Airbag Performance

(This post was published in 2012)

Airbags in automobiles are intended to provide additional protection to a vehicle’s occupants in a crash, the term ‘additional’ implying that seatbelts must be used as the primary safety device. Modern cars usually have multiple airbags at different locations in the vehicle and each of these airbags is designed to deploy when the crash is of pre-determined type(s) and of certain severity. On the other hand, if the crash parameters, measured by onboard sensors, are not within this pre-determined range for a specific airbag, the deployment is suppressed.

As a general principle, an airbag must always deploy when such deployment is likely to reduce the probability and the severity of injuries to the vehicle’s occupant(s) and in addition, the airbag’s design (size, shape, inflation rate, etc) should optimize the protection of car’s occupants. In practice, airbag(s) and sensors are engineered by each manufacturer for a ‘library’ of crashes that are likely to happen on roads. Some of these ‘likely’ scenarios have been prescribed by government agencies and by test organizations, but the rest are determined by each manufacturer.

Some Airbag-Related Reports:

  • A report from the Insurance Institute for Highway Safety (Braver et al “How Have Changes in Air bag 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 the federal safety standard.
  • According to reports in the New York Times and other publications,, airbag-related claims dominate the cases of accidents 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 NHTSA for the same period was of 576 fatalities occurring in frontal crashes where the front airbags didn’t deploy. ?
  • Advances in airbags continue and new possibilities continue to be explored by manufacturers (“Air bag arms race: Carmakers see who can add most safety advances ”http:// apps/pbcs.dll/article?AID=/20070503/ AUTO01/705030385″.

Below is an overview of sensor and airbag design principles. Statistical analyses of accident data are also presented to assess future trends in airbag performance.

AIRBAG OPERATION: When a vehicle is involved in a crash, onboard sensors measure the crash parameters from the vehicle structural response (e.g. acceleartion, displacement, etc.) and transmit these to an onboard proecssor. This processor compares the measured sensor data to pre-programmed values and determines whether deployment is desirable. In case of deployment, an electrical signal is sent to an inflator where propellants are ignited, generating gas and infalting the bag. Frontal- or Side-airbags remain inflated for a fraction of a second. For rollover airbags, the duration can be several seconds. One of the engineering chlaanges in airbags  is extremely short time intervals available for decision making by the onboard processor. Dependiing on the type and severity of the crash, this available time interval can be as short as a few milliseconds between the instant when crash occurs and the time at which bag must be fully inflaterd.

PERFORMANCE EVALUTION: Concerns regarding airbag performance fall generally into two categories –  (a) did the bag deploy properly,  and (b) did the bag provide optimal protection. Many of the complaints about airbags are of non-deployment when such a deployment would have provided additional safety. There have also been cases where the airbag deployed unnecessarily (‘unwanted deployment’) and caused harm to the vehicle’s occupant. In other cases, claims are of improperly designed airbags which deployed but provided inadequate amount of protection.

Some of the non-deployment complaints may be based on expectation that airbag(s) should deployed in all crashes that appear to be severe based on the visible damage to the vehicle. However, the correct engineering approach is to deploy based on probaibility of reduction in injury to the vehicle’s occupants. In the absence of this, deployment is suppressed, the reason being that an airbag can be deployed only once and for a few milliseconds only and it is therefore essential to avoid any other deployment.

How is Crash Severity Perceived? The current parctice in accident reconstruction is to measure the severity of a non-rollover crashe as its ‘ΔV’,  where ‘ΔV’  is defined as ‘the change in the vehicle’s speed during the crash’. There have been several studies of relationships between ΔV and the severities of occupants’ injuries and these have generally attempted to demonstrate that statistical relationships exist between these two parameters. However, significant questions remain regarding ΔV as the proper measure of crash severity and using it as the deciding factor in airbag deployment decisions.

It can be observed everyday driving events that ΔV is insufficient to describe a crash severity by itself. As an example, when a car travelling at 50 mph is stopped by using its brakes in an emergency, it takes a few seconds to come to a complete halt. In this case, the ΔV is 50 mph but no injuries will have normally resulted. But if the same car had come hit a rigid object and came to a stop, the same ΔV would have caused significant injuries.

Proper Definition of Crash Severity: A useful definition of accident severity requires that both the ΔV and the time during which this ΔV occurs be measured.

It should be stated that modern automobiles have onboard ‘event data recorders’ which retain much of the data about a crash when it occurs including airbag deployment data. Such data can be  measurements

Important Factors in Airbag Design

For detailed discussion of important factors that must be taken into account in investigating airbag design and performance, please see the full article here.. This article describes current crash sensing technologies and deployment decisions. Since these decisions are based on injury probability, data are also presented from crashes contained in national database. This is useful in discussing whether airbags should have deployed in a particular event.

Properly Working Airbag: Two factors must be considered in deciding whether the airbag performed properly in an event:

1. Reliability: Airbags and other occupant protection systems must have the highest possible degree of reliability that they will perform as intended. Such reliability is also implicit in US laws for crashworthiness (FMVSS) in form of requirement that all the sold vehicles pass mandated test(s). The total system reliability is the product of the constituents’ reliabilities and therefore, the vehicle’s manufacturer must consider the multiple components that are involved in airbag performance.

2. Performance: This implies that the airbag will meet the governing principles of deployment and when deployed, will optimize occupants’ protection. In order to achieve this, it is necessary to optimize the engineering of all the components that have significant influence in the injury outcome, such as – the vehicle’s structure, doors and pillars, seats, seatbelts, interior components, sensors, bag parameters, etc.

The performance of airbags in crashes is mandated by government regulations. Also, other agencies publish results of tests performed on automobiles.

Read the full article here