hisluvmonkey said "I had always heard it was illegal to flash one’s lights to warn other drivers of radar patrols. If you were caught, a fine would ensue. Is that still the case? "
have tried that in the US, but usually got dismissed. Though cops have created other ways to charge a person, including charging someone with illegal flashing lights. But the law they tried to use only applied addon lights, like orange caution lights, not high beam lights.
Don't know what applicable law would exist here, denying police revenue maybe?