Voters in Colorado and Washington State legalized marijuana for recreational use in November 2012. While the Colorado initiative didn't make any changes to the state's driving-under-the-influence (DUI) laws, Washington's measure established a legal impairment level for tetrahydrocannabinol (THC), the psychoactive chemical in marijuana. Under Washington law, anyone driving with a THC blood content of five nanograms per milliliter or higher is considered per se impaired.

Some states with medical marijuana laws have similar limits. Ohio and Nevada define two nanograms of THC per milliliter as impaired, while Pennsylvania's health department has a guideline of five nanograms that can be used in driving violation cases. Many other states where medical marijuana is legal, such as Arizona, Illinois, and Pennsylvania, have a zero-tolerance policy when it comes to driving and lighting up. Some, such as California, leave it up to police discretion.

Under the new Washington law, police must suspect impaired driving before stopping a vehicle. The driver's blood would be drawn by a medical professional, and anyone that tested above five nanograms is automatically subject to a DUI conviction. The provision, added to the Washington initiative to make recreational marijuana legalization more palatable to voters, is not without controversy.

While proponents cite scientific evidence, including an Australian study of over 3,000 fatal automobile crashes that found risk increased with a THC blood level between 3.5 and 5 nanograms, marijuana activists express concern that medical users may face legal punishment that doesn't take into account a person's size, metabolism, or reaction to the drug, in the same way that drivers are not necessarily impaired by alcohol simply because they are above an arbitrarily set "legal limit."

Authorities face several problems when it comes to judging impaired driving due to marijuana use. One problem is that THC levels aren't as easy to measure as blood alcohol concentrations. Police can get instant blood alcohol information with portable breathalyzers, but there is no similar roadside test for THC. Officers instead use drug recognition training, observation, and sobriety tests.

In Washington, if a driver shows signs of impairment and fails a standard field sobriety test, he can be arrested and taken to a hospital or clinic for blood testing. However, arrests for marijuana DUI based solely on field sobriety tests is not without controversy either.

The only published study on the subject was unable to establish a reliable correlation between performance on the field sobriety test and marijuana consumption, especially at the 5 nanogram level. In addition, the National Highway and Traffic Safety Association has never established a correlation between the ability to drive and the standardized filed sobriety tests.

Another issue is that THC levels may not actually reflect a driver's impairment. Some studies show that five nanograms of THC is roughly equal to .05 percent blood alcohol content, below the state DUI limit for alcohol in Washington. The National Highway Traffic Safety Administration (NHTSA) says that the concentration of THC in the bloodstream spikes within seconds of smoking marijuana, but drops to baseline levels in three to five hours.

It's difficult to know how much marijuana consumption equals five nanograms of THC in the blood, however, since THC concentrations in the drug vary depending on where and how the marijuana is grown and how it is consumed. For example, marijuana taken orally affects a user more gradually than smoking, resulting in THC levels that plateau and then slowly fall off.

The result is that someone who eats a marijuana-laced brownie, for example, may be more physically impaired than someone who smoked a joint in the same amount of time, even if the brownie-eater's blood THC levels are actually lower than the smoker's.

Another potential issue is that, while THC in the bloodstream is detectable for a few hours, the chemical is stored in fat and can be detected in the body for weeks, long after any potential impairment from immediate use fades.

There is also anecdotal evidence that how long THC stays in marijuana users' systems may be affected by how often they smoke, how much marijuana they use, and how long they've been smoking. Regular and heavy marijuana users have reported positive urine drug test results up to three months after stopping drug use.

Medical marijuana users worry that standard urine drug tests might be used to bolster a DUI charge even if they were not impaired while driving. Urine tests typically test for inactive THC metabolites and, as such, are only evidence of consumption sometime in the past. These tests generally cannot be used to determine if a driver has any active THC, the substance that actually causes impairment, in one's system.

Multiple studies show the drug can affect both coordination and perception, resulting in slower reaction times when driving and increased likelihood to weave and drift. Studies also show, however, that marijuana users are more aware of their impairment than those affected by alcohol and may make an effort to slow down or pay more attention to the road.

Marijuana is also still illegal under federal law, so for now, wherever you live, it's probably a good idea to stay off the road if you've recently lit up.