The percentage of drivers on the road who are impaired by alcohol has plunged during recent decades, but the percentage of alcohol-impaired drivers in fatal crashes has stayed relatively constant. This disconnect has perplexed researchers and policy experts alike. Characteristics of impaired drivers, such as their tendency not to buckle up, may offer clues to this puzzle and hint at potential countermeasures.
Searching for possible explanations, Institute researchers recently compared results of national roadside breath-test surveys conducted on Friday and Saturday late nights in 48 states in 1986, 1996 and 2007 with corresponding data on fatal crashes and driver alcohol use from the U.S. government's Fatality Analysis Reporting System (FARS). In the roadside surveys, a representative sample of drivers across the U.S. is asked to voluntarily take a breath alcohol test.
Researchers looked at differences in roadside survey samples and protocols, along with trends in rates of drivers with blood alcohol concentrations (BACs) of 0.08 percent or higher in the roadside surveys and FARS cases. All 50 states and the District of Columbia have laws defining it as a crime for people to drive with a BAC at or above 0.08 percent. Zero tolerance laws in all states and D.C. prohibit drivers younger than 21 from operating a vehicle with any detectable blood alcohol.
The percentage of alcohol-impaired nighttime drivers has declined in each roadside survey, while the proportion of fatally injured drivers with BACs at or above 0.08 percent has remained about a third since 1994, after falling from nearly half in 1982 (see "Drinking continues to decline among weekend drivers," Feb. 6, 2010).
"The conundrum is that trends in alcohol-related fatal crashes aren't in line with the downward trend we are seeing in the roadside surveys," says Anne McCartt, Institute senior vice president for research and the study's lead author. "The number of people who still die in alcohol-related crashes remains alarmingly high. In 2012, 10,322 people died in crashes involving drivers with illegal BACs."
Canada has seen a comparable trend in the percentage of fatally injured drivers with positive BACs, a 2012 study by researchers at the Traffic Injury Research Foundation of Canada and Transport Canada reports. Following a pronounced decline during the late 1990s, the percentage of drivers killed in crashes who tested positive for alcohol has hovered between 35 and 39 percent. Meanwhile, there has been more progress since 2007 in the percentage of drivers who admit to driving while intoxicated. The percentage of Canadians who reported driving when they thought they were over the legal BAC limit of 0.08 percent in the past 12 months fell from 8.2 percent in 2007 to 4.8 percent in 2013, a new Road Safety Monitor poll indicates.
If drinking and driving is declining...
Percent of Friday/Saturday night alcohol-impaired drivers, 1973-2007
Source: NHTSA National Roadside Survey of Alcohol and Drug Use
...why is the proportion of crash deaths
involving drinking drivers remaining constant?
Percent of crash deaths involving BACS ≥ 0.08%, 1982-2011
In the new IIHS study, researchers first looked at roadside survey data and controlled for differences in sampling and weighting because the methodology for the 1996 and 2007 surveys was somewhat different. The goal was to see if these differences account for the disconnect between trends in drinking and driving and fatal crashes.
Even after taking into account these changes, there were large declines in drivers with BACs of 0.08 percent or higher that weren't reflected in rates of drivers with BACs this high in fatal crashes. Researchers calculated that driver impairment rates fell 38 percent from 1996 to 2007 and 22 percent from 1986 to 1996. In contrast, the proportions of drivers involved in fatal crashes with these high BACs dropped only 5 percent from 1996 to 2007 and 6 percent between 1986 and 1996.
Drinking and driving declines were especially large for drivers younger than 21, a testament to the success of state minimum 21 age drinking laws and zero tolerance laws. In the roadside surveys, the percentage of drivers younger than 21 with BACs of 0.08 percent or higher plunged from 3.2 percent in 1986 to 0.6 percent in 1996, then rose slightly to 1 percent in 2007. Minimum legal drinking age laws were in place in 43 states and D.C. by the end of 1986 and in all states by July 1988. Looking at fatal crashes on Friday and Saturday nights, the percentage of young drivers with 0.08 percent or higher BACs was 47 percent in 1986. Nearly a decade later, the percentage had declined to 38 percent. There has been less progress since then. In 2007, the percentage stood at 37 percent.
Driver characteristics could be factor
Could something about the drivers themselves or the vehicles they drive be the root of the discrepancy?
Looking at driver characteristics, the researchers found that impaired drivers in fatal crashes on weekend nights were more likely than other drivers to be unbelted, drive older vehicles, have been speeding and have prior moving violations and crashes or license suspensions. Except for belt use, however, the differences were roughly consistent over time.
Drivers with BACs at or above 0.08 percent were 30 percent more likely not to use safety belts than drivers with BACs less than 0.08 percent in 1986 and 2 times and 2.4 times more likely not to buckle up in 1996 and 2007, respectively. Prior research has shown that belt use is lower among drinking drivers, even though belt use among all drivers has been on the rise. Buckling up is the first line of defense in any crash. Safety belts reduce the risk of dying in a crash by 45 percent, the National Highway Traffic Safety Administration (NHTSA) estimates. While the vast majority of drivers and front-seat passengers use belts, most people who die in crashes don't.
When researchers controlled for belt use, they found that lack of belt use helped account for the discrepancy in trends between 1986 and 1996 but not between 1996 and 2007. They calculated that the 22 percent decline in impaired drivers on the road during 1986 to 1996 should have produced a 6 percent decline in impaired drivers in fatal crashes, and that is what they observed. On the other hand, the 38 percent decline in impaired drivers on the road during 1996 to 2007 should have reduced the number of impaired drivers in fatal crashes by 24 percent after accounting for belt use. The observed decline, however, was only 5 percent.
The fact that drinking drivers have been slower to buckle up than sober drivers appears to account for some of the discrepancy in trends between 1986 and 1996, McCartt notes, but there appear to be other unknown factors at work between 1996 and 2007.
One could be that other drugs besides alcohol are playing a bigger role in fatal crashes, but so far the data are inconclusive. The rate of reported drug use in fatal crashes was markedly higher in 2007 than it was in 1996 among both drinking and sober drivers.
Teasing out the effects of drugs in crashes is difficult due to data limitations. Although FARS includes information on illegal, over-the-counter and prescription drug involvement in fatal crashes, a large percentage of fatally injured drivers are never screened for drugs other than alcohol. The test rate varies widely among states, and there are inconsistencies in the way localities determine the presence of drugs.
Examining historical trends also is difficult since the national roadside survey didn't begin testing for drugs until 2007. That year, 45 percent of nighttime weekend drivers with BACs at or above 0.08 percent tested positive for other drugs, compared with 33 percent of drivers with positive BACs below the legal limit and 12 percent of drivers with zero BACs. The drugs most often detected were marijuana, cocaine and methamphetamine. Drivers were screened for all drugs, not just illegal ones, and the presence of drugs didn't necessarily mean that these drivers were impaired.
"We can't pinpoint one main cause to explain why drinking and driving is down on weekend nights while fatal crash data aren't showing the same progress. It could be a combination of factors that include lack of safety belt use, drug use and other risky behaviors," McCartt says. "While we're waiting for more data to get a clearer picture of these trends, particularly in drugged driving, it's worth exploring some technological countermeasures to curb alcohol-impaired driving and resistance to belt use."
Technology as a countermeasure
The Institute estimates that 7,132 deaths would have been prevented in 2012 if all drivers with BACs of 0.08 percent or higher were kept off the roads. One way to do so is through the use of alcohol ignition interlocks. An alcohol interlock is a breath-testing unit connected to a vehicle's ignition. The driver blows into it and must register a BAC below a preset level. If the reading exceeds that level, the vehicle won't start. Many people with impaired driving convictions are required to install an interlock, and Institute research shows these are effective at reducing the likelihood that people will reoffend (see "Alcohol ignition interlocks are reducing recidivism among drivers convicted of DUI in Washington," March 6, 2012).
A more effective deterrent would be technology to prevent impaired drivers, regardless of whether they have a prior DUI conviction, from starting any vehicle. The Driver Alcohol Detection System for Safety (DADSS), a joint project of the government and the auto industry, is developing such technology. The goal is to come up with a reliable and unobtrusive system (see "Alcohol-detection device project is now in development phase," Nov. 17, 2011). Current interlock technology, although reliable, is too intrusive for this purpose because it requires a driver to blow into a device and takes at least 30 seconds to compute a BAC. NHTSA expects a research vehicle that incorporates two different approaches to measuring BAC — touch-based and breath-based — to be complete in early 2014.
In-vehicle technology to get more drivers to use safety belts also might help reduce crash deaths. Technologies include enhanced belt reminders and belt ignition interlocks. There is strong support among drivers for auditory reminders that last longer than 8 seconds but little support for belt interlocks to prevent drivers from starting their vehicles if they aren't buckled up. A 2010 IIHS study found that driver fatality rates were 6 percent lower in vehicles with enhanced safety belt reminders, compared with cars without them.
Along with DADSS, safety belt ignition interlocks rank high on NHTSA's research agenda. Federal law prohibits the agency from requiring belt interlocks but allows manufacturers to voluntarily install them to meet a federal safety standard (see "LATCH, belt reminders get lift; highway law supports teen driver laws, impaired driving research," Sept. 20, 2012). The agency in November announced an initiative to work with automakers to speed technological advances to improve highway safety. The three priority research areas include DADSS, belt interlocks and front crash prevention and mitigation systems. NHTSA is exploring whether to allow automakers to voluntarily use interlocks to satisfy certain crash test requirements to meet occupant protection rules. Research is now underway to ensure that such interlocks would be reliable and effective.
Taking drivers out of the equation altogether, as in the case of the self-driving cars being developed, also would help reduce alcohol-related crashes. Whether through technology or enforcement, deterrence is key. Until there are cars that won't operate if an alcohol-impaired driver is behind the wheel, sustained and well-publicized enforcement lets potential violators know they won't get away with it. People are less likely to drink and drive if they believe they will get caught. Sobriety checkpoints are a highly visible enforcement method meant to deter potential offenders, as well as catch violators.