Unlike most makers, Mercedes sees airbags as the final 10% required for severe impacts.
They engineer their airbags to deploy in certain situations:
1. A very high threshold if your seatbelt is buckled. If you're unbelted (Attention Yanks...) then the threshold is reduced.
2. Occupany sensors (in later cars) mean that passenger bags don't deploy if the seat is unoccupied. Saves bucks.
3. Side impacts do not deploy front amounted air bags. Side impacts deploy side/curtain bags when so equipped. Front mounted bags do not deploy in rear-end collisions unless the secondary collision is intense enough to cause forward decelleration high enough to trigger the bag.
The car uses an accelerometer in the passenger compartment and impact sensors in the front of the car. In case of electrical failure, the voltage required to activate the trigger is VERY low.
Mercedes is one of only two manufacturers on record as not having faulty (non-deploying) or unintended deployments. The other is BMW. MB's bags are regarded as the best and most reliable on any car sold. Kinda figures since they were the first to install them...
1998 C230 "Black Betty"