How can I fight Toll road ticket in Irvine? Please help me?

I've just recently moved to Irvine, CA and when I took Walnut/Jamboree to get to Marketplace (freeway5/Jamboree), I was confused when I saw the sign "261/Frwy 5". I thought that toll 261 is on Frwy 5, so I stayed on the left lane to avoid taking the toll road. I then realized that I was actually on the toll road when the divider turned yellow. Immediately, I took the exit on Irvine Blvd. A week or so later, I got a toll road ticket for $50. I've never taken toll road before and this was my first time in Irvine. Does anyone has any suggestion on what I could do to avoid this outrageous fee? If so, please help me.