CNN affiliate KIRO tested drivers before and after they smoked marijuana to see how it affects their abilities. Something that had never occurred to me is how states with varying levels of legal marijuana use have to establish limits on how much would make you legally ineligible to drive a vehicle. They have, and if these test subjects are any indication, one would probably need to consume several times the legal limit before police will observe you driving erratically. That said, you should never operate a vehicle, or any heavy machinery, while impaired, legally or not. -via reddit
There's no sense or use in setting the limit so low that a regular medical user (Abby?) who will have a small level in her blood at all times, and is not impaired at all, would be at risk of arrest whenever she drives. It would be a waste of resources as police officers waste time citing and/or arresting people based on test results where no safety risk is present (though presumably drivers would not generally be tested except in an accident or other obvious way of getting police attention.) Putting the level too low would also feed false expectations into the press hysteria surrounding any traffic incident and could give influence legal proceedings based on a false premise, that a driver was 'stoned' when they really weren't.
As I recall, one of the big issues with alchohol is that drivers tend to drive faster when drunk, while a pot user tends to drive slower (as was born out in this test until Abby got really stoned). So that implies that a minor level of impairment due to pot appears to still be safer than a similar level of impairment due to alchohol.