Black Box Data Retrieval
Background on the Event Data Recorder
Most people are surprised when they are told that a crash data recorder is tucked away out of sight in their car or truck. Even those who are aware of the existence of black boxes don’t realize that some vehicles have been equipped with the units for almost thirty years starting with the 1974 Cadillac. The application of air bags necessitated the existence of these devices in order to store the proper criteria and control the timing for air bag deployment. An assembly of sensors and associated microelectronics had to be mounted on the vehicle to determine when all the conditions were correct, as in a frontal crash, to blow the bag out.
Almost in step with increasing use of airbags was the more elaborate use of microprocessor controllers for engine management, emission reduction functions and antilock brakes. These in turn, required sensors to measure such parameters as engine speed, wheel speed and throttle position. These and other device additions converged on enough vehicle models by the early 1990’s to provide the foundation for recording various data items in a rolling memory. The recorded data in memory is continually revised and updated as old data is discarded so that a five-second snapshot is always available and is frozen in memory when a collision occurs.
The Event Data Recorder
The Event Data Recorder (EDR), as the rolling memory is called, has its functionality actually incorporated into the air bag controller which is the Sensing and Diagnostic Module (SDM) on GM vehicles or Restraint Control Module (RCM) on Ford products. That’s the unit which, in some manner, has always been needed to determine when the bag should deploy in an accident situation. In its present day form, a typical module looks like the one shown. It’s important to be aware that every car or truck which has at least one air bag also has an event data recorder of some type. As with most electronic modules where only the input and output connections are of interest, the EDR is often called a “black box.” Since the accelerometers, the devices which sense the amount and direction of vehicle motion are packaged together with all the electronics, the location of the module in the vehicle must be near its center of gravity. That requirement normally leads to placement under either the left or right front seat as shown, or inside the center console. Not apparent from the photo is the fact that the carpet must be cut away to remove the unit or even get to the multi-pin connector to download the recorded data to an external computer.
Data Downloading
The process of downloading data, which is coming to be called data “harvesting,” can be accomplished either at the vehicle with the black box still in place or by removing the EDR. Regardless of the method chosen to physically connect to the EDR module, the read-out hardware and software remain the same as shown. The data conversion, interpretation and display software is, of course, loaded into a PC of only modest capability. For convenience, a laptop is generally used.
Data Content
The basic information of interest for accident reconstruction is:
1. What was the status of critical vehicle parameters just before the accident took place?
2. How quickly did the vehicle slow down to a stop after the collision?
3. Was the seat belt connected?
4. Did the driver apply the brakes?
5. Did the driver apply the throttle?
6. What was the change in velocity (Delta-V) due to impact?
Court Admissibility
There are many highly visible precedents which clearly show that the courts readily accept EDR data. Even as far back as 1992 in a case involving Jerome Brown of the Philadelphia Eagles, the court denied a claim by his family that his car had hit a pothole and the airbag deployed prematurely causing him to lose control of the car and his life. Reconstructionists, using the EDR, proved that Brown was speeding, lost control and struck a tree before the air bag deployment. More recently, in February of 2003, the court accepted EDR evidence that a woman in Ipswich, Massachusetts was speeding when her car slid off an icy road, struck a tree and killed her passenger. Then, in May of 2003, the EDR was used to prove that a Pembroke, Florida man was traveling at nearly 120 mph when his car hit and killed two young girls.