Law enforcement officials would love to have a clear way to tell when a driver is too drugged to drive. But the decades of experience the country has in setting limits for alcohol have turned out to be rather useless so far because the mind-altering compound in cannabis, THC, dissolves in fat, whereas alcohol dissolves in water.
And that changes everything. “It’s really difficult to document drugged driving in a relevant way,” says Margaret Haney, a neurobiologist at Columbia University, “[because of] the simple fact that THC is fat soluble. That makes it absorbed in a very different way and much more difficult to relate behavior to, say, [blood] levels of THC or develop a breathalyzer.”