The United States Numbered Highway System for Google Earth is research project that covers all the current and past U.S. Routes in the 48 contiguous U.S. states before the Interstate Highway System. The project was started in 2016 as to commemorate the 90th anniversary of the US Routes such as US 66. The project is currently on hold indefinitely.
I will add a background about the US Highways when I write appropriately about it if I release some of the KML routes that I’ve made with Google Earth to ZIP file if possible.