At about 9:00pm, I was merging onto the Gardiner expressway west at York. At that point, a lot of cars are generally changing lanes to the right to gain access to the Spadina exit, and so (and I do this every day), I merged to the left hand lane as soon as I could (ie, when there was enough space to get over). I signaled and made the change, and all was well. I continued for several seconds, and then noticed the car in the lane ahead was moving much slower than traffic, and so signaled, changed to the right and went around it. About a minute later, sirens go off in my mirror, and I'm pulled over. The cop asks if I've been drinking, I say "no" (I hadn't), asks why I was "endangering everyone on the road", and I said that I didn't think I had been, but if I was, then I should not have been", and he writes me a careless driving ticket. There was no accident, no close call, no interaction with any other car on the road.

I am going to fight it, but any input with respect to how best to proceed would be appreciated.

Careless driving is exactly that, careless. It doesn't mean there has to be an accident or you narrowly missed another vehicle. It just means "without due care and attention or without reasonable consideration for other persons". (See: Careless Driving)

The officer seems to think you were careless with your lane changing and merging it seems. Once on the Gardiner Expressway West from the York Street ramp, you have to merge to the left to avoid being forced to exit on Spadina as the right lane is quickly marked as the exit. This is even more difficult when there are people to your left looking to exit on Spadina. You merged to your left, decided to pass on the right in the exiting lane, and then you would have had to merge to the left again. The lane lasts 700ft before it exits. I'm assuming the officer deemed your actions as careless to those on the left and right of you looking to merge in this short distance as you are using it as an opportunity to pass.

If you're looking to go to trial, you will want to request disclosure once you get your notice of trial back in the mail. You can start there. Also, I'd suggest you consider a plea deal with the prosecutor. Careless Driving isn't exactly small potatoes. It might be in your best interest to plead guilty to something a little less drastic.