When a person drinks alcohol, the level of alcohol in their blood generally coincides with their level of impairment. A driver with a blood alcohol content (BAC) of 0.02% (within the legal limit), is less impaired than a driver with a BAC of 0.10% (above the limit). The more alcohol in the body, the more impaired the driver is likely to be. As alcohol is processed by the body, its effects start to wear off. If a driver has a BAC of 0.08%, after about 4 to 5 hours without drinking (depending on sex and weight), their BAC will likely be back down around 0.00%.

Marijuana affects the body in a very different way. The effects of marijuana generally peak after about half an hour, and may wear off completely after about 4 to 5 hours. However, when it comes to testing for marijuana, indicators of the drug stay in the body for days or even weeks. A blood test for alcohol won't show any alcohol a couple days after drinking, but a blood or urine test for marijuana can show THC (the primary active chemical in marijuana), days after the effects have worn off.

The question of how marijuana affects a driver is still hotly debated, with some claiming marijuana has no negative impact on drivers. A recent study from the National Highway Traffic Safety Administration (NHTSA) found that marijuana showed a 25% increase in being involved in a crash than a sober driver. However, when researchers factored in the age and gender of drivers, the increased crash risk rate disappeared.

The lack of clarity as to how marijuana affects drivers is reflected in the way states' guage impairment through THC levels. Many states have no minimum level for marijuana impairment, with any presence of THC in the blood enough for a DUI. Ohio and Nevada have per se limits of 2 nanograms/mL, while Pennsylvania has a 5 nanogram “guideline.”

Oregon will soon make marijuana recreationally legal, following the path of Washington and Colorado. They are prepared for an increase in marijuana DUIs, as was the trend in other legal marijuana states. However, unlike the other two, Oregon still has no base level of marijuana in the blood for a DUI. In the short term, they will rely on urine tests, which one official with the Oregon Department of Transportation admits “does not prove impairment at the time of arrest.”

A woman in Denver was recently acquitted of driving under the influence of marijuana. She was pulled over for an expired license plate tag, and the officer said he smelled marijuana. She admitted to being a medical marijuana patient, but the officer had her submit to a roadside test, which he said she failed. Her blood test showed 19 nanograms, almost four times the state's legal limit of 5 nanograms. A jury found in favor of the woman, finding she was not impaired at the time.

If blood or urine tests are not a good measure of impairment when it comes to marijuana, would other testing be a better measure? A new smartphone app purports to be able to help users decide whether they are too stoned to get behind the wheel. The app called Canary uses four basic tests: a memory test; a reaction-time game; a time-perception test; and a balance test using your phone's accelerometer. Based on the tests, the app will give you a green light if you are okay to drive, a yellow light if you're on the fence, and a red light to tell you to stay out of the driver's seat.

At the Gorelick Law Offices, attorney Lynn Gorelick has dedicated her legal career to defending people facing charges of driving under the influence of marijuana in the East Bay. With more than 30 years of experience, Lynn Gorelick understands the local drug and DUI laws, and will make sure you get the justice you deserve. Contact the local East Bay DUI defense attorney who understands that you do not have to plead guilty just because you were arrested.