They are suppose to go off in crashes where the vehicle is travelling more than 20mph. However, there are many incidences where they have gone off for no reason at all and in come cases they have failed to go off at all. It is also known that they can cause sometimes fatal injuries, especially if someone sits close to a steering wheel. They inflate at a speed of 25 thousandths of a second and 35 litres of gas propellant explodes into the bag. Unfortunately, they cant be tested until the time comes they are needed.