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Marijuana's effects on sdlp were compared to those of alcohol obtained in a very similar study by Louwerens et al. (1987). It appeared that the effects of the various administered THC doses (100-300 µg/kg) on sdlp were equivalent to those associated with bacs in the range of 0.03-0.07 g% . . . THC's effects on road-tracking after doses up to 300 µg/kg never exceeded alcohol's at bacs of 0.08 g%; and, were in no way unusual compared to many medicinal drugs (Robbe, 1994; Robbe and O'Hanlon, 1995; O'Hanlon et al., 1995).By comparison, BACs of 0.03-0.07 [grams of ethanol per 100ml blood] are legal everywhere in the US, where maximum legal BACs are 0.08-0.1. And a dose of 100-300 micrograms THC per kilogram is equal to the THC dose from one-half to one and one-half joints, which is a substantial dose. In other studies, for instance one using a driving simulator, lane-keeping performance is even better with cannabis only, but non-additively worse in combination with alcohol. This is consistent with other studies showing that a small amont of alcohol and a small amount of cannabis impair performance much more than would be expected based on studies of each in isolation. Krüger and Vollrath (2000) found that:
When cannabis was consumed very recently (as indicated by the presence of THC in blood) lane keeping performance improves and neither speed nor performance in secondary tasks is affected. When cannabis consumption lies some time back (as indicated by the presence of THC-COOH), the improvement of lane keeping gets even stronger. Moreover, speed is reduced. When alcohol is consumed additionally, these positive changes are reversed and reaction in secondary performance deteriorates. These effects are especially strong for the THC-COOH group. A similar change results from combining cannabis with amphetamines and ecstasy in high concentrations. Lane keeping performance is worse than in sober subjects when both substances are combined. The deteriorating effect of amphetamines/ecstasy remains also when combined with cannabis. Thus, while for cannabis use alone no impairment of driving performance was found (and even an improvement for THC-COOH), the combination of cannabis with alcohol and / or amphetamines / ecstasy is detrimental.Interestingly, drivers under the influence of cannabis tend to drive more slowly, increase their following distance, and are less likely to pass other cars. Individuals under the influence of cannabis often overestimate their level of impairment and adjust their driving to compensate. Alcohol has the opposite effect, that is, individuals under the influence of alcohol tend to underestimate their level of impairment, drive faster, decrease following distance, and pass more. Using subject self-ratings, Robbe (1998) in their study of driving under real-world conditions noted that "[a]lcohol impaired performance relative to placebo but subjects did not perceive it. THC did not impair driving performance yet the subjects thought it had," and that "[e]vidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments."
Analyses that exclude the presence of alcohol revealed that marijuana's impact was not significant. For example, data from over 1000 drivers involved in fatal accidents in Australia revealed that cannabis was present in 11% of them. Ratings if the accident reports revealed that drivers who had consumed alcohol or the combination of alcohol and cannabis were cuplable more often than drivers who were free of drugs. In contrast, ratings revealed that those who used only cannabis were responsible for accidents less often than those who used no drugs at all (Drummer, 1994).Recent reviews of crash culpability studies also fail to support a significant causal role for cannabis in automobile crashes. Chesher Longo (2002) state:
Curiously, many studies of marijuana and traffic safety found that the odds of causing death or injury were slightly lower in cannabis users than in people who who had not consumed drugs (Bates and Blakely, 1999). For example, the study of Australian motorists mentioned above showed that consumers of cannabis were 30% less likely to cause accidents as drivers who had not used any drug. A study of over 300 drivers involved in fatal crashes in California focused on motorists who tested positive for cannabis and no other drug. Unexpectedly, they were half as likely to be responsible for accidents as those who were free of substances (Williams, Peat, & Crouch, 1985). Another investigation of over 1800 fatal crashes in the United States found that drivers who used only cannabis were only 70% as likely to be responsible for accidents as the drug-free group (Terhune, Ippolito, & Crouch, 1992).
The results to date of crash culpability studies have failed to demonstrate that drivers with cannabinoids in the blood are significantly more likely than drug-free drivers to be culpable in road crashes. … [In] cases in which THC was the only drug present were analyzed, the culpability ratio was found to be not significantly different from the no-drug group.A 1999 review of automobile accident epidemiology also failed to support a causal role for cannabis in automobile crashes. Bates and Blakely (1999) wrote:
There is no evidence that consumption of cannabis alone increases the risk of culpability for traffic crash fatalities or injuries for which hospitalization occurs, and may reduce those risks.
Recent exposure to cannabis can only be safely assumed in the minority of culpability studies that determined cannabis use by the presence of THC in the blood. This latter procedure was only followed in two surveys. Culpability odds ratios in THC positives were generally higher than those in THC–COOH positives. Moreover, culpability odds ratios in THC positives were three to six times as high as compared to drug free drivers, depending on the concentration of the drug detected in blood. Together, these data indicate that recent cannabis use may increase crash risk, whereas, past use of cannabis as determined by the presence of THC–COOH in drivers does not.The two recent studies refered to by Raemakers et al are those of Hunter et al (1998) and Drummer et al (2004). Hunter et al reported that drivers with with blood THC concentrations of 2ng/ml, corresponding to cannabis smoking a few hours in the past, actually had lower odds of being involved in crashes than drug-free drivers, with odds ratios of 0.51 indicating nearly 50% reduction. [The same is true for several of the culpability studies using urinary THC-COOH. For instance, 4 out of 6 culpability studies using urinary THC-COOH listed in Raemaers et al table 1 give odds ratios less than 1, indicating decreased risk of accident for drivers positive for THC-COOH. Note that in these same studies, alcohol was associated with greatly increased risk of accident, from about 300% to 700%]. In the group of drivers with blood THC concentrations greater than 2ng/ml (which may range up to several hundred ng/ml), Hunter et al reported a moderately increased risk of accident, odds ratio 1.74. Drummer et al (2004) also reported an increased risk of crash associated with blood THC, with an odds ratio of 2.7. For the subset drivers with highest blood THC, odds ratio was even higher, 6.6. An additive effect was found for THC and alcohol, such that drivers with blood alcohol >0.05 g/dl + blood THC were several times more likely to experience a crash than drivers positive only for alcohol. Again, this is consistent with several previous studies showing that THC+alcohol is associated with greater risk of crash than alcohol alone, and with experimental studies of cannabis+alcohol on driving ability (e.g. Ramaekers et al, 2000).
In 1998, a field study funded by the Federal Highway Research Institute(BAST) of Germany was conducted with drivers at discotheques. Subjects were contacted by researchers from the Center of Traffic Sciences, Wuerzburg (IZVW) and asked to participate in a short interview concerning drugs and driving. Subjects who had consumed drugs and had either been driving, were going to drive or indicated that they had been driving under the influence of drugs at other occasions were asked for an extended interview, a driving-simulator test and for blood, urine and saliva samples (N = 241). Additionally, sober control subjects (N = 59) and subjects under the influence of alcohol (N = 45) were included. The study examines the effect of cannabis (two groups: acute and previous consumption) and amphetamines and ecstasy (two groups: low and high concentration) alone and in combination with alcohol on driving performance. Driving performance was analyzed be means of a driving-simulator which evaluates the ability to maintain lateral position and speed, tests peripheral and central attention and risk-taking behavior. The analyses show that consumption of cannabis and amphetamines / ecstasy, only, does not adversely affect driving behavior. However, the combination of the two substances with one another and or alcohol leads to a substantial impairment of driving and performance in secondary tasks.Mason and McBay, 1984. Ethanol, marijuana, and other drug use in 600 drivers killed in single-vehicle crashes in North Carolina, 1978-1981. Journal of Forensic Science 29(4), 987-1026.
Although the use of ethanol, marijuana, and other drugs may be detrimental to driving safety, this has been established by direct epidemiological evidence only for ethanol. In this study, the incidences of detection of ethanol (and other volatile substances), delta-9-tetrahydrocannabinol (THC), barbiturates, cocaine and benzoylecgonine, opiates, and phencyclidine were determined in an inclusive population of 600 verified single-vehicle operator fatalities that occurred in North Carolina in 1978 to 1981. The incidence of detection of amphetamines and methaqualone were determined for drivers accepted for study during the first two years (n = 340) and the last year (n = 260), respectively. Blood concentrations of 11-nor-deta-9-tetrahydrocannabinol-9-carboxylic acid (9-carboxy-THC) were determined in THC positive drivers. EMIT cannabinoid assays were performed on blood specimens from all drivers accepted for study during the third year, and the feasibility of using the EMIT cannabinoid assay as a screening method for cannabinoids in forensic blood specimens was investigated. The incidence of detection of ethanol (79.3%) was far greater than the incidences determined for THC (7.8%), methaqualone (6.2%), and barbiturates (3.0%). Other drugs were detected rarely, or were not detected. Blood ethanol concentrations (BECs) were usually high; 85.5% of the drivers whose bloods contained ethanol and 67.8% of all drivers had BECs greater than or equal to 1.0 g/L. Drug concentrations were usually within or were below accepted therapeutic or active ranges. Only a small number of drivers could have been impaired by drugs, and most of them had high BECs. Multiple drug use (discounting ethanol) was comparatively rare. Ethanol was the only drug tested for that appears to have a significantly adverse effect on driving safety.Movig et al, 2004. Psychoactive substance use and the risk of motor vehicle accidents. Accident Analysis and Prevention 36, 631–636.
The driving performance is easily impaired as a consequence of the use of alcohol and/or licit and illicit drugs. However, the role of drugsRamaekers et al, 2000. Marijuana, alcohol and actual driving performance. Human Psychopharmacology 15, 551–558.
other than alcohol in motor vehicle accidents has not been well established. The objective of this study was to estimate the association
between psychoactive drug use and motor vehicle accidents requiring hospitalisation.
A prospective observational case-control study was conducted in the Tilburg region of The Netherlands from May 2000 to August 2001.
Cases were car or van drivers involved in road crashes needing hospitalisation. Demographic and trauma related data was collected from
hospital and ambulance records. Urine and/or blood samples were collected on admission.
Controls were drivers recruited at random while driving on public roads. Sampling was conducted by researchers, in close collaboration
with the Tilburg police, covering different days of the week and times of the day. Respondents were interviewed and asked for a urine
sample. If no urine sample could be collected, a blood sample was requested.
All blood and urine samples were tested for alcohol and a number of licit and illicit drugs. The main outcome measures were odds ratios
(OR) for injury crash associated with single or multiple use of several drugs by drivers.
The risk for road trauma was increased for single use of benzodiazepines (adjusted OR 5.1 (95% Cl: 1.8–14.0)) and alcohol (blood
alcohol concentrations of 0.50–0.79 g/l, adjusted OR 5.5 (95% Cl: 1.3–23.2) and ≥0.8 g/l, adjusted OR 15.5 (95% Cl: 7.1–33.9)). High
relative risks were estimated for drivers using combinations of drugs (adjusted OR 6.1 (95% Cl: 2.6–14.1)) and those using a combination
of drugs and alcohol (OR 112.2 (95% Cl: 14.1–892)). Increased risks, although not statistically significantly, were assessed for drivers
using amphetamines, cocaine, or opiates. No increased risk for road trauma was found for drivers exposed to cannabis.
The study concludes that drug use, especially alcohol, benzodiazepines and multiple drug use and drug–alcohol combinations, among
vehicle drivers increases the risk for a road trauma accident requiring hospitalisation.
Marijuana's effects on actual driving performance were assessed in a series of three studies wherein dose-effect relationships were measured in actual driving situations that progressively approached reality. The first was conducted on a highway closed to other traffic. Subjects (24) were treated on separate occasions with THC 100, 200 and 300 g/kg, and placebo. They performed a 22-km road tracking test beginning 30 and 90 minutes after smoking. Their lateral position variability increased significantly after each THC dose relative to placebo in a dose-dependent manner for two hours after smoking. The second study was conducted on a highway in the presence of other traffic. Subjects (16) were treated with the same THC doses as before. They performed a 64-km road tracking test preceded and followed by 16-km car following tests. Results confirmed those of the previous study. Car following performance was only slightly impaired. The third study was conducted in high-density urban traffic. Separate groups of 16 subjects were treated with 100 g/kg THC and placebo; and, ethanol (mean BAC .034 g%) and placebo. Alcohol impaired performance relative to placebo but subjects did not perceive it. THC did not impair driving performance yet the subjects thought it had. These studies show that THC in single inhaled doses up to 300 g/kg has significant, yet not dramatic, dose-related impairing effects on driving performance.Robbe, 1995. Marijuana’s effects on actual driving performance. In: C. Kloeden and A. McLean (Eds) Alcohol, Drugs and Traffic Safety T-95. Adelaide: Australia: HHMRC Road Research Unit, University of Adelaide, pp. 11-20.