Alcohol use by nighttime drivers on weekends is down sharply since 1973 but remains a major problem in fatal crashes. The latest national roadside breath-test survey indicates 2.2 percent of drivers had blood alcohol concentrations (BACs) of 0.08 percent or more in 2007, marking a 71 percent decline from 1973 when the first survey was conducted. At the same time, 16 percent of nighttime weekend drivers tested positive for drugs, the National Highway Traffic Safety Administration (NHTSA) says. The agency cautions that its first-ever estimate of driver drug use doesn't necessarily imply that these drivers were impaired.
The percentage of alcohol-impaired nighttime drivers in 2007 compares with 4.3 percent in 1996, 5.4 percent in 1986, and 7.5 percent in 1973. There were similar declines during the same period in the percentages of drivers with any detectable alcohol in their systems.
The 2007 survey involved randomly stopping drivers at 300 locations in 48 states on Friday and Saturday nights and during the day on Fridays. The daytime component was new for 2007, along with drug screens, and for the first time the survey included motorcycles.
Drivers were more likely to be impaired by alcohol between 1 and 3 a.m. (4.8 percent) than during the daytime (0.2 percent) or early evening (1.2 percent). This is in line with federal data showing that alcohol involvement in fatal crashes peaks at night and is higher on weekends. All 50 states and the District of Columbia have per se laws defining it as a crime for people to drive with a BAC at or above a proscribed level, 0.08 percent.
"The roadside surveys suggest that the prevalence of alcohol-impaired driving has gone down over time, and that's great news," says Anne McCartt, Institute senior vice president for research. "Fatal crashes tell a different story. The reductions aren't showing up in federal crash data. We can't explain the disconnect, so this merits more research."
Based on fatal crash data, the proportion of fatally injured drivers with BACs at or above 0.08 percent declined by about a third between 1982 and 1994, from 49 percent to 33 percent. Since 1994 the percentage of fatally injured nighttime drivers with BACs at or above 0.08 percent has remained about a third. Likewise, the percentage of fatally injured drivers with 0.15 percent or higher BACs has slid 30 percent since 1982 but with little change since 1996.
A complication in the latest roadside survey may be that drivers were less willing to participate in 2007 than in years past. NHTSA notes that the 85 percent participation rate was lower than the 96 percent recorded in 1996 and 94 percent recorded in 1986. This might reflect driver concerns about litigation and privacy rights. It also might reflect a general reluctance to be interviewed. NHTSA accounted for this by using passive alcohol sensors to estimate refusers' BACs.
Surveyed male drivers were more likely to have illegal BACs than females (2.6 percent versus 1.5 percent). Compared with 1996, a lower percentage of males had illegal BACs in 2007 (3.5 percent in 1996). The percentage of female drivers with illegal BACs didn't change between 1996 and 2007. Fatal crashes among male drivers are much more likely to involve alcohol than those among females.
Motorcycle riders in the 2007 survey were more than twice as likely as car drivers to have BACs at or above 0.08 percent (5.6 versus 2.3 percent), followed by pickup truck drivers (3.3 percent). However, crash data indicate that alcohol is a bigger factor in passenger vehicle driver deaths. Thirty-five percent of fatally injured passenger vehicle drivers versus 30 percent of cyclists had BACs of 0.08 percent or more in 2008.
Drivers were asked to complete a questionnaire to estimate the prevalence of binge drinking, defined as consuming 6 or more drinks on a single occasion at least monthly, and heavy drinking, defined as 5 or more drinks a day 4 or more times a week. About 26 percent of drivers said they don't drink.
Binge drinking was widely reported by nighttime drivers with high BACs. Among people who said they drink, about 19 percent met the criteria for heavy drinking and 18 percent for bingeing. These two groups accounted for the largest percentage of drivers with positive BAC results in the roadside survey.
Since this questionnaire was new for 2007 NHTSA can't compare responses with prior surveys. The agency says the results suggest the need to focus on binge drinkers through tougher enforcement of DUI/DWI laws and prevention programs.
"Another option is requiring alcohol detection devices for all drivers once the technology is fully developed," McCartt says. The devices would prevent any driver from starting a vehicle after drinking too much. This idea has strong public support (see "Alcohol detection devices for all drivers garner widespread support," Sept. 17, 2009).
Previous roadside surveys estimated only alcohol use, but NHTSA expanded testing in 2007 to include screening of saliva and blood samples for over-the-counter, prescription, and illegal drugs. More nighttime than daytime drivers tested positive (14 versus 11 percent). The drugs most often detected were marijuana (8.3 percent), cocaine (3.9 percent), and methamphetamine (1.3 percent).
It's difficult to tell whether the drivers were impaired by the drugs because some drugs can be detected in the body weeks after use. Also unclear is the dose at which driving is impaired. NHTSA is conducting more research to understand the impact of drug use on highway safety, including which drugs impair driving ability and at what dose levels and which drugs are linked to higher crash rates.