In this photo taken on Tuesday, March 6, 2012, Angeline Chilton a suburban Denver woman with multiple sclerosis who smokes pot twice a day to ease tremors, holds her pipe as she sits on the front porch of her home in Lakewood, Colo. Chilton insists that she never drives high, but she fears that officials will rush to set an unproven blood-level standard that would put her at risk of breaking the law. In Colorado and Washington, the debate over how to tell whether a driver is high is more than academic. The states are struggling to come up with a blood-level standard for marijuana that would be analogous to the blood-alcohol standard used to decide who's driving drunk. (AP Photo/Ed Andrieski)

Associated Press
In this photo taken on Tuesday, March 6, 2012, Angeline Chilton a suburban Denver woman with multiple sclerosis who smokes pot twice a day to ease tremors, holds her pipe as she sits on the front porch of her home in Lakewood, Colo.  Chilton insists that she never drives high, but she fears that officials will rush to set an unproven blood-level standard that would put her at risk of breaking the law. In Colorado and Washington, the debate over how to tell whether a driver is high is more than academic. The states are struggling to come up with a blood-level standard for marijuana that would be analogous to the blood-alcohol standard used to decide who's driving drunk. (AP Photo/Ed Andrieski)
In this photo taken on Tuesday, March 6, 2012, Angeline Chilton a suburban Denver woman with multiple sclerosis who smokes pot twice a day to ease tremors, holds her pipe as she sits on the front porch of her home in Lakewood, Colo. Chilton insists that she never drives high, but she fears that officials will rush to set an unproven blood-level standard that would put her at risk of breaking the law. In Colorado and Washington, the debate over how to tell whether a driver is high is more than academic. The states are struggling to come up with a blood-level standard for marijuana that would be analogous to the blood-alcohol standard used to decide who's driving drunk. (AP Photo/Ed Andrieski)
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