Airbags in modern automobiles have been around for many years, and their record in improving occupant safety in crashes is generally well accepted. However, many questions still arise regarding their deployment and performance in crashes. Some of these questions and the underlying issues, as well as statistical analyses of accident data, are presented in this article.
Recent Airbag-Related Reports:
A report from the Insurance Institute for Highway Safety (Braver et al., “How Have Changes in Airbag Designs Affected Frontal Crash Mortality?” IIHS, February, 2010) found that the newest airbags provide ‘suboptimal protection’ for belted drivers. This study concluded that the mortality rate for belted drivers was higher in airbag-equipped vehicles that comply with the law for frontal crash protection (FMVSS208, Advanced Airbag Rule) than in vehicles not certified as meeting this latest version of FMVSS.
According to a recent story in the New York Times (McGinty, J. and Maynard, M. “Speed Control a Small Factor in Car Claims.” NYtimes.com. 8 Apr. 2010.http://www.nytimes.com/2010/04/09/business/09warning.html?_r=1&emc=eta1), airbag-related claims dominate accident cases with injuries or fatalities as reported in the Early Warning Records submissions.
Two reports in the Kansas City Star (Casey & Montgomery, “Airbags raise new alarms – Some didn’t deploy in fatal crashes, newspaper learns”, October 21, 2007; “Front airbags don’t inflate in hundreds of crashes”, October 22, 2007) estimated 1400 fatalities during the years 2001 to 2006 in frontal crashes due to non-deployment of frontal airbags.
The estimate from the NHTSA (National Highway Traffic Safety Administration) for the same period was for 576 fatalities occurring in frontal crashes where the front airbags didn’t deploy.
The concerns regarding airbag-equipped vehicles generally fall into three categories:
- airbag(s) did not deploy in the crash where it was expected (by the vehicle occupants) to deploy (‘non-deployment’ claims)
- airbag(s) deployed in the crash where they were not expected to deploy and caused injury to the occupants (‘unwanted deployment’ claims)
- airbag(s) deployed properly in the crash but did not provide the expected amount of occupant protection (‘deficient performance’ claims)
In order to consider these claims, it is necessary to understand the engineering principles governing airbag deployment and performance. A brief and highly generalized overview is presented below. Due to the multiple types and configurations of sensors, airbags, and control units in the field, specific statements will require detailed accident analysis.
How Do Airbags Work?
It is not generally understood that a vehicle’s structure is the most important factor in making deployment versus non-deployment decisions. In a crash, the vehicle’s structure experiences high deceleration (or acceleration) as the vehicle’s velocity changes rapidly. Sensors attached to structural components (such as floor, pillars, etc.) measure these responses and transmit them as electrical signals to a central control unit. This unit compares the gathered data to pre-programmed information and decides whether deployment is required. If deployment criteria are met, an electrical signal is sent to the inflator unit which ignites the propellants. The gas thus generated inflates the airbag. For frontal or lateral impacts, the bags remain inflated for a fraction of a second. For rollover airbags, the duration can be several seconds.
Front crash sensors are usually accelerometers attached to the vehicle’s floor underneath the front seat(s). In many vehicles, the sensor and the control unit are integrated into one box referred to as a ‘sensing and diagnostic module.’ Some automobiles also incorporate additional forward sensor(s) that are attached to structural members underneath the hood and closer to the front end of the vehicle. These additional sensors are intended to detect localized impacts. Sensors for side airbags also are generally accelerometers that measure the lateral response at the attachment location. The sensors for determining imminent rollovers are often angular velocity or gyroscopic sensors, usually mounted on the vehicle floor near the center of gravity.