While it's now perfectly legal to light a joint in nine U.S. states, In the US (29 if you count medical marijuana), there is no precise way to know how high a person is. A drunk driver can be evaluated with a breathalyzer, but for several reasons, a similar device cannot be used to detect marijuana poisoning. Of course, the police can take you back to the station and take a blood sample to determine exactly how much THC is in your system.
Interpretation is the most difficult problem. The problem is that THC, which is believed to be the main psychoactive compound in marijuana, interacts with the human body in a fundamentally different way than alcohol. It goes and stays in the tissues. The molecule can remain for up to a month, while alcohol is rapidly eliminated.
The CHP says it is also evaluating the use of oral fluid detection devices to aid in these drug investigations. The CHP refuses to say exactly what devices they are. But it could be argued that even using THC to confirm drunkenness goes too far. The root of the problem isn't really measuring THC, but understanding the galaxy of active compounds in cannabis and their effects on the human body.
Chemistry is fraught with problems in terms of people's individual genetics and their tolerance levels. But maybe one day road testing for marijuana won't really matter. So don't hold your breath because a magic device tells you that you're high. The state has strict laws that prohibit drunk driving, and the police can check a person's degree of drunkenness with a rapid breathalyzer test.
Traffic studies that rely on blood THC measurements may also be inaccurate if the blood is drawn too late and the THC has already left the body. Measuring the volume of alcohol in one part of the body can predictably indicate how much is in any other part of the body, as well as how much is affecting the brain at any given time. As scientists continue to think about how to establish a biological measure of marijuana intoxication, legislators want to find a way to quickly identify and punish people who are too high to drive. For example, in Colorado and Washington, the legal limit is defined as five nanograms of THC per milliliter, measured in a driver's blood.
It has been shown that THC can be measured in the brain even though it can no longer be measured in the blood. This may not always matter, as some 16 states have zero-tolerance laws that make it a crime to drive with any measurable presence of drugs in the body, marijuana, or otherwise.