"In 2011, 9.4 million persons or 3.7 percent of the population aged 12 or older reported driving under the influence of illicit drugs during the past year. This was a decrease from the rate in 2010 (4.2 percent) and the rate in 2002 (4.7 percent). Across age groups, the rate of driving under the influence of illicit drugs in 2011 was highest among young adults aged 18 to 25 (11.6 percent); this rate for young adults in 2011 was lower than the rate in 2010 (12.7 percent). Additionally, the rate of driving under the influence of illicit drugs during the past year decreased among adults aged 26 or older (from 2.9 percent in 2010 to 2.4 percent in 2011)."

Source

Substance Abuse and Mental Health Services Administration, "Results from the 2011 National Survey on Drug Use and Health: Summary of National Findings," NSDUH Series H-44, HHS Publication No. (SMA) 12-4713. Rockville, MD: Substance Abuse and Mental Health Services Administration, 2012, p. 27
http://www.samhsa.gov/data/NS…