(Perhaps) Surprising results from NHTSA's 2014 NATIONAL ROADSIDE SURVEY of Alcohol and Drug Use by Drivers

All of the following text and statistics have been taken from the NHTSA's website:

Prescription drugs, over-the-counter medications, and illegal drugs may cause impairment alone or in combination with each other and/or with alcohol. In every State and the District of Columbia, impaired driving is illegal. Whether by drugs — legal or illegal — alcohol, or a combination of both drugs and alcohol, impaired driving puts the driver, their passengers, and other road users at risk. In NHTSA’s National Roadside Survey conducted in 2013-2014, 20 percent of drivers surveyed tested positive for potentially impairing drugs.

[That study] found an increase in the number of drivers testing positive for marijuana and other drugs that can impair driving skills compared to the 2007 survey findings. In the 2013-2014 survey, nearly one in four drivers tested positive for at least one drug that could potentially affect safe driving skills.

** Overall, 22.3 percent of daytime drivers were drug-positive and 22.5 percent of nighttime drivers were drug- positive in the combined oral fluid or blood. 

** The Midwest had the highest percentage of daytime drug-positive drivers...

** The percentage of daytime drug-positive drivers was significantly higher in the Midwest than in the West...

** The Midwest had the highest percentage of nighttime drug-positive drivers as well (30.4%) which was significantly higher than in the West...

** Nighttime drivers in the South (22.9%) were also significantly more likely than drivers in the West to be drug-positive...

** Results of overall drug prevalence by time of day and gender [] reveal no statistically significant differences...

** Both males and females aged 21-34 had the highest rates of drug presence....

** Comparisons by race and ethnicity [] show that the prevalence of drug positive driving was significantly lower for Asian drivers compared with White drivers in the daytime sample (23.4% versus 6.4%, respectively, []), but not in the nighttime sample...

** The prevalence of drug-positive driving was significantly lower for Hispanic drivers compared with White drivers in the nighttime sample (13.5% vs. 22.2%, respectively)... 

** There were no statistically significant drug prevalence differences among daytime drivers of various vehicle types []. At night, pickup drivers had a lower drug prevalence than passenger vehicle drivers, which was statistically significant (16.5% versus 24.3%, respectively.)...

** The prevalence of THC-positive, THC-positive-only, and THC-positive plus any other drug in the Midwest was significantly higher than in the West among both daytime and nighttime drivers...

** In the daytime, there was a significantly higher prevalence of THC-positive and THC-positive-only among male drivers than female drivers (12.1% males versus 5.5% females, and 9.7% males versus 4.1% females, p < .05). Males also had a significantly higher THC-positive and THC-positive only prevalence in the nighttime sample (14.6% males versus 9.4% females, and 11.7% males versus 7.4% females, p < .05) There were no other statistically significant differences by drug class, gender and time of day. 
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... The prevalence of illegal drugs-only was significantly higher among males compared with females among both daytime and nighttime drivers 

​The full report can be found here.