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An increasing number of Americans are concerned about driving while under the influence of marijuana.

How much pot can a person smoke and then drive safely? It’s a question more and more states are grappling with as the legalization of marijuana becomes more widespread.

Those states are not alone as nearly half of Americans express similar concerns and report feeling that drug-impaired drivers are a bigger problem today compared to three years ago, according to the AAA Foundation for Traffic Safety.

While 85% of Americans support marijuana-impairment laws, according to its latest survey, the definition of “impairment” is a tough one to nail down when it comes to John Q. Public as well as the law enforcement community.

“While all states prohibit driving under the influence of drugs, there’s significant variation in the minimum acceptable levels of marijuana or its traces in a driver’s system,” said Peter Kissinger, president and CEO of the AAA Foundation for Traffic Safety.

“Sixteen states forbid any presence of prohibited drugs, while five others have specific limits for marijuana. With a lack of uniformity, it’s no surprise we found that more than half of American drivers are unaware of the laws that exist in their state.”

With alcohol, the rules seem pretty clear, especially legally. Most states use a blood-alcohol content of 0.08 as the benchmark for legally drunk, although some still use 0.1. Impairment begins at 0.05 for most states.

However, it’s not just the finite numbers for alcohol compared with marijuana. American drivers are significantly less concerned about the threat of drug impairment behind the wheel. The survey found that while two-thirds feel that those who drive after drinking alcohol pose a “very serious” threat to their personal safety, just over half feel the same way about drug use.

In fact, one-in-six Americans report that, where they live, most people feel it’s acceptable to drive one hour after using marijuana.

“Federal government research suggests that marijuana can impair driving performance for up to three hours,” warned Kissinger. “Decreased car handling performance, increased reaction times and sleepiness have all been documented driver impairments that result from marijuana use.”

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When it comes to prescription drug use and driving, Americans report feeling even less concerned, with just over a quarter reporting feeling the same “very serious” threat to their personal safety.

(Click Here for details about how legal marijuana is changing traffic laws.)

As the use of medical marijuana increases, the lines between that and other prescriptions as well as over-the-counter medications get blurry. All of the aforementioned can impair a driver in similar ways as alcohol. Previous studies have found that a single dose of some cold and allergy medications can have the same effect on driving as being above the legal limit for blood alcohol concentration, and certain antidepressants have been shown to increase crash risk by up to 41%.

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“Just because a doctor prescribes a drug, or you can purchase it over-the-counter doesn’t necessarily mean it is safe to use while driving,” says Jake Nelson, AAA’s Director of Traffic Safety Advocacy.

“Always discuss potential side effects and interactions with your doctor or pharmacist before getting behind the wheel.”

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