Abstract
In order to overcome the shortcomings of crash data a number of surrogate measures of safety have been developed and proposed by various researchers. One of the most widely used temporal indicators is time-to-collision (TTC) which requires the road users to be on a collision course. Road users that are strictly speaking not on a collision course actually might behave and take evasive actions as if they were, thus indicating that such near-miss situations might also be relevant for safety analysis. Taking that into account, a more flexible indicator T2, which does not require the two vehicles to be on a collision course, describes the expected time for the second road user to arrive at the conflict point.Recently extreme value theory (EVT) offering two approaches, block maxima (BM) and Peak over Threshold (POT), has been applied in combination with surrogate indicators to estimate crash probabilities. Most of this research has focused on testing BM and POT as well as validating various surrogate safety indicators by comparing model estimates to actual crash frequencies. The comparison of collision course indicators with indicators including crossing course interactions and their performance using EVT has not been investigated yet. In this study we are seeking answers to under what conditions these indicators perform better and whether they are transferable. Using data gathered at a signalized intersection focusing on left-turning and straight moving vehicle interactions our analysis concluded that the two indicators are transferable with stricter threshold values for T2 and that POT gives more reasonable results.
Highlights
We provide a literature review on surrogate measures of safety focusing on two particular indicators time-to-collision (TTC) and T2, give an introduction to extreme value theory as well as specify the research gap and questions.1.1
A more flexible indicator T2, which does not require the two vehicles to be on a collision course, describes the expected time for the second road user to arrive at the conflict point
The supplementary indicator proposed by Laureshyn et al (2010) measures the expected time that it takes for the second road user to arrive at the potential collision point, it is called T2
Summary
To improve traffic safety and to make sure that it is done in an efficient way, one has to be able to quantify safety in order to support evidence-based policy making. Other conditions, such as vehicle type, road users, collision angle and speed (Laureshyn et al, 2010) and weather may affect this relationship It has to be noted, that Svensson limited the events in the hierarchy only to interactions with a collision course (Svensson, 1998; Svensson and Hydén, 2006). The supplementary indicator proposed by Laureshyn et al (2010) measures the expected time that it takes for the second road user to arrive at the potential collision point, it is called T2 The logic behind this indicator is that TTC assumes the two road users to be on a collision course, which sets a limitation to the situations to be considered in safety analysis. A general introduction to EVT can be found in the subsection
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