At what speed does the airbag deploy?
At what speed does the airbag deploy?
The airbag does not deploy at a specific driving speed, but rather at a specific deceleration (braking force). In a frontal collision with a rigid wall, the deployment threshold is approximately 25 to 30 km/h impact speed. The severity of the impact is what matters – not the speed before the crash.
A common misconception: Many people think the airbag deploys at a fixed speed – for example, at 30 or 50 km/h. In reality, the control unit measures deceleration (negative acceleration) in g. Only when this deceleration exceeds a threshold value over a certain time period is the airbag triggered.
Why is speed not the deciding factor?
The severity of the impact depends on the speed differential at the moment of impact, not on the driving speed. A car traveling at 150 km/h that hits a vehicle traveling at 130 km/h experiences only a differential of 20 km/h – the airbag may not deploy. An impact at 30 km/h into a concrete wall, on the other hand, is significantly more severe.
| Scenario | Driving speed | Speed change | Airbag deployment? |
|---|---|---|---|
| Head-on into concrete wall | 30 km/h | 30 km/h (immediately to 0) | Yes |
| Rear-end collision with stationary car | 40 km/h | approx. 20-25 km/h (car is pushed) | Possible |
| Rear-end collision with moving car | 150 km/h to 130 km/h | approx. 15-20 km/h | Unlikely |
| Head-on into tree | 40 km/h | 40 km/h (rigid obstacle) | Yes |
| Side impact with guardrail | 80 km/h | Low (guardrail gives way) | No (side airbag possible) |
What exactly does the crash sensor measure?
The crash sensor (acceleration sensor) measures deceleration in the unit g (gravitational acceleration). The deployment threshold is approximately 3 to 5 g – this means the vehicle must be decelerated at 3 to 5 times the force of gravity. Only when at least two independent sensors confirm this value does the control unit trigger the airbag.
The plausibility check through multiple sensors is an important safety measure: it prevents false deployments caused by potholes, curbs, or individual sensor defects. Only in a genuine impact do all relevant sensors simultaneously report high deceleration values.
Measurement time window
The sensor does not only measure the peak deceleration value, but also its duration. A short, hard impact (e.g., pothole) produces high g-values, but only for fractions of a millisecond. A genuine impact typically lasts 50-100 milliseconds with sustained high deceleration – the control unit recognizes this pattern.
Severity levels in airbag deployment
Modern airbag control units distinguish between four severity levels of an impact and adjust the deployment accordingly. In a minor impact (severity level 0), no airbag is deployed; in a very severe impact (severity level 3), all available restraint systems are activated at full strength.
| Severity level | Accident severity | Airbag response |
|---|---|---|
| 0 | Minor impact / parking bump | No deployment |
| 1 | Moderate impact | Seatbelt pretensioner + possibly 1st airbag stage |
| 2 | Severe impact | Seatbelt pretensioner + airbags 1st stage |
| 3 | Very severe impact | All restraint systems, both airbag stages |
Two-stage airbags (standard in modern vehicles) can adjust the inflation speed and force to the severity of the impact. In a moderate impact, only the first stage is deployed – this protects against injuries from the airbag itself in lighter accidents.
What factors influence deployment?
Airbag deployment is influenced by the type of obstacle (rigid vs. yielding), the angle of impact, the speed change during impact, whether the occupant is restrained, and seat occupancy. The control unit processes all this data in milliseconds before making the deployment decision.
- Type of obstacle: Concrete wall vs. guardrail vs. another vehicle – the more yielding, the lower the deceleration
- Angle of impact: Head-on, offset, side – determines which sensors respond
- Speed differential: Only the change matters, not the absolute speed
- Seatbelt status: With an unrestrained driver, the deployment threshold may be different
- Seat occupancy: Passenger airbag only when seat is occupied (seat occupancy mat)
Frequently asked questions about airbag deployment speed
Here you'll find answers to the most important questions about airbag deployment.
Can the airbag deploy at walking speed?
Theoretically yes – if the deceleration is high enough. An impact at 15-20 km/h into an absolutely rigid obstacle (e.g., concrete bollard) can be sufficient. However, in typical parking bumps at 5-10 km/h, the deceleration is too low.
Does the airbag deploy in a rollover?
Modern vehicles have rollover sensors (gyroscope) that detect a rollover. In this case, curtain airbags and seatbelt pretensioners are activated to protect occupants from head injuries – even without a classic frontal impact.
Why didn't the airbag deploy in my accident?
The most common reasons: The deceleration was too low (e.g., rear-end collision with low differential speed), the impact angle was unfavorable (side rather than head-on), or the obstacle was too yielding. This does not mean the system is defective – it made the correct decision that no airbag was necessary.
Does the control unit store data even without airbag deployment?
Yes. Even in lighter impacts where the airbag does not deploy, crash data can be stored in the control unit. This can activate the airbag warning light and must be cleared by a specialist. Airbag24 offers this repair for all makes and models.