phdhorn is correct in that their only recourse is the registration. It's a civil infraction, which just means that the legislators that bowed to the lobbyists and legalized these cameras knew that the tickets couldn't be criminal without violating due process laws.
Several years ago when Round Rock had them, I purposely rolled through a right turn on red (entering my neighborhood) and got one just to see what it said. The "officer" that signed my violation under penalty of perjury was actually retired and not accredited anymore. I contested the ticket. That consisted of sitting in a room at RRPD for 15 minutes with another cop who didn't give 2 shits what I had to say. I threw everything away when I got home. Never got anything from a collection agency or had an issue with registration. If they had sent it to collections, that's probably when I would have paid something just to get them off my back.
A couple years later, Round Rock suspended the program because they were paying Redflex too much money and hadn't been able to collect on 14,000 outstanding tickets.