A mere five seconds' worth of data retrieved from a microprocessor embedded in a vehicle can affect the outcome of a trial. That's why attorneys would do well to appreciate the intricacies and limitations of event data recorders. EDRs installed in vehicle safety systems can reveal the details of crash events, and that information can help determine precisely what happened. Typical data collected by EDRs includes vehicle and engine speeds, throttle position, velocity changes, seat belt and airbag status, and the time elapsed from impact to airbag deployment. The newest generation of EDRs offers expanded recording intervals and data sets that include yaw rates (related to a vehicle's spin) and steering angle (the steering wheel's degree of rotation). But as with any emerging technology, the benefits of an event data recorder come with caveats that must be addressed. After all, when someone's fate rides on a data point, the accuracy of the information must be beyond reproach. The EDR's challenge is that it is technology developed by automakers but appropriated by an external audience demanding expanded functionality. The initial regulatory mandate for these devices in the 1980s was to verify the performance of airbag safety systems. But soon other parties - including transportation researchers, regulators, insurance companies, and attorneys - started to see EDRs as a source of real-world accident data. Indeed, today's EDRs developed by car companies address standards established by the National Highway Transportation Safety Administration (NHTSA) (49 C.F.R. §§ 563.1563.12), which have a compliance deadline of September 1, 2012. As it turns out, the EDR embedded in the airbag safety system is the only module the regulation affects - even though a tidal wave of microprocessor-governed safety, performance, and luxury features has overtaken the newest cars, trucks, and SUVs. Even if additional EDR data is available, however, there is no guarantee that the data will be accurate or complete. The technology is still developing, and current automotive EDRs are not yet as durable as the aviation models. At present no standards have been established for the type of data collected, sample rates, communications protocol, or module connectors. That is the goal of the NHTSA rule. But even as new vehicle models conform, U.S. roads are filled with 250 million or more cars that, even if they have EDRs, are not covered by federal rules. For the time being, EDR data is best used as an adjunct to a thorough accident investigation. These devices, while extremely helpful, are not substitutes for live testimony by an eyewitness or a professional accident reconstructionist. The analysis and interpretation of the module's data should be left to automotive engineers and accident investigators who understand the technology's limitations and will bring an expanded toolset to the analysis. What is more, crash data retrieval is not a do-it-yourself procedure, especially if the data is destined for a courtroom appearance. Issues of the chain of custody and spoliation pertain because claims of data loss or tampering are almost sure to emerge if the process is mishandled. Regardless, it is not realistic to assume the modules will deliver insights into every crash: Even if the vehicle involved has an accessible EDR, there are some accident circumstances where the module cannot deliver useful data or conclusive evidence. Such cases include accidents with known causes such as tire failure or known trigger events such as sideswipes. Crash data is most useful in accidents where the air bags deploy and the driver's behavior is in dispute. Typically, this will involve efforts to determine vehicle speed and seat belt use when traditional accident reconstruction practices, such as scene and vehicle inspections and momentum analysis, are incomplete or inconclusive. As for the admissibility of EDR data in court, it has cleared the Daubert threshold. (See Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993).) In one Florida criminal case, the downloaded data was used to prosecute a person who had been driving recklessly before a fatal collision (Matos v. State, 899 So. 2d 403 (Fla. App. Ct. 2005)). And in civil vehicle-defect actions, the data has served to clarify airbag-malfunction claims and allegations of sticking gas pedals (Bachman v. General Motors Corp., 776 N.E. 2d 262 (Ill. App. Ct. 2002)). Broader questions about EDR usage relate to data ownership and privacy issues. Thirteen states, including California, have consumer protection statutes that require an owner's consent to download EDR data. (See Cal. Vehicle Code § 9951.) Privacy issues have been addressed in several recent criminal cases dealing with the applicability of Fourth Amendment protections to warrantless seizures of the EDR. To date, there is no consensus among the courts. A year ago in People v. Xinos (121 Cal. Rptr. 3d 496 (2011)), the Sixth District Court of Appeal held that the defendant had a reasonable expectation of privacy in the data contained in his vehicle's EDR. As such, the data was protected by the Fourth Amendment. However, last May the California Supreme Court voted to depublish the Xinos opinion. Although the case cannot be cited as precedent, the unpublished opinion gives insight into how a future high court might rule when faced with this issue. (Bear in mind, though, that the state supreme court earlier upheld a warrantless search of the contents of an arrested individual's cell phone (People v. Diaz, 51 Cal. 4th 84 (2011)).) Clearly, the law in this area will continue to evolve with the expansion of retrievable data on electronic devices. It is crucial for attorneys to remember that an EDR provides facts, but not context. Stripped of the greater crash narrative, an event data recorder delivers only a portion of the truth. Peter R. Thom is the president of a national firm of consulting automotive engineers based in Orinda, California (www.prtassoc.com).