1. U.S. Department of Transportation, Bureau of Transportation Statistics.National Transportation Statistics 2008; Table 4-10;Washington, DC, 2008; http://www.bts.gov/publications/national_transportation_statistics/2008/(accessed 4/2009).
2. California Department of Transportation.2007 Public Road Data;Sacramento, CA;http://www.dot.ca.gov/hq/tsip/hpms/datalibrary.php(accessed 4/2009).