1. Bureau of Transportation Statistics, Condition of U.S. Highway Bridges: (1990–2008)
http://www.bts.gov/current_topics/2009_03_18_bridge_data/html/bridges_us.html
2. U.S. Department of Transportation, Comprehensive Truck Size and Weight (CTS&W) Study,
http://www.fhwa.dot.gov/reports/tswstudy/Vol1‐Summary.pdf
(pp- 15, Table 5).