Stealing Street Signs
In certain states, like Kentucky, it is illegal for a private individual to even posess a street or road sign.
In other states, it is not illegal per se just to have one in your possession.
In these other states, it depends upon how you acquired the road sign and the condition of the sign.
If you took it down yourself, or someone else took it down, then it's stolen property and it's illegal if the authorities can prove you have stolen property in your possession, but I believe they must prove that you are knowingly in possession of stolen property.
If you find a sign lying along the road where it has fallen off a post and it's still in reasonable condition, it's possible the jurisdiction might want to re-install the sign, rather than install a brand new one. Better just leave it there.
If you find a road sign that a reasonable person would believe has been obviously discarded and nobody wants, then there is nothing wrong with taking it. For example, you may find old discarded bridge signs and sometimes they are still attached to the old rusty poles, but the point is, they were thrown over the side of the bridge into a creek by the state/county/township/city workers who replaced them with new signs. Or you may find another type of old sign that has outlived its usefulness that has been thrown into a ditch. These jurisdictions did not want them anymore, and they just pitched them away.
There are companies in Illinois that recycle old aluminum street/road signs and make key rings and the like from them, so it is certainly not illegal in Illinois to simply be caught in posession of one. After, all these companies are allowed to have them in their possession.