Flashing Headlights to Warn Other Drivers of Cops is Free Speech
Truck drivers often signal each other to warn about road hazards. For example, truckers would double-flash their headlights when they just passed a police speed trap to warn other drivers in the oncoming lanes. Th...
https://www.neatorama.com/2014/04/12/Flashing-Headlights-to-Warn-Other-Drivers-of-Cops-is-Free-Speech/