回复：我住LA，那天去参加一个考试，出来时迷路了，到一个十字路口绿灯左转，被拍了照，当时不明白为什么？

you can say something like:

1) the sign is not obvious, it is so nonobvious that a reasonable person cannot see it or easily recognize it when they drive through. (if it was dark in the evening when you drove through, you should also mention it)

2) you were not familiar with that area. 

3) You have never intentionally violated any traffic rules. 

4) Combining factors 1, 2, 3 above, the court should reduce the fine.