Methods of Prospective Evaluation

This section discusses a variety of rather theoretical methods of prospective evaluation fault tree, traffic conflict technique, operational field and effectiveness in the field, and scenario technique. Section 2.6 focuses on case-by-case prospective methods. 

The basic idea of analyzing the traffic system with respect to the genesis of mistakes, conflicts, and accidents is stractured in he fault tree method [45], see Fig. 1.1 (p. 3). This scheme follows a process-oriented approach as displayed in Fig. 1.2 (p. 4). The basic concept of Reichart was thus extended beyond the basic elements of driver, vehicle, and environment to include, for example, driver assistance systems [42, 46], One advantage of a fault tree is that once a critical set of probabilities is known, the calculation of the other probabilities is straight forward using the Boolean connections in the tree. 

In this context, the top event in a fault tree can be an accident or a conflict [45]. The probabilities for accidents can come from classic accident analysis [42], the corresponding ones for conflicts or mistakes (being at the other end of the tree structure) are not generally known and hard to extract [35, 42]. An example of such calculations as well as further information, for example, on validity of the method, can be found in [45]. The method seems to be able to generate sound results, especially on the connections between conflicts and accidents, although many assumptions are basically needed during evaluation [36]. 

As conflicts can be top events in fault tree analysis or generally constitute rather high level events, the probabilities and nature of conflicts is regarded an important issue within the literature. One way to assess conflicts in traffic is the so-called traffic conflict technique [35]. A traffic conflict may be characterized by considering approaching object trajectories which, extrapolated in time, would exhibit an increased probability for collision unless one of the participants changes his current state of motion [35]. This definition could be extended on non-observable situations and single vehicle conflicts. 

The objectives of this standardized observational technique are risk assessment as well as effectiveness evaluation of traffic facilities, not estimations regarding the quantity of accidents [35]. Thereby, conflicts have a probability to become accidents, which does not mean that accidents can be predicted with the method [35]. The transition probabilities between conflicts and accidents, as needed, for example, in the above-mentioned fault tree analyses, can be assessed [42]. Compared to accident analysis, investigating conflicts has the following advantages [35]  

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