The answer, it turns out, has been anything but simple in Colorado, which last fall became one of the first states in the country to legalize marijuana.
Prosecutors and some lawmakers have long pushed for laws that would set a strict blood-level limit for THC, the key ingredient in cannabis. A driver over the limit would be deemed guilty of driving under the influence, just as with alcohol.
Such legislation has failed several times in recent years in the face of fierce opposition from marijuana advocates and defense lawyers, who claim a one-size-fits-all standard doesn't work for marijuana because it affects the body differently than alcohol.
On both sides, passions run high.
"I haven't had a car accident since I was 18, and I've had marijuana in my system for most of that time," said Paul Saurini, 39, one of numerous weed activists, or "wactivists," who spoke out against setting a firm blood-level limit during a public hearing in the state capital this week.
"We have to create some standards to protect public safety. Not doing so, in my opinion, is reckless public policy," said John Jackson, the police chief in nearby Greenwood Village. "Any time you legalize things like this, you'll have more of it on the roadway. If we had vending machines with Oxycontin, there'd be more people on Oxycontin driving on the roadways. And that's not safe."
Since the passage of Amendment 64 in November, Colorado has been wrestling with the many questions of how to regulate the new marijuana reality, from how to tax it and monitor its growth to where people can buy it, sell it, smoke it and advertise it.
But drugged driving looms as one of the most critical and controversial issues. The outcome of Colorado's struggle to shape marijuana-related DUI laws could have far-reaching implications, as a growing number of states approve marijuana for medical use and others consider legalizing the drug altogether.
State Sen. Steve King, a Republican who supports a THC limit, insists that driving high is no different than driving drunk. "You're a threat and a hazard," he said. "The consensus should be to err on the side of safety for the traveling public."
Michael Elliott and other marijuana advocates argue that marijuana affects different people differently, and that setting a THC limit would free prosecutors from having to prove their cases and could lead to wrongful DUI convictions.
"When it comes to criminal law, we err on the side of protecting the freedom of our citizens and holding the criminal justice system to the highest standards of proof," said Elliott, a lawyer and executive director of the Colorado-based Medical Marijuana Industry Group.
Though research and opinions vary widely, studies have shown that smoking marijuana tends to affect spatial perceptions. Drivers might swerve or follow other cars too closely, as well as lose their concentration and suffer from slowed reaction times. Such findings have led some researchers to conclude that driving high doubles the chances for an accident, and that smoking pot and drinking before driving is a particularly dangerous mix.
Every state bars driving under the influence. But convictions in drugged-driving cases generally rely on police officers' observations rather than blood tests. The White House in a drug policy paper last year called on states to adopt blood-limit laws in an effort to reduce drugged-driving incidents by 10 percent by 2015.
But different states have taken different approaches.
In Ohio and Nevada, where medical marijuana is legal, the limit for driving is two nanograms per milliliter of blood. In Washington state, that limit is five nanograms. A dozen other states, including Illinois, Iowa and Arizona, have zero-tolerance policies for driving under the influence of marijuana and various controlled substances.
In Colorado, both sides agree that people shouldn't drive impaired; the fight is over what should be used as proof of impairment.
Marijuana advocates argue that, unlike with alcohol, traces of the drug remain in the bloodstream long after an individual has smoked pot, and that a THC test can mistakenly suggest a person is high, especially in a regular smoker who has built up tolerance to the drug. But officials who favor a blood-level limit say tests exist that can pinpoint "active" THC in the bloodstream in the hours immediately after marijuana usage.
People on both sides cite the work of Dutch researcher Jan Ramaekers, who found that marijuana users generally are impaired at a level of five nanograms, but that many cannabis users do develop higher tolerances.
Ramaekers, in an interview, said he supports the five-nanogram limit, noting that lawmakers have long set a legal limit for alcohol in the name of public safety, even though people have different tolerances and impairment varies by person.
"Who should the law serve? The individual or the population?" he asked.
Still, some in Colorado are concerned about drawing a bright line between impaired and unimpaired when it comes to marijuana. The state Senate's majority leader, Democrat Morgan Carroll, said research suggests that impairment can occur with anywhere from two to 20 nanograms per milliliter of blood. "My number one problem is that you could convict someone at five nanograms who wasn't actually impaired," she said.
Lawmakers are working on a compromise to break the long-standing impasse. A bill backed by King and other legislators would set five nanograms as the legal limit, but a test indicating that level would not automatically result in a DUI conviction. Instead, people accused of driving under the influence would be able to argue in court that they weren't impaired. The measure is working its way through the statehouse and appears likely to pass.
Carroll is still not fond of the five-nanogram limit but says she and others might be swayed by the provision that would allow defendants to make their case in court. "It gives the accused the opportunity to come in and offer proof," she said.
At Tuesday's hearing, a string of law enforcement officials and a state toxicologist testified in favor of the legislation. Ed Wood, whose son was killed in a car accident caused by a drugged driver, said he supported the bill but wants an even tougher standard. "We believe Colorado deserves better," he said.
But Saurini and other "wactivists" voiced their opposition, with some arguing that marijuana often induces paranoia and causes people to drive abnormally slowly, as opposed to alcohol, which can provide the "liquid courage" to drive irresponsibly.
King, the lawmaker who has long pushed for a legal limit, grows agitated at the suggestion by some marijuana advocates that they drive as well or even better high. It's a reason, he said, to put a limit in place as soon as possible.
"I heard that argument 25 years ago with alcohol," he said. "If you want to smoke marijuana, smoke marijuana. But smoke and walk, smoke and get a ride, smoke and take a cab. Don't smoke and drive -- that's the point we're trying to make."
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