[US] State Estimates of Persons Aged 18 or Older Driving Under the Influence of Alcohol or Illicit Drugs / National Survey on Drug Use and Health, 17 April 2008
PDF - http://www.baynews9.com/images/news/2008/4/23/stateDUI.pdf
"Combined data from 2004 to 2006 indicate that 15.1 percent of current drivers aged 18 or older drove under the influence of alcohol in the past 12 months (“past year�), and 4.7 percent drove under the influence of illicit drugs in the past year."