2007 4 Runner.

I was at a complete stop behind a car that had stopped for a school bus picking up kids. A big Nursery Dump truck slammed into the back of my car and pushed me into the car in front of me. The front end is completely f-ed, and everything was pushed so far back that the frame is sticking out the front. All the radiator fluid came out and it looked like some transmission fluid as well. The rear end is gone. It was pretty violent as the dump truck appeared to not have even braked before hitting me.

Should my air bags have deployed seeing how I rammed into the car in front of me?

Do you have any pictures of the damage? From my understanding, the sensors are g-force based, so if the impact was great enough, it should've set them off

However, I wonder if the older models like the 4th gen use an impact sensor, and it's possible the crash punctured the front, but avoided the crush sensors?

Hopefully someone with more knowledge can weigh in though

I think the setting on airbags is just about walking-speed (3ish mph).

Think about it: walk into a wall at walking-speed: it hurts and when you have the momentum of a few tons of vehicle it adds up.

Additionally in an accident there CAN be more than just one impact: whiplash/etc.

There's a couple variables I think, a minimum speed and g-force on the sensors. In your case no as you were stopped. See quote below.

Sorry to hear you had this happen, I hope all is well with no neck issues. Sucks seeing that coming up in your rear view with no place to go. Don't let them low ball you on the payout as replacements are not cheap.

"Frontal air bags are generally designed to deploy in "moderate to severe" frontal or near-frontal crashes, which are defined as crashes that are equivalent to hitting a solid, fixed barrier at 8 to 14 mph or higher. (This would be equivalent to striking a parked car of similar size at about 16 to 28 mph or higher.)"

Vehicle Air Bags and Injury Prevention | NHTSA

Getting rear ended, the seatbelt tensioners are first priority to help keep you firmly in your seat. With minimal forward speed, and a rear impact first, it may not have fired the airbags to avoid unnecessarily punching you in the face.

That seems like a slow threshold for detonating the airbags given bumpers, per fed regulation, are to have no impact damage at 2.5mph.

The airbag system has a preprogrammed criteria before deployment. The system factors in speed, decel rate, impact force etc. Airbags will not deploy in a rear end collision as when you get rear ended your body has the illusion moving backwards because you are stationary but the vehicle moves forward. Since the rear impact happened first and you were stopped with no decel the system will not deploy from the ft impact.

The damage is akin to shaping metal in a press vs a hammer.
You were pressed into the school bus.

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Hard to say honestly, there's a ton of variables there that are beyond what most technicians and service personnel will ever deal with. We can pull some data from SRS modules made in the last decade or so via the TIS Techstream, but the really granular data needs a special tool that Toyota sends out with an FTS or engineer in special cases.

I would take a guess and say that your pretensioner was popped at the very least, but the vehicle didn't feel the need to pop the airbags in that specific case because just using the pretensioner was enough (though if you weren't wearing a seatbelt, it's also possible the car would have deployed the airbags simply because it had no other options).

SRS Technology has advanced a lot in the past few decades, so it's hard to really say definitively how/when the vehicle decides to activate it's bag of SRS tricks unless you were the engineer who built it.

That's why I said ish. I was using the walking speed more as an indicator how easy walking into a wall hurts.

Regardless, you are right. I just looked on IIHS' website and this is their statement:

"Typically, a front airbag will deploy for unbelted occupants when the crash is the equivalent of an impact into a rigid wall at 10-12 mph. Most airbags will deploy at a higher threshold — about 16 mph — for belted occupants because the belts alone are likely to provide adequate protection up to these moderate speeds."

Now imagine RUNNING into a wall. Mess yourself up something fierce.

IIHS website

No.
They rarely (=almost never) deploy when rear-ended, mainly cause the actual deceleration rate (or, as in this case, acceleration) is not that high, irrelevant to the amount of damage, etc.

Having said that, if you're not an original owner, I'd double check if the airbags are actually there... although given the car is likely to be written off, it's moot at this point.