This is exactly where I got caught too. Even as a born and bred Kairdiffian it's incredibly confusing in that spot as the signage is very poor, especially at night when I got done. I even tried to avoid the area but was about to turn into a one way street going the wrong way so the only way I could see out of there was towards the Castle (I swore blind that the exit past the New Theatre was for taxis and buses only?? - still not sure on this). As I was fannying about, a taxi came right up behind me and I had to continue to avoid causing an obstruction and so actually tried to pull in before the camera but I just entered that area enough for a ticket.
Anyway, I appealed it and although they ignored my reasoning, they did let me off for a 'first offence' (apparently available to everyone), so defo worth writing a letter.
I think it's high time that area was decluttered / made clearer, otherwise cynical people may begin to think it is simply a money making racket