Research Report: Optimizing Your Advertising Dollars, Improving Your Bottom Line with Signs, Kresky Signs White Paper, November 2011.

U.S. Department of Transportation Pipeline and Hazardous Materials and Safety Administration - Governing Body in charge of hazmat placard regulations

1998 CFR title 45, Volume 4 - Specifications for hazmat placards: relevant section 172.500-172-560

Environmental Chemistry - Explanations and information about hazmat placards

Hazardous Materials Transportation Guide

49 CFR 172.101 Table of Hazardous Matrials - Four-digit material codes on hazmat placards and train cars