Identification and Simulation of a Common Freeway Accident Mechanism: Collective Responsibility in Freeway Rear-end Collisions


Gary Davis, Joel Swenson

April 2006

Report no. CTS 06-02

Determining whether or not an event was a cause of a road accident often involves determining the truth of a counterfactual conditional, where what happened is compared to what would have happened had the putative cause been absent. Using structural causal models, Pearl and his associates have recently developed a rigorous method for posing and answering causal questions, and this approach is especially well-suited to the reconstruction and analysis of road accidents. Here we apply these methods to freeway rear-end collisions. Starting with video recordings of accidents on I-94, trajectory information on a platoon of vehicles involved in a crash is extracted from the video record and these trajectories are then used to estimate each driver's initial speed, following distance, reaction time, and braking rate. Using Brill's model of rear-end accidents it is then possible to simulate what would have happened had, other things equal, certain driver reactions been other than they were. In each of three accidents we found evidence that: (1) short following headways by the colliding drivers were probable causal factors for the collisions, (2) for each collision at least one driver ahead of the colliding vehicles probably had a reaction time that was longer than his or her following headway, and (3) had this driver's reaction time been equal to his or her following headway, the rear-end collision probably would not have happened.

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