Of course you can calculate rate-of-change from the tachometer, but it isn't an instantaneous reading like an accelerometer can do. I was just surprised that they would go tot he extra expense -- it must be that the instantaneous value is necessary to get the driver-experience that they want.

From the stuff that people have sent me so far, and aside from my snarky comment about Fred Flinstone above, it seems to me that this system is surprisingly complicated. They must be very confident of the reliability of all the components, since the MTBF must be much higher than with a simpler system otherwise.