A Texas woman spent 12 hours in the county jail and now faces a misdemeanor charge of “standing in the street where a sidewalk is present.”

The woman, Natalie Plummer, is a resident of Houston who was riding her bicycle home from the grocery store when she noticed the police speed trap. She stopped her bike and used a grocery bag to write a sign warning drivers of the impending speed trap. Unfortunately for her, police noticed what she was doing and arrested her.

In some states it is illegal to warn other drivers about speed traps or DUI checkpoints. Even flashing your headlights is sometimes enough to warrant you a citation or even an arrest for obstruction of justice. Texas does not have such a law but police frown on activity which inhibits their ability to ensure public safety. They might not be able to arrest you for warning drivers of impending speed traps, but they can find other ways of getting you out of their way.

Michael Dirden, Houston’s executive assistant police chief, said in a statement that if Plummer believes the police acted inappropriately, she should file a complaint with the department’s internal affairs division.

After being held in jail for 12 hours, Plummer was released on bond, and will soon appear in court to face her misdemeanor charge.

While Plummer’s method of alerting drivers to police activity might have been unprecedented, state laws covering such warnings are decades old. Their most common form, flashing headlights, is legal in some states but illegal in others.

Laws in New Jersey, New York, Ohio, Pennsylvania, Tennessee, Virginia and Florida allow headlight flashing, while other states, such as Arizona and Alaska, forbid it. In Washington, drivers may be fined $124 for flashing their high beams within 400 feet of another vehicle for any reason. Other states forbid headlight flashing in some circumstances but not in others.