Airbags are designed to deploy only when they might be needed to prevent serious injury. In order for airbags to be effective they must deploy early in a crash; in a frontal crash this typically occurs within the first 50 milliseconds (0.05 seconds). A vehicle's airbag control module relies on feedback from crash sensors to predict whether an event is severe enough to warrant an airbag deployment.
Frontal airbags: Frontal airbags are designed to inflate in moderate to severe frontal crashes. Among airbags with advanced features that include a safety belt sensor, there are different inflation thresholds depending on whether people are using their safety belts. One threshold used by airbag designers is "must deploy" which includes a situation such as an impact into a rigid wall of 10-12 mph for unbelted occupants. The "must deploy" threshold is slightly higher - about 16 mph - for belted occupants because the belts alone are likely to provide adequate protection up to these moderate speeds. Frontal airbags may deploy to help protect occupants in side impacts if there is sufficient forward deceleration during the crash. For example, if a vehicle is moving forward at the time of a side impact, frontal airbags can help prevent serious injuries.
Advanced airbags compliant with government crash performance standards have been required in all passenger vehicles effective with model year 2007. Advanced airbags are designed to suppress deployment if weight sensors in the seat detect that a front-seat passenger is small or in a child safety seat. Advanced airbags also can deploy at a lower energy level or pressure when passengers are small or out of position, or if the crash is of very low severity.
Side airbags: Because of the small space between an occupant and the side of the vehicle, side airbags must deploy very quickly to cushion occupants from intruding vehicles or objects. Some airbags typically deploy within the first 10-20 milliseconds of a side crash. "Must deploy" thresholds can be as low as 8 mph for narrow object crashes (e.g., trees and poles) and 18 mph for the more distributed side crashes (vehicle-to-vehicle crashes). Several auto manufacturers deploy the side airbags in frontal crashes to help control occupant movement during the rebound phase of a crash. Some curtain side airbags may stay inflated longer to protect occupants in rollover crashes. Allowing the airbags to remain inflated or triggering their deployment during a rollover can help prevent full or partial ejection of occupants.
Source: Institute for Highway Safety