Effects of Cannabinoids on Driving Ability

• Author: James G. Wigmore
• Pages: 106-193

 

 

Ef‌fects of Cannabinoids on Driving Ability

The ef‌fects of THC on driving ability has been studied a lmost as much

as alcohol. The study of THC, however, is more dicult than for alcohol,

as THC is such a low concentration in the blood (/,th that of

alcohol) and there was no simple reliable method for measurement in

the breath. In addition, THC shows a biphasic elimination patter n (very

rapid just after smoking and a prolonged end phase), unlike the uniform

linear el imination of alcohol .

Some early studies did not measure blood THC concentrations and

so could not determine how eciently (if any) the test subject inhaled

the cannabis smoke and so may have been measu ring essentially THC-

free subjects. In addition, the early studies used cannabis ciga rettes

legally supplied by US government growing operations and had low

THC concentrations of less than – percent. More recent studies use a

more realistic concentration of – percent THC or even higher.

The studies on the ef‌fects of THC on driv ing ability can be divided

into f‌ive main types:

• Laboratory studies of various components of the dr iving task

• Laboratory st udies using driving simu lators

• Closed-course driving of motor vehicle

• Actual highway dr iving observations

• Field, epidemiological, or case-control studies of motor vehicle

collisions

Physical signs or tests such as the st andardized f‌ield sobriety tests (SFST)

or drug recognition and evaluation (DRE) ca n assist in determin ing

Chapter : Ef‌fects of Cannabinoids on Driving Ability  

Reference Number: 

driving impairment due to cannabis or other drugs. A lcohol combined

with THC causes more signif‌ica nt impairment of driving ability, which

is of concern, as about  percent of drivers recently arrested for DUIC

in Colorado also had a positive BAC.

Synthetic cannabinoids are not detected in the usual oral f‌luid or

urine cannabinoid tests and cause a much greater impairment in dr iv-

ing ability than botanical cannabinoids. Some proposed countermea-

sures for DUIC include a zero tolerance for THC. Proposed per se blood

THC limits in Can ada are a lower penalty limit of  to  ng/mL and a

higher penalty lim it of  ng/mL or greater. Also, the BAC limit is lowered

from . to . g/mL if the blood contains . ng/mL of THC or more.

This is to account for the additional impai rment caused by the combina-

tion of both d rugs.

. LABORATORY STUDIES

The most controlled method of determining the ef‌fects of THC on dr iv-

ing ability is in the laboratory. The well-known time-distorting ef‌fects

of THC can impair driving ability (). Some tolerance to the ef‌fects

of some aspects of THC on driving i mpairment was found in heavy can-

nabis users (, ). Cannabis-impaired d rivers tend to focus more

on self than on the externa l task (). Cannabis also impaired div-

ided attention tasks, which are an import ant safety aspect of driving

(). Accuracy of a visual attention ta sk decreased with increasing

THC dose (). No dif‌ferences in genders were found in cognitive

testing after smoking a joint (). The use of higher potency skunk

or sinsemilla ca nnabis causes more consistent impairment of executive

functioning, tack ing, and motor impulse control (). Impairment in

a critical track ing task occurred at serum THC concentrations between

 to  ng/mL ().

Reference Number: 

, .., . , . -, .. ,  ..

. “Marijuana, Alcohol and Combined Drug Ef‌fects on the Time

Course of Glare Recovery.” Psychopharmacology, : –,  (

table,  f‌igures,  references)

  Wigmore on Cannabis

Reference Number: 

Abstract: Ten male subjects (ages  to  years) were administered a

placebo,  mL/kg alcohol,  and  mg of THC, and . mL/kg a lcohol

and  mg THC, and were tested on a glare recovery task. Pupil size was

not af‌fected by alcohol. BrACs were determined with an Omicron Intoxi-

lyzer, and the mean peak BrACs were . g/mL (alcohol alone) and

. g/mL (alcohol and  mg THC). It is suggested that smoking

THC may delay the gastric absorption of alcohol. Both alcohol and mar i-

juana delayed glare recovery for at least two hours af ter ingestion.

Although our experiments were performed under photopic light con-

ditions, questions about night vision and the e f‌fect of alcohol and

marijuana on glare recovery obviously arise. It is likely that simila r alco-

hol- and marijuana-induced dysfunc tion occur during night-time activity.

Reference Number: 

, ., . , . , -. , . , . -

, -. , . , . ,  . . “Weed

or Wheel! fMRI, Behavioural, and Toxicological Investigations of How

Cannabis Smoking Af‌fects Skills Necessary for Driving.” PLOS ONE :

e,  ( tables,  f‌igures,  references)

Abstract: Thirt y-one male occasional cannabis users (ages  to 

years) smoked an  percent THC joint (approximately  mg THC) or a

placebo and were tested on various psychomotor skills (e.g., CTT), fMR I

tracking task s, and other measures. Various questionnaires were also

completed (VAS). Blood samples were collected, and blood THC con-

centrations were determined. The highest blood median concentra-

tions were . ng/mL (ranged . to . ng/mL). Time perception and

attention were af‌fected by cannabis.

In conclusion, we have shown that in occasional smokers, cannabis

globally altered the activity of the main br ain networks involved in cog-

nition despite the low THC concentrations. Subject s might be more

attracted by intrapersonal stimuli (sel ) instead of orientating atten -

tion to task performance and this res ults in an insucient allocation of

task-orientated resources. Ef‌fects on BOLD re sponse were associated

with the subjective evaluation of the state of confusion. By contr ast,

we failed to f‌ind any quantitative correlation between the THC levels

measured in whole blood and either th e BOLD signal or the psycho-