Supposing you have a 3-axis accelerometer, I would point the accelerometer with the X axis to the front of the car and the Y axis to the left of the car so that the Z axis of the accelerometer is perfectly normal to the ground and car plane. This way, gravity (you know, an accelerometer is also subject to static accelerations, not only dynamic accelerations - for an explaination on this see my thesis) won't impact on anything but the Z axis.
Then, everything > that 1G acceleration on the X axis means a braking.
Just hope not to get 1G acceleration on the Y axis, as that would mean you have been hit by a car or something 