Let me see . . .
year % of evaluations determined to be THC
[*] 2005 64.5%[*] 2006 59.5%[*] 2007 50.1%[*] 2008 58.8%[*] 2009 48.3%[*] 2010 65.8%
I do not know what happened to make the spike at 2010 but I suspect that it was an emphasis on focusing on marijuana due to the legalization. These statistics are nearly worthless since we do not have info confirming this with a test that positively determines if a subject is under the influence by THC.