In Georgia we have a case directly on point. In short, a signal is not required to turn left from a left-turn-only lane because other motorists are on notice of what you plan to do. Such a stop should be found to violate the Fourth Amendment, and any evidence gathered therefrom should be suppressed. That should result in your case being dismissed.
This is a very state-specific question. In WI, for example, the law states you do no not have to use your turn signal, unless you are affecting other traffic. If you are in a turn-only lane, you should not have to use your signal to let other traffic know you are about to turn. So, other traffic should not be affected.
Bottom line: Consult with a local, experienced DUI attorney. You can find such an attorney through the National College of DUI Defense: www.ncdd.com.
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