DUI Marijuana in SC – Does Marijuana Affect Driving Ability Like Alcohol?

Does Marijuana affect driving like alcohol?

Although South Carolina does not have a “DUI marijuana” charge, an “ordinary” DUI can be based on marijuana use or a combination of marijuana and alcohol or other drugs.

Many people, including many SC legislators, assume that marijuana affects driving like alcohol or the use of other drugs, but what evidence is that assumption based on?

Does Marijuana Affect Driving Like Alcohol?

Does marijuana use really affect a person’s ability to drive?

If it does, at what point do the intoxicating effects impact a person’s ability to drive to the extent that they are materially and appreciably impaired?

Studies that have been conducted have resulted in wildly varying estimates about the effect of marijuana on driving ability, with some saying the odds of a crash are significantly more likely, while others have found that the odds of a crash are increased only slightly if at all. Why?

study conducted by the National Highway Traffic Safety Administration (NHTSA) has concluded that previous studies finding an increased likelihood of crashes after marijuana use did not account for multiple variables that affected the outcome of the studies.

More importantly, after accounting for variables like the driver’s age and gender, the US government study found that marijuana smokers have a minimally higher risk of crashing than those who did not smoke marijuana before driving.


How can police say a person’s ability to drive has been substantially and appreciably impaired (the legal standard for driving under the influence under SC Code § 56-5-2930) when there is no “legal limit” for THC in a person’s system? Or when the US government has said that marijuana use creates only a minimally higher risk of crashes if there is any increased risk at all?


There is no doubt that marijuana does not affect a person’s ability to drive in the same way that alcohol does.

If marijuana use does affect a person’s driving ability, and, surely, at some level of use it does, what is the threshold amount of THC in a person’s system that would cause substantial and appreciable impairment?


NHTSA Study Says Marijuana Does Not Impact Driving Ability

Does the use of marijuana affect driving? Not according to NHTSA:

According to a new study commissioned by the National Highway Traffic Safety Administration, people who smoke marijuana have a minimally higher risk of crashing than those who stay sober. During a 20-month study of 10,858 drivers in Virginia Beach, researchers working 24/7 compared drug and alcohol readings from people involved in crashes against similar people (at the same time of day and location) who kept their cars intact. The main takeaway: When factoring age, sex, and race, there was no “significant increased risk of crash involvement” due to marijuana use.

With one hand, the government is putting people in jail for “DUI marijuana.” With the other hand, the government conducts research and concludes that there is no “significant increased risk of crash involvement” due to marijuana use.

Drugpolicy.org also points out that marijuana use has little or no impact on driving impairment and that, when THC is found in the system of a fatally injured driver, alcohol is usually there too:

Large doses of marijuana can affect perception and psychomotor performance, changes that could impair driving ability. But in driving studies, marijuana produces little or no car-handling impairment – consistently less than produced by moderate doses of alcohol and many legal medications.

Mixing marijuana and alcohol together amplifies potential risks. Surveys show that when THC is detected in the blood of fatally injured drivers, alcohol is often detected as well.

But Other Studies Say Marijuana Does Affect Driving…

The NHTSA study also reviewed previous studies, some of which found that marijuana affects driving and some that found marijuana does not affect driving. Why the discrepancies?

They found that many of the studies that have been conducted did not account for other variables that may have skewed the outcomes. For example, a study may find that there is an increased risk of crashes after smoking marijuana, but it does not account for the higher likelihood of crashes among young males – when the data is adjusted to account for demographic variables, the same study may find little or no risk of crash due to marijuana use:

Table 4 examines the odds ratios for the same categories and classes of drugs, adjusted for the demographic variables of age, gender, and race/ethnicity. This analysis shows that the significant increased risk of crash involvement associated with THC and illegal drugs shown in Table 3 is not found after adjusting for these demographic variables. This finding suggests that these demographic variables may have co-varied with drug use and accounted for most of the increased crash risk. For example, if the THC-positive drivers were predominantly young males, their apparent crash risk may have been related to age and gender rather than use of THC.

It appears that at least some of the studies that have been conducted thus far have fallen into one of the most common pitfalls in statistical research – correlation does not equal causation.

What does that mean? A simple example is: When people eat more ice cream, there is an increased risk of shark attacks…

The facts are there, and they are indisputable – as ice cream sales go up, so does the number of shark attacks. What’s the problem? The researchers must account for other variables that can cause the correlation – for example, when it is warm outside, people eat more ice cream. And they tend to swim in the ocean more.

Although there is a correlation between shark attacks and ice cream sales, shark attacks are not caused by ice cream sales.

NHTSA’s conclusion?

There is no “significant increase in levels of crash risk associated with the presence of [THC]:”

This study of crash risk found a statistically significant increase in unadjusted crash risk for drivers who tested positive for use of illegal drugs (1.21 times), and THC specifically (1.25 times). However, analyses incorporating adjustments for age, gender, ethnicity, and alcohol concentration level did not show a significant increase in levels of crash risk associated with the presence of drugs. This finding indicates that these other variables (age, gender, ethnicity and alcohol use) were highly correlated with drug use and account for much of the increased risk associated with the use of illegal drugs and with THC.

DUI Marijuana – What is the “Legal Limit?”

SC’s DUI laws make it a crime to drive while under the influence of alcohol or drugs to the extent that your ability to drive is materially and appreciably impaired.

But what does that mean in the context of marijuana use? Obviously, if a person is combining “hard drugs” or alcohol with marijuana use, they are more likely to be substantially impaired. I would also guess that an extremely high level of THC in a person’s system may also cause the person to be substantially impaired.

But what is the “legal limit?”

Prosecutors must rely on testimony by toxicologists (who work for SLED) or “drug recognition experts” (who are police officers) to establish that a person was driving under the influence of marijuana and how the marijuana use affected their ability to drive safely.


Except, in both cases, their testimony is now contradicted by research conducted by the US government

If marijuana use is to be included in the definition of DUI in SC, we need to set a “legal limit” of blood THC content that is based on actual research and science, rather than the preconceived notions of legislators and police, and law enforcement needs a way to accurately test blood THC content.

Questions About DUI Marijuana in South Carolina?

If you have been charged with possession of marijuana in Myrtle Beach, SC, or the nonexistent criminal charge of “possession of hemp,” do not plead guilty.

Whether it is the legality of the substance that was confiscated, the inability of SC law enforcement to test the substance, or traditional marijuana defenses, you most likely have options. Call me at 843-492-5449 or fill out our contact form to set up a free consultation to discuss your case.

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