As a practical matter, Jack is probably right...you have little chance of beating it, and if you plead not guilty and lose, then you have to pay court fees and costs that could easily run you an additional $50 to $75.
If you want to educate yourself about it, the starting point is getting the municipal ordinance so that you can read the language precisely. Then there are statutes and codes regulating the language and placement of the signs that can be investigated. Followed by measuring the signage and documenting the measurements that would prove the signage to be out of code to be used in court as evidence.
Did you think you had any other defenses? Other folks were doing it doesn't fly.