Common EDR Misconceptions

In the years since the early 1980s when I first began performing traffic crash investigations, I have seen many changes in the techniques used, as well as the attitudes toward those investigations. For one thing, the term “accident” has been largely replaced with the term “crash”. In the late 1980s after I had gone beyond basic investigation into crash reconstruction, I discovered an entirely new world of tools and techniques to use in the craft. I experimented with a number of different tools along the way, trying to improve my methods of data collection and documentation. There were also changes in the way basic analysis was approached. The original computer algorithms that were out there began to generate more advanced computerized programs which allowed one to do more repetitive calculations which allowed one to easily examine what a small error in either the data collection or math calculations would do to the final outcome. In many cases a very small, even minute, error in a particular datum would result in a huge difference in the final answer. In other cases, a pretty wide latitude could be taken with only minor changes showing up in the result. As with any calculation, if you put in garbage, you get out garbage. However, in dealing with a traffic collision, there will invariably be something missing that cannot be accounted for fully. Many attempts have been made to close the gap on any error but I do not believe data errors can be fully eliminated. Any crash reconstructionist is, after all, human.

One of the latest technologies added has been “Event Data Recorders” that are now mandated in all vehicles in the United States. However, grave misconceptions abound concerning exactly what they are and what they do. More and more often, I and my colleagues are being approached by both law enforcement and attorneys or insurance companies looking for someone who can read the “black box” in a car. To begin with, cars do not have a “black box” but they do have a system which includes the ability to possibly record some data concerning a particular “event” such as a crash. This is most often contained in the airbag control module of the vehicle (ACM) or a similar module, regardless of the name used by the vehicle manufacturer. In some vehicles it is called a restraint control module or RCM and may go by other names such as SDM, as decided by the maker of the vehicle. But most often it is a part of the safety restraints installed in the vehicle. Recording data is a secondary function not the primary one. The primary purpose of this module is to monitor what is going on with the vehicle and determine whether or not safety restraints need to be deployed, and if needed to deploy those restraints. For example, the system may decide there has been an event that requires to be monitored but no restraints are needed. In that case there may be a “non-deploy event” recorded but no action is taken. That event would have been written into a “volatile” memory, meaning that it can be overwritten by another event that requires that space in memory. Most modules now can retain multiple events but there is a limit to the number of events recorded. That number of events is arbitrary and may vary from one make and model to another. In any event a section of memory that is volatile is subject to be written over by another event that comes later in time or is more severe. A more severe event requiring the deployment of safety systems such as seatbelt pre-tensioners, airbags, or side curtain airbags, is written into a nonvolatile memory and cannot be overwritten. If for example, the arbitrary number of events that can be recorded by a particular module is three and there are three events already written into the memory before a deploy event occurs, then generally speaking the oldest of those events will be overwritten by the “deploy event”. You will then have one “deploy event” and two “non-deploy events” recorded in the module but they may be separated in time and or “Key Cycles”, which are also recorded. It should also be noted that in most cases an event can be overwritten or erased after a pre-determined number of key cycles, usually about 250 key cycles.

The next misunderstanding that I frequently run across is the idea that whatever is recorded in that module is “the gospel truth” and nothing else need be done. This is by far the biggest misunderstanding that people have. The information recorded in that module is but one piece of a much larger puzzle. If you look at the report generated by the CDR tool? which is the most common tool for passenger cars and light trucks and read the information straight from it without better knowledge you would think that was THE speed of the vehicle. And in some cases, it might be, but in others it is not. You cannot just take that information at face value. Determining what is in that report is reading intensive and each section of the report must be compared with the rest of the report. Not only must each section of the report be compared with the rest of the report, but it must also be compared with other known facts, and data from the scene, the vehicles, witness statements, driver statements, and other factors involved in the total crash scenario. And each crash scenario is different from every other one. Never look at an Event Data Recorder report and think that you have all the answers! If anyone tells you they can look at that report and tell you exactly what happened they are sadly mistaken. Next time you’re looking into a case that involves a vehicular crash remember that we can obtain a lot of information from the electronic data that is contained in the vehicles, but also remember that it does not tell the whole story or fully speak for itself. If you employ me, or any other reconstructionist worth his salt he’s going to tell you that he needs much more than just the CDR report alone to give you the answers you require. An attorney, an insurance company, a law enforcement agency, or any other entity faced with determining what happened in a vehicular crash must not hamstring their expert! You have to give them the rope they need to run, and let them run. It will be well worth your investment to avoid the embarrassment to both you and your expert of being knocked down in the courtroom.