Development of Analysis Methods Using Recent Data (2012) / Chapter Skim
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Pages 78-89
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From page 78... ...
78 Outline of a Causal Theory of Traffic Conflicts and Collisions Gary A Davis, John Hourdos, and Hui Xiong Department of Civil Engineering, Minnesota Traffic Observatory, University of Minnesota Using recent developments in causal analysis, a minimal model capable of rigorously representing traffic conflicts and crashes is constructed.
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79 Hayward (1972) introduced the use of time to collision (TTC)
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80 and combinations of TTC versus RR were used to define boundaries between nearcrash and conflict situations. Since range rate can also be interpreted as closing speed (CS)
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81 how these formal results might be applied to actual crash and nearcrash events. Causal Model of Crashes and Conflicts Our starting point is Pearl's (2000)
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82 The distance separating the major approach from the col lision point at this time is y u x y v t r t y v t a t r, ˆ , ˆ ˆ ˆ( )
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83 Here the notation P(Uj) has been used as shorthand for P(u∈Uj)
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84 This ratio will tend to be large for subsets where the minimum successful evasive action comes from the righthand tail of the distribution of evasive actions. That is, other things being equal, crashes should tend to have extreme values of the eva sive action, and this is what Figure C.4 shows.
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85 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 crash 1 crash 2 crash 3 slope=1 line 1.0 0 A2g 1 A10g 1 A13g 1 Aeq 1 1.00 Amin2g 1 Amin10g 1, Amin13g 1, Aeq 2, Figure C.4. Minimum versus observed decelerations (in g units)
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86 In this paper, we postulated a minimal theoretical structure that incorporates crashes, near crashes, and conflicts; defined rigorously what we mean by a surrogate event; and then derived a relationship between crash propensity and the sur rogate. Our treatment produced a natural, onedimensional method for grading the severity of noncrash events in terms of their minimum successful evasive actions.
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87 Hauer, E
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88 P y yc y u x P x x u P u y u i j j n i j i i<( )
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89 and since ˆ , ˆ , , , , , y u x y u x if u U k j if u U k j j k k − ( )
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