What is an Event Data Recorder (EDR)? In short it is the "black box" of the vehicle. It has the capability to save pertinent crash data before, during, and after a crash. After the crash critical safety functions such as airbag deployment and/or seatbelt pretensioners are powered, then crash data is saved to the computer's memory. If the battery or electrical system is compromised soon enough in the collision the crash data information may not be saved entirely or in part. Depending on the vehicle, it may record a vehicle's critical information before, during or after the crash event. The term EDR is the generic term for all types of vehicle event data recorders. The Airbag Control Module (ACM) controls the operation of the airbag and other involved restraint systems. Two other terms are RCM and SDM, with RCM (Restraint Control Module) being the Ford designation and SDM (Sensing Diagnostic Module) being the General Motors designation for the Airbag Control Module.
"Most vehicles driven today have an EDR which records useful information as to how the crash occurred."
The EDR normally shows information about the vehicle in the seconds leading up to the collision. It generally gives the information in discrete time intervals before the collision occurred. These times are in tenths, half or full second intervals that give a snap shot of what the vehicle was doing leading up to the collision. Common information given includes:
- Vehicle Speed
- Engine RPMs
- Brake Status Switch Circuit State (including brake pressure)
- Engine Percent Throttle
- Accelerator Pedal Position
- Cruise Control Status
- Steering Angle
- Seatbelt Status
The above list is not meant to be complete but is typical of some of the information reported. There is other important information often given as well. With the many different years, makes, and models of vehicles on the road today it is necessary to determine what information is available for the particular vehicles involved.
"Using the speed and time information provided by the EDR, the distance away from the collision at each time increment can be calculated. "
If the steering wheel angle and yaw rate is available the lateral position of the vehicle can be tracked as well. The table below shows the EDR record of the time and speed at ½ second increments starting with the 0.3 seconds before the trigger at the collision. The trigger point occurs very soon after the first touch of the collision. The distance that corresponds to each time period was calculated by the investigator.
Using the recorded speed from one time point to the next permits the calculation of how far the vehicle traveled during that increment. This calculation is made using two formulas.
First the deceleration factor is determined for the time increment:
With this value the distance traveled during the increment can be found using the formula:
With: Vi = initial velocity, Vf= final velocity, t = Time, f = deceleration factor, and g = acceleration due to gravity (32.2 ft/s/s). The speed or velocity is converted from mph to feet per second by multiplying the speed by the conversion factor of 1.467.
The final speed is at the trigger (TRG) point and the starting speed is at the various time points prior to the trigger. The calculation utilizes the final and starting times to calculate the distance that is desired. For instance if the distance away from the collision at which the vehicle was located when the brakes were first applied is needed then the two times used would be the Trigger point time 0 and the start of braking point somewhere between time -.08 and time -1.3 would be used. Although in the table above the brakes first show being applied at -0.8 the vehicle speed drops from 57.3 mph to 47.5 mph at some distance between -1.3 and -.08. This amount of speed reduction in ½ second shows heavy braking for the entire time, so even though the brake switch does not show that the brakes were applied until after the -1.3 second time point it had to be applied a short time before in order for the braking system to be able to drop the speed in the way that it did. The shorter amount of time the brakes were applied would mean that harder braking would be needed. If the time that the brakes were applied is too short then the amount of braking would be outside the capability of the vehicle's braking system. So the distance calculations are approximate but are within an acceptable range of error.
In the illustration above the driver was on the brakes at a point close to -1.3 seconds before the collision and before the driver could apply the brakes the threat would have had to be recognized and the decision made to apply the brakes. Depending on the circumstances, perception / reaction times can vary. Factors such as age of driver, night or daytime, simple or complex decision among other factors can influence a person's perception / reaction time. Using a 1.5 to 2 second perception / reaction time for this example would correspond to a time of approximately -2.8 to -3.3 seconds on the before mentioned EDR information. At -2.8 to -3.3 seconds the vehicle is approximately 212 to 255.3 feet away from the collision. If in this case the speed limit was 55 mph and there was a vehicle that pulled out from a side road in front of the vehicle with the EDR, using a 1.5 to 2 second perception / reaction would have been reasonable and the crash may have been unavoidable. If in another case though the speed limit was 35 or 40 mph, then excessive speed of the oncoming vehicle may have been a factor to consider. At 35 mph with a 1.5 to 2 seconds perception / reaction time a vehicle can be stopped using normal braking in 179 to 205 feet.
The EDR data along with speed / time / distance calculations can help to determine the cause of a crash.