Why should you plead not guilty?
1. if you appear on trial, the cop might not show up, and the ticket is automatically dismissed;
2. if the cop does show up, with some preparation, you have a chance to win the case;
3. in the worst case, if you lose, you still don't pay more than if you have just pleaded guilty. Court costs are already tagged onto the prescribed fine. (Often fines are reduced if you put up a good defence)