Two states place the standard at 2 nanograms per milliliter of blood. Others have zero tolerance policies. And Colorado and Washington state are debating a threshold of 5 nanograms.
Such an attempt failed the Colorado Legislature last year, amid opposition from Republicans and Democrats. State officials then set up a task force to settle the question — and the panel couldn't agree.
This year, Colorado lawmakers are debating a similar measure, but its sponsors concede they don't know whether the "driving while high" bill will pass.
In Washington state, the ballot measure on marijuana legalization includes a 5 nanogram THC limit.
The measure's backers say polling indicates such a driving limit could be crucial to winning public support for legalization.
"Voters were very concerned about impaired driving," said Alison Holcomb, campaign director for Washington's legalization measure.
Holcomb also pointed to a failed marijuana legalization proposal in California two years ago that did not include a driving THC limit.
The White House, which has a goal of reducing drugged driving by 10 percent in the next three years, wants states to set a blood-level standard upon which to base convictions, but has not said what that limit should be.
Administration officials insist marijuana should remain illegal, and Kerlikowske called it a "bogus argument" to say any legal level of THC in a driver is safe.
But several factors can skew THC blood tests, including age, gender, weight and frequency of marijuana use. Also, THC can remain in the system weeks after a user sobers up, leading to the anxiety shared by many in the 16 medical marijuana states: They could be at risk for a positive test at any time, whether they had recently used the drug or not.
A Colorado state forensic toxicologist testified recently that "5 nanograms is more than fair" to determine intoxication. But, for now the blood test proposals remain politically fraught, with supporters and opponents of marijuana legalization hinging support on the issue.
Huestis, of the government-funded drug abuse institute, says an easy-to-use roadside saliva test that can determine recent marijuana use — as opposed to long-ago pot use — is in final testing stages and will be ready for police use soon.
Researchers envision a day when marijuana tests are as common in police cars as Breathalyzers.
Until then, lawmakers will consider measures such as Colorado's marijuana DUI proposal, which marijuana activists say imperils drivers who frequently use the drug such as Chilton, the multiple sclerosis patient.
She says that since she began using pot she has started driving again and for the first time in five years has landed a job.
Chilton worries Colorado's proposal jeopardizes her newfound freedom.
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National Institute on Drug Abuse drugged driving report: http://goo.gl/ZAYwn
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