Airbag Control Module

Case Synopsis:  Operator/Vehicle 1 (OV1) was traveling northbound (NB) approaching a line of stopped cars at an intersection.  Operator/Vehicle 2 (OV2) was also traveling NB in the same lane, behind OV1.  A collision occurred when the last of the stopped vehicles was rear-ended by OV1 and OV2.  OV1 claims he brought his vehicle to a stop behind a line of cars, was rear-ended by OV2, and pushed into the rear of the stopped car.  OV2 claims that OV1 rear-ended the stopped vehicle in front of it and then he hit the back of OV1.  No evidence was left on the roadway as a result of the collision and the damage to the vehicles did not indicate any specific order of impact.  Based on the physical evidence alone, a determination of the order of the collisions was not clear.

Expert Analysis: The airbags of OV1 were deployed as a result of the collision.  The airbag control module [ACM] was preserved and the data was downloaded using crash data retrieval [CDR] software.  The report generated from the downloaded data indicated two separate events were recorded- a deployment event and a non-deployment event.  The non-deployment event was an event in which the ACM “woke up” (sensed a deceleration), but decided that deployment of the airbags was not necessary.  The deployment event was a separate event in which the rate of change of deceleration was such that the ACM made the decision it was necessary to deploy the airbags.

Conclusion: The accelerations sensed by the ACM during the non-deployment event were determined to be consistent with the vehicle being struck from the rear, and the accelerations sensed by the ACM during the deployment event were determined to be consistent with a frontal impact.  The report further indicated that the non-deployment event occurred approximately 2 seconds prior to the deployment event.  This data recorded by the ACM supported the claim of OV1 that he was rear-ended prior to striking the stopped vehicle in front of him.