The drug showing the greatest increase among drivers from 2007 to 2013/2014 was marijuana (THC). The percentage of THC-positive drivers increased from 8.6 percent in 2007 to 12.6 percent in 2013/2014, a proportional increase of 47 percent.

The drug showing the greatest increase among drivers from 2007 to 2013/2014 was marijuana (THC). The percentage of THC-positive drivers increased from 8.6 percent in 2007 to 12.6 percent in 2013/2014, a proportional increase of 47 percent.

(NHTSA 2013-2014 Roadside Survey)