> I see no way to differentiate between braking and going downhill. Any
> ideas ?

You seem to be hung up in the process and forgot to do a reality check.
First, going up or down hill by itself isn't relevant.  Your implicit
assumption is that you are going at a constant speed down that hill.  If
you're coasting there will be no front/back accelleration.  If you are in
fact going down the hill at a constant speed, then you *are* braking, either
with the brake or some other means like with a low gear.  Does it really
matter which if you are trying to alert cars behind you?  Will they be less
likely to rear end you if you are braking with a low gear than with the
brake, even though the resultant car motion is the same?

Another simple reality check you have missed is the magnitude of the
deccelleration when going down a hill at constant speed.  6% grade is
considered a "steep" hill for a normal paved road where speed is expected to
be several 10s of miles/hour.  Such a 6% down grade will have truck warning
signs at the top and will require using low gear or braking most of the
time, else your speed will get unacceptably high in just a few seconds (you
will pick up about 13 MPH in 10 seconds).  Now 6% grade is just what it
says, so inside a car going down that hill at constant speed it will feel
just like braking at 0.06g.  This is rather mild braking, and probably not
far from the noise floor for an accellerometer bouncing around in a car
anyway.


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