DOT & Sign Regulations
The Department of Transportation
DOT - The Manual on Uniform Traffic Control Devices (MUTCD)
DOT - Standard Highway Signs
Richard C, Moeur's Web Page Pertaining to Traffic Sign Regulations

Regional Sign Information
California Traffic Signs

State DOT Information
California Department of Transportation
Florida Department of Transportation
Illinois Department of Transportation
New York Department of Transportation
Texas Department of Transportation

Note: Most State DOT web sites are http://www.dot.state.XX.us (replace XX with your states two letter postal abbreviation)

Sign Making Material Specifications
3M Reflective Sheeting Specifications

Humorous / Special Interest
Road Trip America
Strange Signs

We only ship to North American addresses, so if you live in Europe, then here are some European retail sign companies...
Green Magic Co.