Highway Engineering, Rural Roads and Pavement by Chatburn on Google Books is 1920's tech. Most is still used today.
Gravel Roads Maintenance and Design Manual US Dept Transportation, Federal Highway Admin
Tennessee's Survey Report of Historic Highway Bridges, Chapter Two, has a good history of road building and development in Tennessee. Maybe there is a similar reference for Pennsylvania.
US Army Manuals include TM 5-623, TM 5-600, FM 5-35, FM 3-34.343 (FM 5-446) are useful resources.
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