Each impact can have it's force broken down to an x-component and a y-component. Air bag sensors only sense decceleration forces in these two directions.

The data you provided from IIHS and HLDI is probably a result of extensive research showing that real world crashes are rarely as simple as barrier testing. I'm glad to see some real world situations are considered when designing air bag deployment.

One thing that hasn't been mentioned is the importance of seat belts. Air bags protect you from a single hit. Some crashes involve multiple hits. In these situations air bags fail to do thier jobs.

One tidbit that I've learned is that the human head is less tolerant of side forces than forward forces, meaning side forces are more fatal than an equivalent forward force. This is why I'm glad to see side impact bags gaining in popularity.

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