From the National Highway Traffic Safety Administration (pdf file):

Over the last four decades, the National Highway Traffic Safety Administration (NHTSA) and/or the Insurance Institute for Highway Safety have conducted four national surveys to estimate the prevalence of drinking and driving in the U.S. These surveys utilized a stratified random sample of weekend nighttime drivers in the contiguous 48 States. The first National Roadside Survey (NRS) was conducted in 1973, followed by national surveys of alcohol use by drivers in 1986, 1996, and 2007.

The 2007 NRS included, for the first time, measures to estimate the use of other potentially impairing drugs by drivers. Prior roadside surveys had collected breath samples to determine blood alcohol concentration (BAC). Due to developments in analytic toxicology, NHTSA determined it would be feasible in the 2007 survey to collect oral fluid and/or blood samples to determine driver use of a wide variety of other potentially impairing drugs.

The 2007 NRS was designed to produce national estimates of alcohol and drug use by drivers. Thus, the use rates shown below are national prevalence rates calculated from the percentage of subjects using alcohol or drugs and adjusted with an appropriate weighting scheme.

Results of the 2007 Survey: Alcohol

The 2007 NRS found a dramatic decline (71%) in the number of drinking drivers with BACs at or above the current legal limit of 0.08 g/dL* on weekend nights compared to previous surveys. Similar declines were found at other BAC levels. For example, the percentage of drinking drivers (any positive BAC) declined almost as much over this time period, but one cannot infer impairment at very low BACs.

The percentage of male drivers with a BAC over the current legal limit of 0.08 g/dL was 42% higher than the percentage of female drivers with illegal BACs. Over 2% of the weekend nighttime drivers had illegal BACs (>0.08g/dL) while only 0.1% of daytime drivers had illegal BACs.

Time of day made a big difference in the likelihood of drivers having illegal BACs. Looking just at Friday daytime (9:30 a.m. to 11:30 a.m. and 1:30 p.m. to 3:30 p.m.), early nighttime (10:00 p.m. to midnight), and late nighttime (1 a.m. to 3 a.m. Saturday), only 0.2% of drivers had illegal BACs during the daytime, while 1.2% had illegal BACs during the early nighttime and 4.8% had illegal BACs during the late nighttime.

Substantial differences were observed in the percentage of drivers with illegal BACs by vehicle type. Motorcycle riders were more than twice as likely as passenger car drivers to have had BACs > 0.08 g/dL (5.6% compared to 2.3%). Pickup truck drivers were the next most likely vehicle type to have illegal BACs (3.3%).

Underage drivers are of special interest since they have been shown to be a high risk of crash involvement when drinking and driving. The percentage of underage drivers in fatal crashes with a 0.08 g/dL or higher BAC decreased from 1973 to 1996. However, from 1996 to 2007, there has been a slight increase. The NRS data do not show this same trend; the percentage of underage drivers with 0.08 g/dL or higher BACs has been decreasing throughout this time period.

Results of the 2007 Survey: Drugs

Participants in the 2007 NRS were asked to provide an oral fluid and blood sample in addition to a breath sample. The oral fluid and blood samples were tested for the presence of a large number of potentially impairing drugs. The list of impairing drugs covered illegal, prescription, and over-the-counter products, including stimulants, sedatives, antidepressants, marijuana, and narcotic analgesics.

Based on the oral fluid results, more nighttime drivers (14.4%) were drug-positive then were daytime drivers (11.0%). Based on the blood test results which were administered only at nighttime, 13.8% of the drivers were drug-positive. Using the combined results of either or both oral fluid and blood tests, 16.3% of the nighttime drivers were drug-positive.

The most commonly detected drugs were Marijuana (THC) at 8.6%, Cocaine at 3.9%, and Methamphetamine at 1.3% of nighttime drivers.

The full significance of these findings for highway safety will only become clear when ongoing and additional research conducted by NHTSA and others is completed. NHTSA is responding to these findings with programs to enable law enforcement officers to recognize drug impairment, and education for prosecutors and judges on factors associated with drug-impaired driving cases.

Under the Drug Evaluation and Classification program, NHTSA has prepared nearly 1,000 instructors and trained more than 6,000 officers in 46 states. Officers receive extensive training to recognize symptoms of driver impairment by drugs other than alcohol.

NHTSA has also initiated a follow-on study to the 2007 NRS to identify which drugs are associated with higher crash risk. This case-control study will include in-depth investigations of a large number of crashes of all severities. The proportion of drug use by crash-involved drivers will be compared to that of a similar sample of non-crash involved drivers to determine if drug use is associated with crash involvement. Findings from this large-scale study are expected in 2012.