NHTSA Studies Show Drugged Driving Increasing, But Drunk Driving More Dangerous

The National Highway Traffic Safety Administration (NHTSA) has released two new studies that highlight the changing landscape of impaired driving on America’s roadways.

NHTSA’s most recent Roadside Survey of Alcohol and Drug Use by Drivers shows a steady and significant decline in drunk driving—rates have dropped by nearly a third since 2007, and by more than three-quarters since 1973. NHTSA largely credits the drop to education and enforcement campaigns that target intoxicated driving.

But the good news on drunk driving is countered by skyrocketing rates of drugged driving, which have risen sharply in recent years. According to the roadside survey, “nearly one in four drivers tested positive for at least one drug that could affect safety,” with marijuana topping the list.

The spike in stoned drivers is catching the attention of states that have or are considering making medical or recreational marijuana use legal. Many law enforcement agencies are concerned that people view drugged driving as more acceptable than drunk driving.

Yet the second study released by NHTSA notes that it is not yet clear the extent to which marijuana impairs a person’s ability to operate a vehicle. Researchers attempted to determine if using pot is associated with a higher level of crashes but were unable to definitively show a link. The study shows a correlation between drivers who test positive for THC and a slight increase in crash rates. However, the research was unable to determine if the increase was due to marijuana usage or other risk factors.

In contrast, research has established that drivers with a blood alcohol concentration over the legal limit are 5 to 200 times more likely to be in a crash than someone who is sober. Experts speculate these findings could be explained in part by the differences in how the body processes alcohol and drugs. Drivers with a positive BAC are actively impaired at the time of the test. In contrast, evidence of drug use can remain in the body long after the impairing effects of the drug have worn off.

NHTSA emphasizes that far more research on drugged driving is needed to get a better understanding on how drug use affects a person’s ability to operate a car. What is your jurisdiction doing to address the issue of drugged driving?

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Field Sobriety Test Studies Found to Be Flawed

Proponents of the so-called “standardized” field sobriety tests (SFSTs) have long pointed to federally-funded field studies which indicate a high correlation between performance on the tests and actual blood alcohol concentrations (BAC). 

Subsequent studies, however, have called those conclusions into question.

Originally, the National Highway Traffic Safety Administration (NHTSA) paid a private group, the Southern California Research Institute, to conduct studies to find which among the various field sobriety tests used by police were most effective and to develop a standardized 3-test battery.  SCRI subsequently reported to NHTSA that a battery of walk-and-turn, one-leg-stand and nystagmus provided a strong correlation with breath test results.

Confronted with questions about those conclusions, NHTSA later commissioned the same researcher who had conducted the original studies, Marcelline Burns, to  corroborate the accuracy of her own tests of the SFSTs – rather than commission an independent source. 

Burns accompanied a small number of San Diego officers conducting actual DUI investigations in the field.  After administering the SFSTs, the officers were asked to guess whether suspects had blood alcohol  concentrations (BAC) over or under .08%.   Burns reported a 91% correlation between SFSTs and BAC over-under estimates, thereby validating the battery of tests she had helped create.

A subsequent scientific article challenged Burns’ corroboration of her own research.  In Hlastala, Polissar and Oberman, “Statistical Evaluation of Standardized Field Sobriety Tests”, 50(3) Journal of Forensic Sciences 1 (May 2005), the raw data used in the validation study were obtained from NHTSA through the Freedom of Information Act.  The methodology used was then reviewed and the data subjected to statistical analysis.

The methodology was found to be seriously flawed in a number of respects.  For one thing, many of the suspects had very high BACs, making estimates of whether a suspect was over .08% obvious regardless of SFST performance.  For another, there was no attempt to isolate the influence of SFST performance from other factors:  officers estimated BACs after the field sobriety tests, but they also took into account earlier observations, such as erratic driving, slurred speech, odor of alcohol, flushed face, admissions as to amount of alcohol consumed, etc.

The most glaring defect in Burns’ corroborative study was that “all police officers  participating in the study were equipped with NHTSA-approved portable breath testing devices”.  In other words, the San Diego officers already had the results of portable breath tests before they were asked to estimate the BACs later obtained at the station!

After reviewing the flawed methodology, the raw data was then statistically analyzed.  The conclusions:

If we consider three ranges of MBAC [measured blood alcohol content], 0.00% to 0.04%, 0.04% to 0.08%, and 0.08% to 0.12%, the officers’ EBAC [estimated blood alcohol content] overestimated the MBAC 76%, 67% and 48% of the time, and underestimated it 14%, 26% and 28% of the time. 

In other words, officers relying upon field sobriety tests were far more likely to overestimate  BACs than underestimate — particularly with those suspects having very low BACs. 

(T)he utility of the SFST depends very much on how intoxicated an individual is.  Accuracy (and specificity) are low when individuals are close to 0.08% MBAC, but if the individuals are quite intoxicated, such as above 0.12%, then accuracy is high.

In borderline cases involving persons at or under the legal limit, then, officers were very poor at estimating blood-alcohol levels based upon SFSTs.  And it is these cases, of course, that are critical.  Suspects with high BACs are relatively easy to single out without the help of field tests; it is for the closer cases, particularly those who are innocent (below .08%), that the SFSTs are designed.  And it is with these very cases that the tests apparently fail. 

Put another way, accuracy in using field sobriety tests is high when they are not needed — and low when they are.

For another independent study conducted by Professor Spurgeon Cole of Clemson University, in which he found field sobriety tests to be worthless, see Are Field Sobriety Tests Designed for Failure?.

This entry was posted on Monday, June 18th, 2012 at 12:29 am and is filed under Duiblog. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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