This is the final report of a project entitled "State of Knowledge of Drug-Impaired Driving." The project was conducted by Mid-America Research Institute, Inc., of New England for the National Highway Traffic Safety Administration (NHTSA). David Shinar of Ben Gurion University of the Negev, Israel, and J. Michael Walsh of The Walsh Group, Bethesda, Maryland, made significant contributions. This review examines research published during the 1981-2001 period and references some of the earlier material contained in prior reviews.
The scope of the review included foreign as well as U.S. literature with a direct bearing on highway safety. The review emphasizes controlled substances to include marijuana, benzodiazepines, non-benzodiazepine sedative and hypnotic drugs, and others such as amphetamines, cocaine, hallucinogens, and narcotic drugs. However, research related to any other drugs having the potential to significantly impair driving is also included in the review. Applicable research conducted in foreign countries, and documented in the English language, is included.
This report is presented in six substantive chapters. Chapter 2 contains a description of the methods followed in determining the topics and issues of concern in the review, identifying, acquiring and screening the documents to be reviewed, and reviewing individual documents. Chapter 3 deals with research pertinent to the detection and measurement of drugs in drivers, and Chapter 4 reviews the experimental literature, including research conducted in a laboratory testing human performance on tasks believed to be related to driving, and research conducted either in a driving simulator or on a closed course testing performance in actual driving tasks. In Chapter 5, we examine literature flowing from epidemiologic studies of drugs and traffic crashes, including literature on the drug use of various subgroups of drivers such as drivers arrested for drunk driving or "drugged" driving. Chapter 6 deals with literature on countermeasures for drug-impaired driving, and Chapter 7 presents our conclusions and recommendations. An index of terms and a bibliographic listing of references follow at the end of the report.
Our conclusions and recommendations are organized by the four major types of scientific literature examined in the review, namely:
- Detection and measurement of drugs in drivers,
- Experimental research on the effects of drugs on performance of driving-related tasks.
- Epidemiologic research on the drugs in driving populations, including drivers in crashes, on-the-road drivers not in crashes, and drivers suspected or convicted of drug-impaired driving.
- Research on countermeasures for drug-impaired driving.
The conclusions and recommendations are presented below. Examples of documents supporting the specific conclusions are cited, and cross references to pages of this report discussing more general conclusions are provided.
DETECTION AND MEASUREMENT OF DRUGS IN DRIVERS
- A variety of specimens can be assayed for drugs, including urine, blood, sweat, saliva, and hair, among others. Each specimen is unique, and each offers different patterns of information about drug use over time (page 11).
- Most laboratories use immunoassay screening technology with gas chromatography-mass spectrometry (GC/MS) confirmation. Over the last 20 years the cost of using these technologies have become affordable, and most laboratories now have the equipment, the assays, and the expertise to identify the most commonly used drugs (see reference).
- While there have been significant improvements in laboratory assays for drugs of abuse, the value of such improvements to highway safety specifically is limited by an insufficient number of laboratories incorporating these improvements.
- The reliance solely on the forensic laboratory to assay all specimens in all cases limits the number drug-impaired driving cases that can be prosecuted, because there are simply not enough forensic resources currently available.
- Point-of-contact-testing (POCT) devices offer promise for alleviating this problem. For example, these POCT devices could be used by police officers to routinely screen DUI suspects for illegal drug use and obtain drug test results immediately, as they currently do with alcohol tests (page 15).
- Until there is adequate capability for rapid, cost-effective drug testing, many drugged drivers will not be identified or prosecuted.
- Federal and state agencies concerned with traffic safety should provide additional support to enhance forensic capabilities to detect and measure drugs in drivers.
- The forensic community should give more attention to the new POCT technology and work to integrate this technology with laboratory testing into a more efficient and cost-effective system for detecting and quantifying drugs other than alcohol in drivers.
Selected literature on the effects of a wide range of drugs on performance of driving-related tasks and performance of actual driving tasks was reviewed. Classes of drugs considered were:
- central nervous system (CNS) depressants,
- CNS stimulants,
- antihistamines, and
- other drugs that have been investigated in a few individual studies.
- The amount of research in these classes varies widely, with the most attention given to CNS depressants and the least given to narcotics. We found essentially no experimental research on some other classes of drugs not listed above, for example, hallucinogens and inhalants.
- With respect to the acute effects of drugs, it appears that the following drug classes have a high potential for significant impairment of driving and driving-related performance:
- narcotics (Stevenson, Pathria, Lamping, et al. (1986),
- long-life benzodiazepines in therapeutic doses (Soames, 1982),
- short-life benzodiazepines in high doses (Kunsman, Manno, Przekop et al., 1992),
- barbiturates (Mintzer, Guarino, Kirk, et al. (1997)),
- 1st generation H1 antihistamines (Moskowitz and Wilkinson, 2003; Starmer, 1985), and
- certain anti-depressants, that is, amitriptyline, doxepin, and mianserin (see page 44).
- Drugs classes with a relatively low potential for significant impairment after acute usage are:
- CNS stimulants (which actually may improve performance at low doses in some instances) (Ward, Kelly, Foltin, and Fischman, 1997),
- 2nd generation H1 antihistamines (Starmer, 1985) , and most other anti-depressants (page 47).
- The literature suggests that acute use of cannabis has a moderate potential for impairment (Lamers and Ramaekers, 1999).
- Very few studies have examined the chronic and sub-chronic use of the above classes of drugs, and most of those that have suggest little effect on driving and driving-related performance.
- All-in-all, the literature supports the common-sense notion that drugs with a strong sedative action taken in the highest doses have the highest potential for significant impairment, while others have the lowest potential. Other meta-generalizations about which tasks and functions are impaired by which doses of which drugs cannot be made on the basis of the literature we examined.
- Current experimental research should be continued, with emphasis on newly emerging drugs with potential to impair driving performance.
- More research should be performed to determine the effect of chronic as well as acute use of drugs on the performance of realistic driving-related tasks. Such research should include both closed-course studies, and also simulator studies of the types possible in the National Advanced Driving Simulator at the University of Iowa.
- A significant amount of new information has been added to the pool of scientific knowledge about the role of several classes of drugs in traffic crashes since the last state of knowledge update. However, gaps still exist on certain drug classes that are in widespread use, for example, antihistamines and antidepressants.
- The literature suggests that the prevalence of the drugs that have been studied in driver populations, while not negligible, is much smaller than the prevalence of alcohol in such populations.
- The literature indicates that chemical tests of drivers in North American crashes were performed most often for narcotics, benzodiazepines, barbiturates, cocaine, amphetamines, and cannabis.
- Of these drugs, cannabis/marijuana has been found the most often by a wide margin. This should not be surprising, given the findings of the 2001 National Household Survey on Drug Abuse (U.S. Department of Health and Human Services, 2002) that 76% of current users of illicit drugs were users of this cannabis/marijuana.
- For fatally injured drivers, cannabis had the highest percentages testing positive, ranging from 7% to 37% with a mean of 14%. The mean percentages of each of the other five drugs amounted to about 5% or less (page 82).
- Few of the reviewed studies examined the percentages of various drug classes found in non-crash-involved drivers (page 81). Only two drugs were found to be present in more than 1% of the drivers: benzodiazepines (4% in a Canadian study and a mean of 3% in other foreign studies), and cannabis (5% in the Canadian study).
- Except for benzodiazepines, the percentages of drug-positive drivers suspected by the police of driving under the influence of drugs were about the same in foreign studies as in U.S. studies, ranging from an average of about 13% for barbiturates to 28% for cannabis (page 85). Benzodiazepines appeared in an average of 30% of suspected drivers tested in foreign studies versus 14% in the U.S. studies. Only one foreign study (in Switzerland) had data for cocaine use (11%), and the U.S. studies indicated an average of about 16% of the tested suspects were positive for cocaine.
- The role of drugs as a causal factor in traffic crashes involving drug-positive drivers is still not understood. Drug risk factors are still not known with acceptable precision, with some evidence suggesting little or no increase in crash risk at drug levels being detected by current chemical test procedures. Available evidence (page 83) suggests a maximum risk factor of about 2.0 occurring for benzodiazepines and cannabis, followed closely by narcotics at 1.5. CNS stimulants (including cocaine and amphetamines) were associated with either no increased risk factor (cocaine) or even a decreased risk factor (other stimulants).
- Current research does not enable one to predict with confidence whether a driver testing positive for a drug, even at some measured level of concentration, was actually impaired by that drug at the time of crash. This is in sharp contrast to alcohol where BAC measurements can provide a good estimate of impairment.
- With respect to drug prevalence, the state of knowledge about the prevalence of drugs in traffic crashes in the U.S. should be updated periodically. Drugs of interest should include those currently in vogue among user populations.
- With respect to drug-crash risk, a program of research should be undertaken to assess the traffic-crash risk associated with the potentially impairing drugs that current knowledge suggests are the most prevalent in serious traffic crashes in the United States. This research program should compare the drug use of drivers who were involved in crashes with that of a similar group of drivers who were not involved in crashes. The program should concentrate first on fatal crashes and should be of sufficient geographic scope to enable some reasonable assessment of the general magnitude of any drugged-driving problem nationwide. Clearly, such a research program poses some formidable difficulties, especially with respect to drugs in on-the-road, non-crash involved drivers. Nevertheless, work must begin if further progress is to be made in defining the drug-crash problem in this country.
COUNTERMEASURES FOR DRUG-IMPAIRED DRIVING
- Countermeasure approaches in the United States and Europe have involved the use of the Criminal Justice System to enforce drugged driving laws using methods similar to those used in enforcing alcohol-impaired driving laws.
- The major emphasis in these countermeasures is the identification of impairment among stopped drivers using chemical tests and / or clinical assessments.
- We found no evaluations of the impact of any drugged driving countermeasure on crashes, either in the United States or Europe. This might be expected, given the lack of any databases containing objective measures of the presence of drugs in crash-involved drivers.
- Determine the effect on traffic crashes of existing drug-impaired driving countermeasure programs in selected jurisdictions.
- Develop ways of improving the response of the Criminal Justice System to drug-impaired driving, including legislation, enforcement, adjudication, and sanctioning.
- Identify new, more innovative approaches to dealing with drug-impaired driving with initial emphasis on drug classes known to have higher potential for creating drug-crash risk.
- Increase the extent and intensity of research and development efforts to apply technology to drug-impaired driving.
- Provide more funding support to the efforts of operational agencies involved in current drug-impaired driving countermeasure efforts.
- Establish an integrated, long-term drug-impaired driving program at the federal level incorporating the above elements in a phased approach.