(Ed. note: Part 1 examines the current state of THC testing and the challenges for certifying a DUI)

America’s history with marijuana is lengthy and fraught with hiccups. After decades of prohibition, the doorway of decriminalization paved the path for legal cannabis.

Marijuana is legal for recreational use in eight states plus DC. 20 states allow medical marijuana, and most of the remaining states decriminalized the drug. More states are currently pushing for full legalization.

The change in legality has led to some growing pains for lawmakers and police. Chief among them is how to handle stoned drivers. Many current testing methods have inherent flaws. Accurately diagnosing and prosecuting drivers impaired by marijuana is a challenge.

Current Climate of Cannabis and Cars

As legalization picks up steam across multiple states, studies and stats trickle in. Here are key findings:

• Pew Research survey reveals that 57 percent of Americans currently support legalization. That’s a huge leap from 16 percent in 1990. This gap will increase.
• Columbia Universityfound that drunk drivers are 16 times more likely to get in a fatal accident than drivers who test positive for pot.
• Center for Disease Control says that marijuana users are 25 percent more likely to get in an accident. They stress that factors like age and gender may play a role in higher crash rates.
• Insurance Institute for Highway Safety says collision claim frequency in Colorado, Washington, and Oregon rose by three percent. They say that recreational cannabis is the main culprit behind the increase.
• American Journal of Public Health, on the other hand, found no increase in fatality or crash rates in Colorado and Washington.

It’s difficult to draw conclusions about the affects of legalization. More studies will paint a clearer picture as recreational cannabis reaches more states. Some current findings seem to contradict each other, perhaps due to study controls and conclusions.

It’s also not yet clear how cannabis use impacts motor skills. The NCBI found that “most marijuana-intoxicated drivers show modest impairments on actual road tests.” A driving simulation by the NIDA shows that stoned drivers suffer slight cognitive difficulties.

Law enforcement, as a result, struggles with accurately testing and applying laws to marijuana users. It’s a double faceted problem: We don’t have proper drug testing methods largely because we don’t completely understand cannabis. It’s no surprise that many current cannabis DUI testing methods are under fire.

Today, states use two approaches to test stoned drivers: per se or effect-based laws. Both tests face criticism. “Neither type of test is accurate because neither is validated to detect marijuana impairment,” Jolene Forman, staff attorney with the Drug Policy Alliance, says.

Per Se DUI Laws Lack Science

Per se laws, or THC threshold tests, are controversial. They set a limit for how much cannabis can be in a person’s system while driving. If tests find a specific trace amount of THC in the driver, they are deemed impaired.

Contrary to other tests, per se DUIs do not require any signs of impairment. And the science behind them is contentious.

“THC threshold tests only measure, at best, whether a person has consumed marijuana at some point in the past few days,” Forman says. Many others share her opinion.

“Such laws are generally unpopular because science has not validated the presence of specific thresholds of substances other than alcohol as predictors of driver impairment,” says Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws (NORML).

Failing a per se sobriety test doesn’t guarantee that a driver is high. Most states with per se DUI laws have set the limit at five nanograms of THC per milliliter of blood. Critics of the laws claim that limit is arbitrary. They also contend that THC measurements are inherently inaccurate.

The National Highway Traffic Safety Administration agrees. “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects,” their study concludes. “It is inadvisable to try and predict effects based on blood THC concentrations alone.”

People metabolize marijuana and alcohol differently. Daily cannabis users will have THC in their blood long after use. They can fail a per se DUI test without recently using cannabis. Cases where a sober driver gets a drug driving charge are already happening in per se states.

“Marijuana may be detected hours, days, or even weeks after use, long after any possible impairment has dissipated,” Forman says. “This would be equivalent to a test showing that a person had a glass of wine or some beer nights or even weeks prior. It would tell you very little about whether the person is presently impaired.”

“[Users] can test positive for four days for infrequent users, and up to ten days for daily users,” Ravi Spaarenberg, CEO of Sensi Seeds, says. “This clearly does not provide a conclusive answer to the question if the person was in fact under the influence of cannabis while driving.”

Here are the biggest criticisms of per se pot laws:

• Not scientifically sound. There aren’t widely accepted levels of impairment for cannabis like alcohol. Heavy users can have illegal amounts of THC in their system without using any cannabis.
• Doesn’t account for tolerance. Five nanograms of THC, the legal limit, impacts people differently.
• No consensus. There are various sobriety tests like hair, urine, or blood analysis. Each tests may analyze different cannabis compounds, and they can contradict one another. Standardized tests backed by science are necessary.
• Risk of false positives. Equipment and technology to determine intoxication aren’t foolproof.

Disputing Effect-Based DUI Laws

Some states forego THC threshold tests in favor of effect-based tests. This requires evidence of impaired driving. Courts must also prove that impaired performance was due to cannabis. Like per se laws, effect-based laws come with their fair share of criticism and debate.

When a cop pulls over a suspected drunk driver, they look for visual clues. Slurred speech, bloodshot eyes, decreased motor skills, and the smell of alcohol are some indicators. With the exception of red eyes and a potential smell, stoned drivers don’t give off such clues.

“When law enforcement stop a vehicle and interact with the driver, it is often very difficult to tell if a person is under the influence of marijuana,” Attorney Matt Pinsker says.

Even with obvious signs of impaired driving, it’s not easy to tie it cannabis. Law enforcement doesn’t have standardized THC test kits and detection techniques. This is partly due to the recent legalization moves in states.

“There are a host of problems with drug testing techniques and analyses, including the substantial risk of false positive test results, false negative test results, specimen contamination; and chain of custody, storage, and re-testing issues,” Forman says.

“THC tests are unreliable and often incorrect,” she continues. “Chemical testing, like many other forensic disciplines, is highly technical and imperfect.”

The discrepancies with these tests make prosecuting cannabis DUIs a challenge.

(Ed. note: Part 2 examines why courts struggle with marijuana-related DUIs, and what you can do to fight a DUI arrest)