The Accuracy of EDR Information
Every HV EDR reports something it calls “Vehicle Speed.” However, it is not as simple as that. The EDR is actually measuring the rotational speed of the output shaft of the transmission. This measured speed is then used along with other inputs (such as the tire size and rear axle ratio) to compute a wheel speed. This wheel speed is interpolated to produce a "Vehicle Speed."
In many circumstances, these data are very accurate. Most HV EDRs sample data between 5 and 100 times per second, and all HV EDRs except Mack report in whole-second increments. Several individuals have studied HV EDR accuracy and have found that HV EDRs can be accurate to within (+/-) 1 mile per hour under the right conditions.
There are, however, several known cases where HVEDR data are inaccurately reported. Some Caterpillar Quick Stop data may be reported at 1 sample per second when it was actually measured and recorded at 0.48 samples per second. Late model Cummins engines may report data at 1 second intervals which were actually recorded at 5Hz.
Ideally, a thorough inspection of the vehicle should be performed in order to help corroborate the EDR data. This inspection should be performed by an individual who has significant knowledge of HV EDRs and how they work. When it is impossible to conduct a thorough inspection, it may be necessary to review maintenance records of the vehicle to determine whether physical characteristics of the vehicle match their corresponding programmed parameters.
HV EDR data should match up very closely with the available physical evidence and an accident reconstruction based on valid scientific or engineering principles. It is important to understand that although HV EDR data are extremely useful, they do not always provide the investigator with a "What You See Is What You Get" type of situation, and therefore, data should not be interpreted in a vacuum.
When interpreting HV EDR data, the investigator should follow the advice given by W. R. Haight regarding passenger car ACM data: "Always perform a situationally appropriate accident reconstruction."
**Every crash and every EDR is different, and not all crashes will provide the specific data an investigator may hope for. However, in some cases, the absence of data may be very significant.**
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