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Smoke and Mirrors: Driving While High on Marijuana Doubles One’s Chances of a Serious Car Crash

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Booze is behind an estimated 2.1 million car accidents each year in the U.S.—which cause almost 11,000 traffic fatalities annually. But many drug users have claimed that a few puffs of pot before getting behind the wheel are perfectly harmless. A new study, however, shows that drivers who smoke marijuana within a few hours of hitting the road are almost twice as likely as stone-sober motorists to be in a crash that results in serious injury or death.

Authors of the new paper, published online Thursday in the British Medical Journal (BMJ), sifted through nine previous studies to develop a clearer picture of the risks to users who light up before revving up. Previous studies have left the effects of marijuana on its own—when not combined with alcohol or other drugs—a little hazy.

But the researchers’ findings make sense to others in the field. “Their results are consistent with experimental evidence that cannabis use leads to dose related impairments in simulated driving, psychomotor skills and on-road driving,” Wayne Hall, of the University of Queensland’s Center for Clinical Research who was not involved in the new research, wrote in a related essay in BMJ.

In addition to the finding that drivers who had recently smoked pot were substantially more likely to be involved in a serious accident, the researchers found that those who had died in these crashes had higher amounts of the drug’s compound tetrahydrocannabinol than those who survived. But there was not enough data to link concentrations of the compound to various outcomes in order to suggest a threshold for dangerous intoxication, noted the researchers, who were led by Mark Asbridge, of Dalhousie University’s Department of Community Health and Epidemiology.

Driving while stoned has become a hot topic as more states allow for medical use of marijuana. The 2009 National Survey on Drug Use and Health found that more than 10 million people admitted to having driven while on at least one illegal drug—with pot being the most common. More than a dozen states currently have roadside drug tests for cannabis that sample drivers’ saliva for traces of tetrahydrocannabinol. But, as Hall noted, ascertaining a dangerous level—as is currently used in a breath-based test for alcohol (0.05 percent)—is less clear cut. So far many governments are using a zero-tolerance rule, but, as Hall pointed out, “researchers have proposed a concentration of tetrahydrocannabinol below which driving is not impaired.”

Assessing the definitive risk for actually being in a car crash because of marijuana use is also tricky because studies haven’t always looked at drivers who were not determined to be “at fault”—or passengers of vehicles or people involved in minor accidents.

Nevertheless, Asbridge and his colleagues noted, “This information could be used as the basis for campaigns against drug impaired driving.” But the roadside testing has not been as widely publicized as rapid alcohol breath tests have been, so the chance of getting “caught” with pot in one’s system doesn’t seem to have scared very many people into not smoking before driving. As Hall wrote, the idea that roadside tests for pot will reduce traffic fatalities as drastically and as rapidly as breathalyzers did for alcohol “is probably too optimistic.” But that doesn’t mean that attempts to stub out the dangerous habit should be written off just yet. “Better evidence is essential,” Hall said of the attempts to fight pot-impaired driving with more roadside testing.

Katherine Harmon Courage About the Author: Katherine Harmon Courage is a freelance writer and contributing editor for Scientific American. Her book Octopus! The Most Mysterious Creature In the Sea is out now from Penguin/Current. Follow on Twitter @KHCourage.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. phillip66 7:59 pm 02/9/2012

    Please feel free to correct me if i am wrong, but i recall someone mentioning a study a few years ago that driving abilities improved after smoking cannabis, How long after i do not know due to being able to detect cannabis in the body for up to 28 days.

    Another report stated that in the USA where medical marijuana was legal, that there was fewer road accidents/road deaths than in states with no medical marijuana.

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  2. 2. ASHIK 9:43 pm 02/9/2012

    I dont want to consume drugs my entire lifetime.

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  3. 3. Mr.Tie 10:02 pm 02/9/2012

    See and this is why I don’t get high before I drive…. I wait till I’m driving before I start doing bong hits, remember responsibility first people.

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  4. 4. Alenz 11:06 pm 02/9/2012

    Here in brézil, drunk drivers kill and they don’t will be punished, why is an ‘acident’

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  5. 5. lilolme 11:44 pm 02/9/2012

    It seems logical to me that if you are under the influence of any mind altering chemical, marijuana, alcohol, or something prescribed, you should not be driving 2,000 pounds of steel around other people. The selfishness and indifference to the risks you put other people in should be charged as murder because it is not an “accident.” People want to let their politics on a plant guide their judgment on this matter. Anyone else know someone killed by a driver under the influence?

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  6. 6. NettlesAndBurrs 2:33 am 02/10/2012

    I wonder what ‘doubling your chances’ means in absolute terms. For example, if your chance of being involved in a serious crash is (say) one in a million before smoking, and it doubles to two in a million after smoking, then it hardly appears significant.

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  7. 7. mandingueiro 4:47 am 02/10/2012

    Good question Nettles. Have you checked the source article?

    Can’t say I’m having much luck answering your question from this. They mention use of control-comparison studies. Any idea how you can make control comparisons for vehicle collision reports? Asbridge Et Al. make use of reports where culpability is not addressed – ‘being in a car crash’ doesn’t necessarily implicate the cannabis user as the driver at fault for those reports.

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  8. 8. mandingueiro 4:50 am 02/10/2012

    Can’t seem to edit my previous post. Want to clarify that some of the reports in the meta-analysis take culpability into consideration, but not all of them.

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  9. 9. malcolmkyle 6:12 am 02/10/2012

    The often cited statistic that 6-8% of drivers in motor accidents test positive for marijuana is a case-book example of mistaken causality. A positive test merely indicates that the driver has used marijuana sometime in the past 90 days. Since roughly 7% of the population uses marijuana on a monthly basis, the 6-8% statistic, far from proving anything about the effects of marijuana, simply affirms what should be expected.

    Here is a graph which indicates that the presence of certain amounts of cannabis in your body actually appears to REDUCE the risk of being involved in an accident:


    * Fact: When combined 2002 to 2005 data are compared with combined 2006 to 2009 data, the Nation as a whole experienced a statistically significant reduction in the rate of past year drugged driving (from 4.8 to 4.3 percent), as did seven States: Alaska, California, Florida, Hawaii, Iowa, Michigan, and Pennsylvania. Four of these seven States have legalized medicinal marijuana, Alaska, Hawaii, Michigan and California.

    * Fact: California led the US to a nationwide, statistically significant reduction in the incidence of “drugged” driving during a time period when the number of patients claiming the protection of the California Compassionate Use Act and SB-420 increased by at least a factor of 10.

    * Fact: The Journal of Psychoactive Drugs, Hartford Hospital in Connecticut and the University of Iowa Carver College of Medicine document states that MARIJUANA DOES NOT CAUSE DANGEROUS DRIVING:

    Google MARIJUANA DRIVING STUDY. You’ll see two common findings:

    1. Drivers under the influence of marijuana are VERY SLIGHTLY impaired.

    2. Unlike those under the influence of alcohol, marijuana consumers are aware they are VERY SLIGHTLY impaired and they CONSISTENTLY ADEQUATELY COMPENSATE by slowing down and being a little more cautious. That doesn’t mean they get in the fast lane on the interstate and drive 15 miles per hour. Marijuana makes you cautious, not crazy. – Those Cheech and Chong movies were comedies, NOT documentaries!

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  10. 10. malcolmkyle 6:13 am 02/10/2012


    On 29-Nov-2011, a study was published by University of Colorado Denver Professor Daniel Rees and Montana State University Assistant Professor D. Mark Anderson showing states that have legalized medical marijuana experience fewer fatal car crashes compared to states that have not. The researchers suggest that there may be fewer fatal drunk driving accidents in those jurisdictions because more people may be choosing to smoke marijuana instead of making the more dangerous choice of consuming alcohol – both traffic fatalities and alcohol consumption declined.

    The rate of fatal crashes in which a driver had consumed any alcohol dropped 12% after medical marijuana was legalized, and crashes involving high levels of alcohol consumption fell 14%. The study thoroughly accounted for other contributing factors regarding this decrease, such as changes in the number of miles traveled each year and new traffic laws.

    “Our research suggests that the legalization of medical marijuana reduces traffic fatalities through reducing alcohol consumption by young adults,” – Daniel Rees, professor of economics at the University of Colorado Denver who co-authored the study with D. Mark Anderson, assistant professor of economics at Montana State University.

    So, the prohibitionist scare-tactic of claiming there would be an increase in traffic fatalities if marijuana were to be legally regulated for all adults should now be banished to the fantasy realm from whence it came.

    * Hall & Hommel (2007) considered whether there was “sufficient evidence to discourage cannabis users from driving by conducting roadside drug testing. They concluded that there was “no scientifically persuasive evidence that” random drug testing has saved lives.

    Similarly, Weatherburn et al. (2003) argued “there are no solid grounds for asserting that cannabis intoxication is a major cause of road trauma”. Consideration needs to be given to evidence that THC serum concentration does not always denote impairment (Bedard et al. 2007). — Laumon et al.’s (2005) conclusion corroborates these findings and reports that the role of cannabis in “fatal crashes is significantly lower than that associated with [any] positive blood alcohol concentration.”

    Whilst Grotenhermen et al. (2007) do suggest that a concentration of 7-10 ng/ml is comparable to a blood alcohol content of 0.05%, Bedard et al. report that the “frequency of drinking and driving and the severe impact of alcohol on driving abilities are well beyond what has been shown with cannabis”.


    Hall, W. & Hommel, R. (2007). Reducing cannabis-impaired driving: is there sufficient evidence for drug testing of drivers? Addiction, 102(12), 1918-9.

    Weatherburn, D., Jones, C. & Donelly, N. (2003). Prohibition and Cannabis Use in Australia: A Survey of 18- to 29-year-olds. Australian and New Zealand Journal of Criminology, 36(1), 77-93.

    Bedard et al. 2007. The impact of cannabis on driving. Canadian Journal of Public Health, 98: 6-11.

    Laumon, B., Gadegbeku, B., Martin, J.L. & Biecheler, M.B. (2005). Cannabis intoxication and fatal road crashes in France: population based case-control study, British Medical Journal, 331, 1371-1374.

    Grotenhermen, F., Leson, G., Berghaus, G., Drummer, O.H., Krüger, H.-P., Longo, M., Moskowitz, H., Perrine, B., Ramaekers, J. G., Smiley, A. & Tunbridge, R. (2007). Developing limits for driving under cannabis. Addiction, 102, 1910–1917.

    Anderson, B.M., Rizzo, M., Block, R.I., Pearlson G.D. & O’Leary D.S. (2010). Sex Differences in the Effects of Marijuana on Simulated Driving Performance. Journal of psychoactive drugs, 42(1),19-30.

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  11. 11. malcolmkyle 7:01 am 02/10/2012

    * Relative risk of vehicle collision whilst under the ‘acute’ influence of cannabis is 1.92 whilst the relative risk of a similar vehicle accident with a blood alcohol content of 0.8 g/100 mL (the legal limit in many places) is 2.69.

    In a 60 km/h zone the risk of involvement in a vehicle accident with casualties increases exponentially (doubles) with each 5
    km/h increase in traveling speed.

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  12. 12. Paleoecologist 10:02 am 02/10/2012

    malcolmkyle – The article you cite finds that states with medical marijuana have fewer alcohol-related accidents, presumably due to reduced alcohol consumption amongst young people. Less accidents due to alcohol consumption does not have a bearing on the rate of accidents due to mariguana; there could still be twice as many serious accidents when high than when sober. The mechanism may just be different.

    Incidentally, I find it interesting that the article finds a reduction in alcohol-related accidents amongst young people– they’re presumably not the people with medical marijuana prescriptions.

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  13. 13. Durazac 11:19 am 02/10/2012

    I agree primarily with those who suggest that it really doesn’t matter – mind altering chemicals and driving don’t mix.

    That said. I have a lot of experience with stoners and drunks and there are huge differences in the way tolerance affects their motor skills. No drunk can hide the fact that they are drunk – no matter what their experience with alcohol, but there are many, many stoners who have such a high tolerance and outward control that they could be on the moon in their minds while working a job right next to you – and you would never know. In fact, many prefer to operate at complex levels in that condition – in science, music and sports especially. I am sure that even in the lofty Sci-am offices, some to the people there would be looking around nervously saying “don’t tell people that there are stoners among them – they may find us”….

    Did I mention that paranoia is a common side effect of pot.

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  14. 14. CougarB 8:39 pm 02/10/2012

    There is a lot of information missing from the study. In earlier research, being stoned was an impairment for novice stoners, but not for people who were experienced in being stoned.

    For anyone who uses marijuana, medical or otherwise, this is just common sense. But for the researchers, the experience of the users of pot was not evaluated as a confounding variable, which means that they did not do their homework.

    There has been a long history of research that shows the bad side effects of the weed, and later, after the research has been evaluated, those negative effects have usually evaporated. It usually comes down to researchers seeing what they want to see and hearing what they want to hear.

    Meanwhile, extremely objective results, such as the fact that marijuana is a neuroprotector for multiple sclerosis in both humans and animals, is blown away by those who look at marijuana with such a strong initial bias that they cannot comprehend facts that don’t match their preexisting prejudices.

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  15. 15. CougarB 8:40 pm 02/10/2012

    I meant, in paragraph 2, the experience LEVEL of the users of pot was not evaluated…

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  16. 16. CougarB 8:46 pm 02/10/2012

    One more point. If people who use mind-altering substances should not be driving, that would eliminate almost everyone on the road, because almost everyone is using caffeine, which enhances read rage, anti-depressants, which can slow reaction time, and a host of other legal drugs which impair mental function.

    Why don’t they do some research that compares the number of destructive road rage incidents with high serum levels of caffeine with the number of destructive road rage incidents of people who do not use caffeine? Maybe they’d find that caffeine is as dangerous as pot.

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  17. 17. Daniel35 9:39 pm 02/10/2012

    But how can we talk about a minimum dose, such as from 2nd hand smoke? Is there any amount that we can say doesn’t affect us at all? And is there any chemical that can be proven to not have any mental effect?

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  18. 18. dhbone 10:30 am 02/11/2012

    I’m glad to see some skepticism about this article. I too have read reports that it helps make you a safer driver. Maybe not. But marijuana is always under the attack…

    Is there a way currently to tell how much is in your system at one given time? Or an established legal limit? No. also, I think it’s important to note the differences in what “the drug” does to you– I have no evidence for this but I am curious: I find that alcohol gives one magic wings and people think they can do anything–including driving. Well, also, as someone else said, why is pot demonized when caffeine and nicotine are not? And alcohol I would say is far worse for you and more dangerous, yet it’s perfectly legal.

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  19. 19. CO Buddyboy 3:46 pm 02/11/2012

    We need to revisit the whole concept of “impaired” driving. In our society it is extremely rare to have drivers who are totally “unimpaired.” People use drugs of choice with impunity and we don’t differentiate them as impaired and unimpaired. I am a legal user of amphetamines and I know I drive more aggressively because of it, yet no one calls my driving “drugged” because I have a prescription. I think we need a comprehensive study of ALL drugs to try to get at what we as a society think is “OK” when it comes to driving under the influence. That includes caffeine, tobacco, alcohol, cannabis, and the myriad prescription drugs. Driving under the influence of cannabis has its risks, but it is far safer than driving under the influence of alcohol, and probably less safe than driving under the influence of tobacco. Cannabis has been “on the road” for many years now and it will be in the future. Let’s see if we can come up with a good method of evaluating stoned driving that fits with all the other drugs we consume regularly…

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  20. 20. Me_Yow 2:16 pm 02/12/2012

    ive been delivering pizzas in a 5speed with a pipe in one hand and a lighter in the other since 2006

    driving while under the “influence” or marijuana makes me drive slower, and if i become paranoid i become the perfect driver assuming im being watched.

    i wholeheartedly disagree with the study in the article, and id like to see the tolerence of the people whole regularly use cannabis in this study, instead of a person who has never taken an oil dab out of a 3 foot bong alone on the highway

    if im stoned 24/7 then obviously any accident i get into will be blamed on the weed, but in reality it helps me drive, when im sober its easier to develop road rage, but a little hash pipe sesh before my commute make the whoel ride like butter.

    ive only hit things in my cars when sober. FACT

    f tha poleese

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  21. 21. tonbo 2:31 pm 02/12/2012

    I’d have to say that *most* people can barely drive PERIOD, with no distractions or absorbed substances at all.

    People under 25 generally drive incredibly recklessly while completely sober and with no distractions; I’ve seen it hundreds of times. The elderly are equally dangerous.

    I’m incredibly nervous being in a car at all times (though it wasn’t always so) because I no longer look at these pictures of twisted metal that used to be cars and say “that’s only a picture — it never actually happens and will certainly never happen to me.” I KNOW that it happens *with sickening regularity* (it just happened in the time it took for me to write this post, and somebody just died).

    I look at some people in grocery stores handling a grocery cart — a GROCERY CART, ferchrissakes — completely absent-mindedly, blocking aisles, being completely oblivious to everyone around them — and I imagine that person behind the wheel of a 2,000-pound car going 70 miles an hour on a highway and I CRINGE with horror.

    No need for any marijuana or ANYTHING ELSE. We’re pretty much farragoed the moment we get behind the wheel as it is.

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  22. 22. r0b3m4n 6:26 pm 02/13/2012

    Me_Yow and tonbo are spot on.

    I am (was?) a young male with very high hyper activity and testosterone levels. Going 90 in a 70 was not uncommon for me at one time. But not when stoned! Always slowed the roll when smoking, and I even experienced much lower blood pressure levels due to my fellow incompetent drivers.

    More blockers and similar variables need to be reported to to better put things in perspective, try looking at comparables to put this in perspective like:
    Stick shift vs automatic
    Size of vehicle
    Age of vehicle
    post cramming for finals session?
    improper shoes (sandals / heels)
    number of other people in car
    infant in car? (wife drives batty with kid since she can’t handle the crying for long periods…)
    AWD vs Front vs rear
    Tread level on tires
    weather conditions
    GPS vs using a Map
    tolerance/drug history
    other drugs in the system
    DRIVING HISTORY (speeding and accidents reported separately plz)

    And for godssakes when making a report on topics like this DO NOT NEGLECT TO MENTION THE NUMBER ONE KILLER IN THE SYSTEM!!! CELL PHONES! It makes the article come off as written by MADD, which is the most biased organization on the topic possible! And go ahead and report talking hands free/not, separately from texting. Perhaps even list the top 50 most likely variables to cause a car accident. Lets face it risk is a part of all of our lives and when the odds are approaching that of dying from falling space debris it becomes not even worth discussing (and definitely not worth holding up traffic to test for!).

    Personally I am far more worried about being killed by a road rager or a cell chatter than by a stoner.

    I suggest all members of MADD support and develop and App that turns your cell phone off automatically (except GPS?) when it’s velocity is determined to be above 30mph.

    I also urge Cops to start enforcing the hands free laws better. And law makers to make the repeat penalties stiffer. Also COPS, PLEASE start pulling people over for impeding traffic/driving like an a$$, these inconsiderate people cause road ragers to blow up and are the other side of the same coin and should also be punished for their detriments.

    **please do not read following cost analysis**

    Too many assumptions to list:
    I have had many speeding tickets in my life – luckily always in Cali and 18 months apart so I never get a point on my record. My total cost from speeding is ~$1,200. I make a bit over $30/hr. So my current break even point is 1200/30=40 hours of my life saved by speeding. So have I saved that much time by speeding? Heck yeah! Probably more like 10 or 20 times that number of hours have been saved. I’m sure others will point out the decrease life expectancy due to speeding reducing the break even point and to those people I say driving period has risks, so does walking biking and busing and since the fed govt has out lawed the one job I used to do from home (online poker – which you would puke if you knew my ROI or hourly rate on, hint: ROI was infinite since I started with zero dollars!) I have only been in one car accident when I hit a telephone pole at ~10 mph in a vehicle older than me that lost power steering and brakes when the electrical system fried a wire, I was sober at the time.

    I am now a drug free Engineer (ok fine, except for I avg 1/4 a cup of caffenated diet soda a day), so please keep the slapping to a minimum. doh – my post is longer than original article…

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  23. 23. lesizz 3:38 pm 02/15/2012

    I think this article would be more appropriate for USA Today than a scientific magazine. How much THC affects driving how much? A better approach would be to subject people to various amounts of THC and then measure their driving responses on a trainer.
    How many of the proclaimed “drug free” people ingest refined sugar or chocolate, which has at least 4 mental modifiers? Or alcoholic beverages? Drugs of all kinds are part of the human experience in most cultures. Like anything else, use of drugs can be done irresponsibly. It’s the individual’s choice.

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  24. 24. sedwin 2:27 am 02/17/2012

    Over the years I have personally been required (because of my age) to ride as a passenger when others were under the influence of marijuana and I was not. This happened dozens and dozens of times and the drivers were numerous. In case anyone is wondering I grew up in California in the 70′s and 80′s. I was not the only passenger, there were a number of friends and relatives who also actively observed the reactions of the people under the influence. For the record, in the overwhelming majority of cases those under the influence if anything drove slower and safer than everyone else. They almost couldn’t help it. The drug slows down time which is why people become so introspective and paranoid. It also makes people see and obey every traffic sign and signal. Even if they try to speed or disobey traffic laws they can’t because this attempt at law breaking will only last for a few seconds or perhaps a minute at the most and then without the person under the influence even realizing it they fall right back into following the posted speed limit and traffic signs. Believe me I and all of my sober friends and family saw it over and over for years and years. We were not biased and we did not have an agenda like most (if not all) of these “studies” being produced we were just simply extremely intelligent children living in a So. Cal surf community.

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  25. 25. richardfg7 2:35 pm 12/7/2012

    Where are the actual statistics on how many people have actually died from driving stoned ? Personally I’ve smoked and drove for about 35 years now and have never had an accident except for twice being blindsided by non-stoned drivers. I really wish they would of lit-up first. Maybe they would of been more “in-tune”. I think the biggest problem is the belief that pot sends you off into this Alice in wonderland “trip” where reality doesn’t apply. It’s really not that intense at all. Just a general feeling of well being is all it is. You don’t see things that are not there or get violent. The only problem really is all the fear the government has created.

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  26. 26. richardfg7 2:42 pm 12/7/2012

    I almost forgot.What is this “time slows down” thing all about ? I’ve heard that for years along with all the other side effects like rape and murder but never really figured it was real either. Fun to laugh about though. GO SEAHAWKS!!!

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  27. 27. robinrow 8:23 am 11/17/2014

    Some of the most dangerous drunk and driving reduction in the rate of past year drugged driving
    fighting tickets

    Link to this
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  29. 29. briankizer 12:14 am 12/2/2014

    Drunk and drive is one of the worst case…this will cause bad to all people. Delaware traffic lawyer

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  30. 30. paulshaver 12:51 am 01/14/2015

    It usually comes down to researchers seeing what they want to see and hearing what they want to hear.
    traffic ticket lawyers

    Link to this

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