What Makes Testing for Marijuana Impairment So Challenging?

What Makes Testing for Marijuana Impairment So Challenging?
A marijuana plant is seen in Vancouver in a file photo. Don Mackinnon/AFP/Getty Images
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In a deadly crash last year, a 20-year-old pick-up truck driver from Texas crossed a double solid-yellow centerline, drove into oncoming traffic, and collided with a bus carrying 13 passengers.

All but one of the passengers and the bus driver were killed in the collision, while the truck driver and the remaining passenger were seriously injured.