For future reference, CA limit is 55mph, unless otherwise specified. All of those reasons: down-hill, dark ... go against you because all of those are good reasons to go slower, not faster. I wouldn't bring those up in my defence and hope that the officer did not record them on the ticket. An admission that you did not notice the speedometer further strengthens the charge. If you don't know how fast you were going, how could you deny the charge of 96, regardless of your lack of credibility as a witness to your own violation. There is always the small chance that the officer will not respond to your TBWD. I recently saw a very badly written & frankly moronic speeding defence get dismissal.