I was traveling behind 3 cars and slowing for a traffic light. The 5th car hit me from behind and pushed me into the #4 car in front of me. I was sandwiched between 2 cars. The 5th car that hit me was going 40-45mph while I was at 15 or 20mph with brakes on. While most of the impact came from the back, I still hit the front end, but my driver side air bag did not deploy. I am wondering why it didn't deploy and how fast do you need to be going for the sensor to go off.