Dear Tom and Ray,

I was involved in a very serious accident a month ago. An 18-wheeler hit me from behind and sandwiched me into a van in front of me. The entire front end of my car (1996 Cadillac Eldorado) was smashed in, yet the air bag did not inflate. Even though I had my seat belt on, my head swung forward and hit the steering wheel. I was very upset that the bag did not inflate, but the dealer claimed that even though all the sensors were hit (to put it mildly), air bags will not inflate unless you are going at least 35 mph. And they will rarely inflate if you are hit from behind. Is what he told me true? -- Lena

RAY: Sort of, Lana. Normally, when you're hit from behind, you're pushed backward into the seat -- rather than forward into the steering wheel -- and once you're pushed back into the seat, you're supposed to be held in place by the seat belt. How your head hit the steering wheel, I'm not sure. Perhaps the seat-belt mechanism failed and didn't lock to hold you in place?

TOM: But since the primary impact of a rear-end collision moves you backward, the air bag is not designed to go off in that case.

RAY: Now, if you're in a "sandwich" collision -- like my brother was with a pastrami on rye the other day -- where you're hit from behind and pushed into something, the air bag CAN go off. But it will only go off if the frontal part of the collision occurred while the car was traveling more than 15-20 mph or so. In other words, if the front end was slowly mushed, like yours was, the air bag would likely not go off -- especially if the car used inertial sensors.

TOM: Without an accident reconstruction team, we might never know exactly how fast the front of your car was traveling when it hit the back of the van.

RAY: So, we can't say for sure whether the air bag performed correctly when it didn't go off, but circumstances suggest that it did just what it's programmed to do.

TOM: And the fact that you're alive to write to us is further evidence that things worked as they should have. Aside from the Cadillac wreath emblem that's permanently embossed in your forehead, it sounds like everything has worked out all right. We hope so, Lena.