Riding High: Pot-Smoking Drivers Evade Blood Tests

People who drive after smoking marijuana are at greater risk of car crashes, but blood tests to check for the drug may not be a reliable way to catch impaired drivers, a new study suggests. Researchers found that levels of marijuana's active ingredient — tetrahydrocannabinol, or THC — decrease very quickly in the blood. This means that a person who was impaired by marijuana while behind the wheel might not have a positive test result by the time a test is administered a few hours lat