Why not just use the handbrake to disable the video?
Use the speedometer? or even use the GPS (not very reliable though)
The accelerometer is not really that reliable for measuring true acceleration of the car. Rather it measure the total acceleration felt by the sensors, thats including the gravity. Go and buy one, tilt it up and you see it register 1g. Put the car on a hill and you see it register some acceleration even though the car is parked up. A bump on the road will see a change in acceleration and I dont think theres a way to remove this easily. Hit a bump once, a small acceleration is registered and it may continue seeing the car travellling at some speed even though you have already stopped.
There is a long thread about this, dont know if you already read it.