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Marijuana is legal in Missouri but driving while high is not. How do police detect impaired driving?

Jeff Barnard

Updated at 5:15 p.m. Feb. 6 with updates about recreational marijuana sales

Marijuana dispensaries in Missouri began selling recreational weed on Friday, the first day of sales since Missouri joined more than 20 other states in legalizing the drug in November.

Legalization opened the door to easier access to the drug for medical purposes and an opportunity to expunge past marijuana-related crimes. It also raised the risk of someone consuming cannabis products and then choosing to drive.

In 2017, about 1 in 8 high school drivers reported driving after using marijuana at least once during the previous month.

While it's hard to measure precisely how many crashes are caused by drugged driving, estimates from the National Institute on Drug Abuse suggest almost 44% of drivers in fatal car crashes tested positive for drugs. Drunk driving accidents make up about a third of all traffic-related accidents, according to the National Highway Traffic Safety Administration.

The traffic safety administration's 2013-2014 National Roadside Survey revealed alcohol use by drivers between 1973 and 2013-2014 is trending down. By contrast, the percentage of weekend nighttime drivers who tested positive for marijuana rose from 8.6% in 2007 to 12.6% in 2014.

What the science says about driving under the influence of marijuana

According to the Centers for Disease Control and Prevention, marijuana affects the areas of the brain that control how your body moves — balance, coordination, memory and judgment.

When you consume cannabis products, it can slow reaction time, impair coordination and distort perception, all of which are essential skills for safe driving.

Studies show an association between marijuana use and car crashes, but the CDC reports more research is needed. That is because it is difficult to connect the presence of THC — the compound that causes the psychoactive "high" — with impaired driving performance.

Testing for drug impairment is difficult because of a lack of drug-detecting technology and no exact number to determine impairment like there is with blood alcohol concentration level.

Marijuana can also stay in a person's system for weeks, and could appear in roadside tests despite no longer causing impairment.

How do police officers determine impaired driving in Missouri?

Even before an officer pulls someone over, they will check for telltale signs of impairment, like swerving, speeding or other sorts of reckless driving. Donna Drake, a spokesperson for the Kansas City Police Department, said officers also check for the smell of marijuana to decide if a field sobriety test is necessary.

But testing for weed isn't as easy as a breathalyzer test, so the officer conducts three tests which are standard practice across the country.

The first test is the horizontal gaze nystagmus, where the officer looks for if the driver's eye is involuntarily jerking. Next, the walk-and-turn test measures a person's divided attention skills. And lastly, the driver is asked to stand on one leg while counting or answering questions.

If the driver cannot pass two of the three tests, the officer will often determine them to be impaired.

"Our Traffic Division including our DUI Officers will be out on patrol during this holiday season, as they are every day, looking for impaired drivers as well as traffic offenders," Drake said. "The most prevalent contributing factors to fatality crashes are speed, impairment and lack of seat belt use. It is so important to slow down, wear your seatbelt and do not drive while under the influence."

Some police officers are specially trained to recognize signs of drug impairment. These Drug Recognition Experts, or DREs, are called to the scene if the patrolling officer can not determine if the person is impaired or what substance is impairing them.

The DREs use standardized tests in addition to non-standard options. They will also check the driver's pulse, body temperature and blood pressure to determine what category of drugs they are using.

Field officers can undergo training that focuses on drug impairment.

Can officers search your car?

Currently, officers in Missouri cannot search your car only because of the smell of marijuana. However, in driving cases, the odor can still be used as reasonable suspicion to conduct a DUI investigation.

There are still possession limits — currently three ounces — that law enforcement could use as a reason to search a vehicle if there is evidence a person is exceeding the limit.

However, in Kansas, the courts have long held that the smell of marijuana or any illegal substance is enough to search the car.

What are the penalties?

Driving under the influence in Missouri is a misdemeanor for first-time offenders and can result in up to six months in jail and a $500 fine. Subsequent infractions can lead to a year in jail and no more than $2,000 in fines.

In Kansas, the first offense is punishable by no less than 48 hours but no more than six months imprisonment, or 100 hours of community service and a fine of $500 to $1000. A second offense carries slightly increased penalties, but a third offense constitutes a felony with a mandatory minimum of 90 days to one year in jail and a fine of $2,500.

Any subsequent offenses will be considered among the most severe felony levels.

Noah Taborda is the Health Reporter at KCUR in Kansas City, Missouri.