Impaired driving is a big problem throughout the country. While many people automatically attribute impairment to alcohol, it can also come from other sources. One of these is marijuana, and even though this drug hasn't been decriminalized here, police departments around the country are finding that it is increasingly difficult to determine when marijuana is a factor in impairment.
Officers who stop someone for the suspicion of alcohol impairment can perform a breath test to determine whether the person has a blood alcohol concentration (BAC) level that exceeds the legal limit. There hasn't been a test for marijuana until recently, but now that one is available, there are opponents who note that it isn't what it seems.
Marijuana remains the human body for longer than alcohol, so it is possible that a person will still test positive for tetrahydrocannabinol even after the impacts of the THC have abated. This point could possibly be used as a defense strategy in cases that have to do with marijuana impairment.
As is the case with alcohol, the timing of the test could have an impact on the results. The University of Pittsburgh notes that THC becomes "virtually undetectable" in a person's breath within three hours after using marijuana. The issue here is that some people consume marijuana via edibles instead of through smoking, which could render this test useless if edibles are the cause of the impairment.
Another issue is that really isn't a good reference about what limit should be established for THC. There haven't been definitive studies that denote a specific level at which a person's ability to drive is impacted.