Thanks for guiding me through this. 
I agree that 1g might be a theoretical value. The actual value will need some empirical foundation, I am afraid. However, I am still thinking about a way to detect an emergency braking (what's the correct expression for that?) state. One thing that came to my mind is if I will see some increasing g (or m/sec2) values due to reduced speed during ongoing braking. The stopping distance increases quadratically in relation to speed (sorry for phrasing it this way, never tried to articulate something in a foreign language). Maybe a very basic quadratic function can help me to express myself: s = (v/10)2. What are your thoughts on this?
I suppose I can neglect aerodynamical downforce since most road cars will rather produce upforce.