The arguments raised against Proposition 19 are really quite strange. The opponents of the measure don't say that marijuana should stay illegal - they just claim that the authors of the proposition made some "huge mistakes." The reality, however, is that the arguments against Proposition 19 are misleading.
My favorite is this one:
"The California Police Chiefs Association opposes Proposition 19 because proponents 'forgot' to include a standard for what constitutes 'driving under the influence.' Under Proposition 19, a driver may legally drive even if a blood test shows they have marijuana in their system."
Guess what - right now, under existing law, you can legally drive even if a blood test shows that you have marijuana in your system. Just like you can legally drive even if you have alcohol in your system (as long as it is less than .08% by weight). (See California Vehicle Code section 23152(a).) You just cannot be impaired by drugs or alcohol.
Proposition 19 changes nothing about existing DUI law. In fact, it even says: "This act shall not be construed to affect, limit, or amend any statute that forbids impairment while engaging in dangerous activities such as driving..."
I am guessing that the standard the proponents "forgot" to include is a numerical standard, like the one used for alcohol, which makes it illegal to drive with .08% or more alcohol in your blood. (See Vehicle Code section 23152(b).) But the reason there is no similar numerical standard for marijuana is because marijuana is illegal - no one has been able to do studies like those done for alcohol that led to the adoption of the .08% standard. (That is because you can't give a bunch of volunteers an illegal drug. But you can give them alcohol.) If marijuana is no longer illegal, then we can do studies that will lead to a valid numerical standard for marijuana.
So if you want clearer and stricter laws against driving under the influence of marijuana, you should actually vote for Proposition 19.