merging zone while \(C\), \(D\), and \(E\) are or will be crossing it. However, \(A\), \(D\), and \(E\) lie outside the motion prediction time horizon, so only interactions between \(A\) and \(C\), \(C\) and \(D\), and \(C\) and \(E\) are considered at this time. The time horizon is calculated using the motion prediction employed. The above example illustrates the methodology at an instant in time and describes interaction instants. These road users are in fact interacting over several time steps, termed “user pair” or simply “interaction”. Analysis of the evolution of indicators over these time steps is time-series analysis.
Some other points should be mentioned:
visual obstructions or distractions could affect the outcomes of indicators, by adding noise or preventing road users from being partially or completely tracked.
the region of analysis (i.e. camera space if obtained from video data) can have a bounding effect on indicators. Because indicators are predictions, it is important that the analysis area be completely enclosed by the camera space and that sufficient upstream distance be provided for indicators to be generated at the merging zone.
it is possible for \(C\) to obstruct the movement of \(D\) in relation to \(A\). These are special cases involving interactions with more than two road users that will have to be handled in the future, when the methodology matures with more sophisticated prediction methods.