A comparison of collision probability threshold is also performed on motion patterns and demonstrated in Figure \ref{fig:hourly_traffic_events_mp_prob}. Constant velocity yields a consistent, though relatively rare, collision probability of exactly 1.0 (a single collision point), while normal adaptation yields anywhere between 1 to 100 collision points with sum of probability of collision of 1.00. They are both therefore ignored. For clarity of comparison, the sites have been reordered by motion pattern, using the unique 15th percentile indicator aggregation and thresholds \(p_{threshold} = 0.001\) and \(i_{Threshold} = 1.5\) s. In comparison, a \(p_{threshold} = 0.01\) yields important decreases in the number of hourly events reported, and, more importantly, changes site ranks.
As depicted in Figure \ref{fig:ttcfreq_aggreg_comparison}, unique minimum values have the problem of over-representation of low-TTC outliers (especially in the the 0-0.25 s TTC range) due to data noise and possibly instantaneous tracking errors. The effect seems slightly worse for motion pattern prediction, which does add a second layer of discretization to the analysis. Unique 15th percentile generally corrects this error.
Overall, normal adaptation offers little benefit over constant velocity. Meanwhile, the increase in low TTC detection can be seen for all indicator aggregation methods.
Finally, sites are clustered by geometric and built-environment similarity in Figure \ref{fig:ttcfreq_cluster_comparison} to illustrate some minor discrepancies in interpretation, although not nearly as prominent as in the case of site ranking by event thresholds. Most prominently, following the methodology outlined in \ref{indicator_distributions}, interpretation of cluster _cl_1 changes significantly between aggregation and prediction methods, while the interpretation of cluster _cl_5 changes according to the prediction method. The interpretation of the other clusters are mostly unaffected.