U.S. Road Sign Design Catnip

99% invisible, which is my #1 favorite podcast

https://99pi.org

also posts articles. Here are two wonderful ones about U.S. highway signs:

American Highways 101: Visual Guide to U.S. Road Sign Designs & Numbering Systems

https://99percentinvisible.org/article/american-highways-101-visual-guide-to-u-s-road-sign-designs-numbering-systems/

Tricky Business: Appearing on Blue Highway Exist Signs is Harder Than It Looks

https://99percentinvisible.org/article/tricky-business-appearing-blue-highway-exit-signs-harder-looks/