It depends on what highway sign(s) you are considering. But most likely, the sign(s) is (are) trademarked by the respective government entities. For examples, "CHP" is owned by the California Highway Patrol and "Metrolink" is owned by Southern California Regional Rail Authority.
You can find out by doing a trademark search at www.uspto.gov or you can contact the City / County / State attorneys and speak with the trademark or possibly procurement office. You may be able to pay a royalty to use it.
However, you should consult an IP attorney because maybe you can still use the sign if there would be no likelihood of confusion between your webstie and CalTrans, etc.
DISCLAIMER – No legal representation is being sought by the response to this question. Please note that this answer is for informational purposes only and does not constitute legal advice. An attorney should be consulted for more in depth legal advice as more facts and information are necessary. No attorney-client relationship is hereby intended. However, if you have additional questions concerning this matter you may reach Dominique N. Thieu, Esq., by: phone 714.435.9500 or e-mail me
[email protected], THIEU VIRTUAL LAW GROUP, 17150 Newhope Street, Suite 213, Fountain Valley, CA 92708.