Marijuana's impairing effects on driving are moderate when taken alone but severe when combined with alcohol

Abstract

Previous experimental and epidemiological studies failed to provide unequivocal evidence that marijuana, either alone or in combination with alcohol, impairs a driver's performance to the extent that it will compromise trac safety. We investigated the eects of marijuana, alone and in combination with alcohol, on actual driving in four, single-blind, randomized, cross-over studies. In Study 1, 24 subjects performed a road-tracking test on a closed segment of a primary highway after smoking marijuana that contained 0, 100, 200 and 300 mg/kg D 9 -tetrahydrocannabinol (THC). In Study 2, 16 new subjects smoked the same THC doses before they performed a roadtracking and a car-following test; however, this time in the presence of other trac. In Study 3, two groups of 16 subjects performed a city driving test. One group smoked marijuana delivering 0 and 100 mg/kg THC prior to driving; the other group drunk orange juice mixed with or without a low dose of alcohol. In Study 4, 18 subjects performed a road-tracking and a car-following test in each of six conditions where they smoked marijuana with 0, 100, or 200 mg/kg THC after they had consumed orange juice with or without alcohol. In these studies, marijuana alone signi®cantly increased lateral position variability in the road-tracking test and distance variability during deceleration manoeuvres in the car-following test. Reaction times during car-following were not signi®cantly aected, and a THC dose of 100 mg/kg did not impair city driving performance. Blood plasma concentrations of THC and THC-COOH were not related to the degree of impairment. A low dose of alcohol (i.e. blood alcohol concentrations around 0 . 04%) impaired performance in all driving tests. Whereas marijuana's eects on driving performance were small (100 mg/kg THC) or moderate (200 and 300 mg/kg) when taken alone, they were severe when combined with a low dose of alcohol. In conclusion, marijuana alone impairs driving performance, with the degree of impairment increasing from small to moderate as the THC dose increases from 100 to 300 mg/kg. However, when low to moderate doses of THC (100 and 200 mg/kg) are taken in combination with a low dose of alcohol sucient for attaining a BAC of about 0 . 04% actual driving is severely impaired. #

Cannabis smoking impairs driving performance on the simulator and real driving: a randomized, double‐blind, placebo‐controlled, crossover trial

“…Results show that driving stability is impaired after THC intake, both in simulated and real driving conditions. Globally, these results are coherent with the results of previous studies involving simulation [46,47] or real driving conditions [16][17][18]28]. In Lenn e et al's study, SDLP was significantly increased, as compared to placebo, after smoking cigarettes with 19 and 38 mg of THC, in occasional smokers [46].…”

“…In Lamers & Ramaekers's study stable degradation of SDLP was observed after 40 min and 100 min of driving on a closed-course road [16]. Dose-dependent impairment was observed in Robbe's study [17].…”

The effect of alcohol, THC and their combination on perceived effects, willingness to drive and performance of driving and non-driving tasks

“…As in our previous study (Ronen et al, 2008), when THC alone was administered, subjects drove significantly slower than when they were under the influence of alcohol, which actually made them drive faster. This is consistent with Robbe (1998), which according to his findings noted that they support the belief that drivers become overconfident after drinking alcohol but may become more cautious and self critical after consuming small amounts of marijuana. Lenne et al (2010) also found that performance following inhalation of cannabis was associated with increase of headway and decrease in mean speed while alcohol at the low levels studied in their experiment (0.02 and 0.05 BAC) had fewer effects including a slight increase in mean speed.…”

“…Robbe (1998) found that marijuana alone impairs driving performance mainly under "medium-high" doses, and when combined with alcohol the impairment was greater than with alcohol or marijuana alone. Lenne et al (2010) tested the effects of cannabis and alcohol on simulated arterial driving and found that their higher level of cannabis (Mean of 12.01 ng/ml) caused greater levels of impairment than lower levels (mean 7.40) while alcohol at the doses used (mean 0.02, 0.05 BAC) had few effects and did not induce synergistic effect when combined with cannabis.…”

Analysis of Δ9-tetrahydrocannabinol in oral fluid samples using solid-phase extraction and high-performance liquid chromatography–electrospray ionization mass spectrometry

“…There is strong evidence from performance studies that D 9 -THC has significant effects on the cognitive and psychomotor tasks associated with driving [2]. Moreover, cannabis is often taken together with other drugs, such as alcohol, resulting in enhanced psychoactive effects and more severe impairment of driving performance [5,6]. New, easy and sensitive methods to detect impaired drivers are needed in our country.…”