Smoking marijuana and driving — when is it a crime...

... to a state, like Colorado, that legalizes marijuana?

Prosecutors and some lawmakers have long pushed for laws that would set a strict blood-level limit for THC, the key ingredient in cannabis. A driver over the limit would be deemed guilty of driving under the influence, just as with alcohol....

But what's the level that's comparable to the blood alcohol level that's deemed to be too much impairment? It shouldn't just be arbitrary!

Though research and opinions vary widely, studies have shown that smoking marijuana tends to affect spatial perceptions. Drivers might swerve or follow other cars too closely, as well as lose their concentration and suffer from slowed reaction times....

Every state bars driving under the influence. But convictions in drugged-driving cases generally rely on police officers’ observations rather than blood tests. The White House in a drug policy paper last year called on states to adopt blood-limit laws in an effort to reduce drugged-driving incidents by 10 percent by 2015.

There are so many things that impair driving — sleepiness, distraction, low intelligence — why single out anything (including alcohol) for a special law as opposed to relying on observation of actual impaired driving? You could say single out the things that are measurable in the blood. Or you could say: Alcohol deserves to be singled out the way it is because there is a huge, specific, and much-studied problem.

How are people who use marijuana supposed to know when they're over the limit? With alcohol, the products are labeled and there is at least some rough information about the number of drinks and the amount of time that must pass before you can drive. What if you had a drink with dinner and then couldn't figure out whether it was legal to drive the next morning? Isn't that what will happen with marijuana? You'll have no idea what your blood level is, even days later.

Or will there be little blood test kits so you can check? Will the government provide all sorts of labeling requirements for marijuana? They might make legal marijuana production very expensive by requiring predictable calibration of THC and accurate labeling. Then the product could be very expensive, and lots of sales tax could be collected. Isn't that the real puzzle: how to regulate and collect taxes when you've got a product that already thrives as contraband?