Here's The Speed That A Vehicle's Air Bags Will Typically Deploy At

Fatalities in car accidents grew alongside the rising popularity of cars during the early 20th century. While general regulations like speed limits and traffic signals made driving safer, the biggest leaps in automobile safety came in the form of seat belts and air bags. The modern safety features like assisted driving, blind spot warning, and electronic stability controls aim to avoid accidents altogether, while seat belts and air bags remain the last line of defense during a crash.

Air bags are particularly fascinating, considering they inflate into a full-size cushion within milliseconds of detecting a potential crash. As per the National Highway Traffic Safety Administration, air bags deploy in case of a moderate to severe crash, which is roughly the equivalent of crashing into a fixed obstacle (such as a wall) at 8 to 14 mph, or crashing into a stationary car at 16 to 28 mph. It is important for air bags to inflate only in serious crashes, as air bags are single-use products that need expensive replacement after inflation. Let's explore how airbags make the life-saving, split-second decision to inflate during a crash with such accuracy.

Air bag sensors first determine whether the impact is significant when a crash occurs. If it is, the sensor triggers a chemical reaction that produces a gas to inflate the air bag. Once inflated, the gas slowly escapes the air bag while absorbing the momentum from the passenger's body. Seat belts play a huge part in assisting air bags in gradually absorbing the passenger's momentum. This multistep process must happen automatically and within milliseconds of a crash to be effective.

Developing such a fast-acting autonomous safety system was a huge challenge in the early air bags of the 1950s. The early systems used spring-based sensors and compressed air to inflate the air bags; however, they were both slow and inaccurate.

The breakthrough came when engineer Allen K. Breed developed an improved ball-based sensor and used sodium azide to inflate the air bags. In Breed's design, a steel ball held in place with strong magnets would detach in case of a crash, triggering a reaction that produced multiple liters of nitrogen gas in a fraction of a second. That said, the sensor could sometimes misfire in benign scenarios such as driving over potholes or speed bumps. 

Modern cars now use MEMS-based sensors, which are much better at identifying crashes. Moreover, the chemical compound sodium azide (which was one of the reasons behind the Takata air bag recall) has been replaced with a safer guanidine nitrate, making modern air bags much safer and more reliable than their predecessors.