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Publication Number: FHWA-RD-03-050
Surrogate Safety Measures From Traffic Simulation Models
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There is a significant amount of additional effort required to validate that the proposed surrogate measures can adequately assess the safety of particular intersection conditions. The proposed surrogate measures are largely not observable by an independent roadside observer with only visually subjective information on vehicle locations and speeds. Past studies on TTC estimation have used categories to judge the value of TTC in bins (i.e., "high," "medium," "low" to correlate with 0-0.5 s, 0.5 s-1.0 s, 1.0 s-1.5 s) (33). Video analysis could be used to improve the estimated speed, acceleration, etc. of vehicles involved in particular conflict events so that better estimates of TTC, PET, etc. could be produced. The issue, however, is not whether the surrogates can be replicated in a field study, but rather whether the surrogates are correlated with observable behaviors that indicate the safety of a traffic facility. This does not mean that the surrogates need to be correlated directly to the actual number of crashes or conflicts at a particular intersection, but rather that the relative differences (or perhaps rank order) of various intersection designs as evaluated by the surrogate safety methodology are correlated with a similar study with real-world conflict measurements.
Three hypotheses for surrogate safety measures from simulation models and a corresponding validation test approach for each are listed in this section. Each validation test includes an estimate of the level of effort (LOE) required for executing the test activities. The hypotheses for the utility of the surrogate measures are:
Hypothesis: Two different intersection designs produce different frequencies of traffic conflict events predicted by a simulation model. This indicates that one intersection design or strategy is more or less safe than another.
Positive Result: Validation that traffic simulation results could be used in evaluating proposed future alternatives for intersection redesign. Conclude that surrogate measure distributions are appropriate discriminators of relative intersection safety performance.
This hypothesis must be satisfied before the other hypotheses can be tested.
Hypothesis: High frequency of traffic conflict events predicted by a simulation model is correlated with high frequency of traffic conflicts as measured in a real-world study by the traffic conflicts technique.
Positive Result: Validation that traffic simulation results could be used to replace or augment traditional data gathering for safety analysis.
Hypothesis: Frequency of traffic conflict events predicted by the simulation model for a particular intersection improvement alternative is correlated with the actual change in the frequency of conflict events in the real world as measured in a real-world study.
Positive Result: Validation that the safety improvements predicted by the simulation model are not only relatively comparable (i.e., percentage improvements) across alternatives, but are also comparable in an absolute sense (total number of conflict events of particular types).Approach