The one time I had to go to traffic court the lady infront of me was in the exact same situation. They were doing bridge construction and the speed limit dropped 20 mph....there was one sign showing the new speed limit.
The local police had the traffic officers lined up...they were writing tickets like shooting fish in a barrel.
After she got her ticket she called the police and they could not give her any information about how many vehicles were stopped and/or ticketed.
So she took the afternoon off and sat in a parking lot close by and counted how many cars were stopped.
She had a log she showed to the judge and showed him how many dozens and dozens of cars the police were stopping.
The judge threw the case out and lectured the officer about "underhanded speed traps"
I will never forget the look on the officer's face when the judge let her go