Legal Question in Criminal Law in Texas

Theft is defined as the appropriation of property without the effective consent of the owner.

So if you don't have the owner's consent, then taking the property is illegal.

The issue you present, that the sign is lying on the side of the road, suggests that the property is abandoned by the owner. The concept of abandoned property makes it lawful for garbagemen to pick up trash, for street cleaners to clean streets, and for you to pick up a coin in the street you found.

So was the sign "abandoned" ? Perhaps. And perhaps not. If the sign lying on the roadside had been replaced, it probably was abandoned. If it was just knocked down, probably not. My suggestion is to call the entity charge, tell them the sign is lying on the roadside, and ask if you can have it. Then you will know for sure.

that would be called theft.