Many images are from photographs mailed to me or images e-mailed to me by various people, listed in the Acknowledgments page. For images not listed there or below, the sources are photos found on the web, such as the Motoways/Highways thread in SkyscraperCity.
The ultimate source for most of the state highway signs shown in this web site is
United States Road Symbol Signs, published by the Federal Highway Administration, 1979
although I didn't get the images from that publication. I assume the folder that I borrowed for the state highway signs got their images from this publication, since the signs are exactly the same.
Most of the Canadian highway signs came from official highway maps from each province.
The Kentucky signs are from the 1975 Official Kentucky Highway and Parkway Map.