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See: Global Marijuana March. ~600 different cities since 1999. First Saturday in May. City lists: 1999 2000 1. 2. 3. 4. 5. 6. 7. 8. 9. 2010. 11 ...Search them. Add city name to search.
With less than 5% of world population the USA has over 2.4 million of 9.8 million world prisoners! The majority of U.S. inmates are in due to the drug war.
Most Republican leaders oppose cheap universal healthcare. 45,000 uninsured Americans die each year due to lack of health insurance.
Relative impairment due to cannabis, alcohol, drugs, fatigue, cell phones, illness, stress, overwork, etc.. Driving and workplace impairment. Drug testing versus impairment testing.
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*Table of Contents. After text loads, click topics below. Click TopLink, back button, or HomeKey to return here fast.

*Intro. THC blood tests for driving impairment.
*Impairment testing versus drug testing.
*Canada and impaired driving.
*Driving and cannabis use. Studies, links.
*Ignition interlock systems.

*Drug War charts, and more.


Introduction. [TopLink]

7% of California Drivers Test Positive for Marijuana, but Are They Impaired? | By Maia Szalavitz. Nov. 20, 2012. Good article. It also refers to a previous study: "Driving within three hours of smoking pot is associated with a near doubling of the risk of fatal crashes."

That review study says: "Canadian researchers pooled the results of nine well-designed, high-quality studies that included nearly 50,000 drivers involved in crashes in multiple countries. They found that recent marijuana use was associated with a 92% increased risk of fatal or near-fatal accidents. ... Driving with a blood alcohol concentration of .08 — the legal limit — nearly triples the risk of crashing; a blood alcohol concentration of .10 almost quintuples it."

That older article had this really interesting fact: "The risk for minor collisions, however, was not raised significantly — possibly reflecting the fact that many stoned drivers attempt to be more cautious and can therefore compensate for some risks. That compensation may fail, however, when quick reflexes are most needed."

That is my experience. Cannabis is not as impairing as alcohol, but quick reflexes are somewhat lacking. From my reading and experience, the first half hour after smoking is the most dangerous time. The concentration of active THC in the blood declines rapidly during that first hour. I think that first half hour (if average to good pot) is deceptively dangerous. I think it approaches or passes the danger of a blood alcohol concentration of .08. There is a timeline chart for THC concerning nanograms per milliliter of saliva or blood here:

You Are Going Directly To Jail (Updated). DUID Legislation: What It Means, Who's Behind It, and Strategies to Prevent It. Paul Armentano, Deputy Director. NORML | NORML Foundation. September 16, 2011. Emphasis added to quote: "Under Pennsylvania’s law, motorists with detectable levels of THC in the blood above 5 ng/ml are guilty of DUID. Under Nevada’s law, motorists with detectable levels of THC in the blood above 2 ng/ml or detectable levels of THC-COOH [THC metabolite] in the urine above 15 ng/ml are guilty of DUID." Quote is from August 25, 2005 version of the article. See the Nevada law here:

From the above-linked NORML report:
"Many European DUID laws rely on blood specimen collection. This is because, unlike urinalysis, both drug metabolites and parent drugs are readily detectable in the blood. In general, peak THC serum levels typically exceed 100 ng/ml minutes after drug ingestion and then fall rapidly. As a result, detection times for marijuana and other parent drugs in the blood at levels above 1 ng/ml is typically only a few hours after past use. Heavy cannabis users, however, may show residual THC serum levels of more than 2 ng/ml up to 48 hours after last use."

From the NORML report:
"Recently, a pair of scientific reviews of automobile crash culpability studies have indicated that THC levels in blood serum below 5 ng/ml are not associated with an elevated accident risk. (Levels below 5 ng/ml are attained in recreational marijuana users, on average, within 1 to 3 hours after cannabis consumption.) Moreover, some studies suggest that "even a THC serum level of between 5 and 10 ng/ml may not be associated with an above normal accident risk." However, additional studies are necessary before reliable THC/blood threshold for impairment may be derived."

So it seems that Nevada's 2 ng/ml limit for THC in the blood will not legally allow driving during the first few hours after using cannabis. One could risk driving and hope that one does not have a serious accident and thus get blood-tested for THC. You would be taking a significant risk, because anybody can have a serious accident, even when completely sober.

Technically, and morally, you would not be in the wrong to drive, especially after waiting an hour after cannabis use. THC blood  levels drop dramatically during the first hour. Unless it was very strong cannabis, or above-average amounts of cannabis ingested, or unless it was combined with some alcohol or other drug, you would not be intoxicated beyond the equivalent threshold for alcohol impairment. Of course, whenever possible it is always better to wait until one is completely sober before driving.

The bottom line seems to be that it takes high doses of THC in the blood to cause legally-significant driving impairment. A level of impairment equivalent to that caused by 0.08% blood alcohol. Most people do not smoke that much cannabis at one time. As concerns driving impairment, the combining of cannabis and alcohol is much more dangerous.

There is very little research because the government rarely provides the cannabis to allow the research. Here is some more info below. The problem with this often-referred-to image below is that the ng/ml amounts shown could be way off since they are estimates of THC blood levels 60 minutes after cannabis use. The estimate is based on the amount of cannabis consumed before the impairment testing. The ng/ml levels are NOT from blood testing. The UK study used actual blood tests to measure THC, and came up with much higher THC levels (ng/ml).

But both studies showed that average cannabis use does not reach the level of impairment caused by a 0.08% blood alcohol level (the legal DUI limit in most places). Both studies also show that the most impairment from cannabis use occurs during the first hour. The studies also show that most people do not smoke to the high level of intoxication necessary to cause legal driving impairment. Even with very strong cannabis, most people recognize its strength, and stop smoking when they reach their normal preferred average high. It takes effort to get legally impaired with cannabis.

For the graph below the THC levels are at 60 minutes after use of cannabis.
The THC levels are estimates based on the amount of cannabis initially consumed.
The THC levels are NOT from actual blood tests.

A URL that you can use to remote load or hot link the above graph image:

"One approach to deriving a legal limit for cannabis during driving has been to set the threshold to the level at which 50% of results show impairment. For alcohol, Berghaus showed a BAC [blood alcohol content] of 0.073% corresponded to impairment on 50% of 923 performance measures examined. The corresponding threshold for THC was 11ng/ml [11 nanograms of THC per milliliter of blood]. This is the closest estimate of dose equivalence to date, although there are recent, well-controlled studies which have not been included in such meta-analyses. A necessary research undertaking would be a thorough meta-analysis of results to date, using statistical measures of effect size related to dose."
 -- Quote above (emphasis added) is from the conclusion page of the 
UK Department of Transport report. Cannabis and driving: a review of the literature and commentary. and and

The Berghaus info mentioned in the above quote is from his 1995 article:
*Behavioral Effects of Alcohol and Cannabis: Can Equipotencies be Established? 1995 by H.-P. Krüger and G. Berghaus, Center for Traffic Sciences, University of Würzburg, Röntgenring 11, D-97070 Würzburg, Germany.

"THC - delta-8-tetrahydrocannabinol - a minor but psycho-active constituent of cannabis.
THC - delta-9-tetrahydrocannabinol - the major psychoactive constituent of cannabis.
THC-COOH - 9-carboxy-THC - the most rapidly produced metabolite, not psychoactive.
CBD – Cannabidiol, the second main constituent of cannabis but not psychoactive, although it may interact with THC to produce effects."

"Several recent papers have sought to correlate plasma THC levels with general impairing effects of cannabis and the specific effects on psycho-motor skills related to driving performance. (Berghaus et al., 1995; Kruger & Berghaus, 1995). The latter paper reviews the available literature to compare the effects of different concentrations of alcohol and THC on various aspects of driving performance. The authors deduced an equi-potency of effects which related a BAC of 73mg/100ml to a plasma concentration of 11ng/ml THC. However, such concentrations relate to the THC levels 60 minutes after smoking a typical cigarette containing 10 mg of THC [average cannabis]. In addition, the levels of THC used in the analysis were in the main not based on measured blood values, but were predicted values calculated from the THC concentrations in the cigarettes and time of testing as obtained from a pharmaco-kinetic model derived by Sticht (1995). There is still considerable debate as to the absolute levels derived from such models and Huestis (1999) gives higher plasma THC values comparable to our own measurements."
-- Above 2 quotes (emphasis added) are from:
*The Influence of Cannabis on Driving. B., Sexton F., et al. United Kingdom: TRL; 2000: pp. 110. The results from a study of different doses of cannabis and the influence on driving and driving related skills are reported. UK's Transport Research Laboratory (TRL). and


What is impairment? Well if you are partying, and not driving, then it can be hard to say if one is "impaired" in one's partying. As in, "I am just an 'impaired' party animal." :)

But if one is driving or working at certain jobs, then there are levels of impairment that can become dangerous, and/or seriously detrimental to productive work.

People make a common error, and that is to extrapolate from one's own experience and to assume everyone else is effected in the same way by cannabis, alcohol, or other drugs.

Because most average moderate-dose cannabis use by most people is so much less impairing than alcohol some people incorrectly say that cannabis never significantly impairs driving. But everyone is effected differently. Smoking different strains of cannabis or different amounts causes different effects on people - even on the same person. Impairment varies greatly for many reasons.  From person to person.

Cannabis has several active ingredients. There are many, many strains of cannabis with many different combinations and potencies of those active ingredients.

Experienced users often stop smoking when they reach their desired state. Inexperienced and/or infrequent users can easily overindulge and get a lot more impaired. And some people are much more impaired by smaller amounts of cannabis. Brain chemistry varies greatly from person to person.

Fatigue adds to impairment. Combining cannabis with alcohol and/or other drugs greatly adds to impairment.

It is not hard to tell if one is too impaired to drive. Just do some of the many roadside sobriety tests. Try them at home.

These tests could save your life. And others.

1. Walk in a straight line.

2. Walk heel to toe.

3. Stand with your feet together.
Then tilt your head back.
Then close your eyes.
Then stretch your arms out to your sides.
Then touch your nose.

4. Then try doing test 3 while standing on one foot.

The last one can be difficult for many people even when sober. It is excellent for this reason. It makes you question whether you are sober enough to drive. This doubt can save lives. The bottom line is that it is better to give a damn about saving others' lives. BEFORE you decide to drive. It is better than the guilt, remorse, and punishment afterwards for killing or disabling someone.

There are many other tests such as the ones above. With many variations. What matters is to do them when completely sober so that you KNOW what is normal for you, and thus to KNOW when one is too impaired to drive.


Impairment testing versus drug testing. [TopLink]

*4-1993. Impairment Testing Effective, DOT Told at Hearing. Impairment Tests test for impairment, fatigue, etc.... without testing for drugs.

*11-1998. NORML News - November 19, 1998. Drug testing negatively impacts employee productivity, study concludes. First major study. 63 companies.


Canada and impaired driving. [TopLink]

This British Columbia Civil Liberties Association (BCCLA) page is very good:
The process outlined in the consultation document consists of several stages. The first stage is triggered when police suspect someone of driving while impaired, typically as a result of poor or erratic driving. Police then observe the driver and ask questions, in an attempt to determine impairment. Standardized field sobriety tests are performed and the driver is required to take a breath test. This breath test either indicates alcohol is present, or rules alcohol out as the cause of any potential impairment.
So far, so good. "Standardized field sobriety tests" include some of the ones listed before: Feet together, head tilted back, eyes closed, arms out, touch nose. Stand on one leg and repeat all the above. And other similar tests.
Blood alcohol content tests are well understood, and are reliable enough to hold up in court.

The webpage then goes on to describe why no other tests after that are reliable indicators of too much impairment.


Driving and cannabis use. [TopLink]

*TRL - Creating the future of transport. UK's Transport Research Laboratory (TRL). Use their site search engine to search for "cannabis" 

*Schaffer Library of Drug Policy. References on Drugs and Driving.

*Erowid Cannabis Vault : Cannabis (Marijuana) & Driving Impairment. Many links and reports.

*Google search shortcuts. Cannabis use and driving. and

*2002. Canadian Senate report on cannabis and driving. First link below is the summary. Second link is the comprehensive full report that analyzes many studies. and

*Drug Policy Alliance: Marijuana and Driving Bibliography.

*NORML Home / Library / Driving and Marijuana.

*Drugs and Driving. Willette, Robert E, Ed. NIDA Research Monograph; March 1977: pp. 152. This monograph presents a critical review of the available literature relating drug use to driving and other complex human performance. It offers recommendations for the future.

*Marijuana's Effects on Actual Driving Performance. 1995. Robbe, Hindrik W.J.. From The 13th International Conference on Alcohol, Drugs and Traffic Safety (T'95) held in Adelaide, Australia from 13 August 1995 to 18 August 1995.

*Marijuana use and driving. 1994. Robbe, Hindrik W.J.. Journal of the International Hemp Association 1: 44-48. 

"I can remember, some 40 years ago, as a young policeman in Harlem, gathering in the bar with my colleagues after work listening to them complain vigorously about the junkies who made our work so difficult. During our discussions, we drank prodigious amounts of beer without the slightest awareness we were consuming a drug that could be as lethal as heroin. In fact, more of my fellow policemen died in driving accidents after these drinking sessions than were slain in the line of duty."
-- Joseph D. McNamara, D.P.A.. Commentary: Criminalization of Drug Use. Psychiatric Times, September 2000, Vol. XVII Issue 9.


Ignition interlock systems. [TopLink]

It might be a good idea to put these in ALL cars. The cost would be low if all new cars came with these. Economies of scale would allow this to happen. In the end the costs would be offset by the savings in accident and medical costs.

Insurance costs for everybody would be dramatically cut too.

An ignition interlock device prevents a vehicle from starting if the driver’s blood alcohol content (BAC) is above a specified set limit. 

*Google search shortcut for ignition interlock. 

*Schaffer Library of Drug Policy. References on Drugs and Driving. See section titled "ALCOHOL IGNITION INTERLOCK DEVICES".


Drug War charts, and more. [TopLink]