If I had to give one piece of advice to legislators, it would be very simple: Do not imagine that a law does exactly what you want. Beware of magical thinking.

It's easy to imagine a world in which the written law is always obeyed, as if by magic. This imaginary world is filled with docile, uncomplaining citizens, who never question the law and never do anything outside of the wise legislator’s intentions. When the legislator decrees something, they do it. When the legislator forbids, they abstain.

No one has ever visited such a world, but it's all too easy to imagine that we live there today. Consider one argument advanced by opponents of California Proposition 19, the ballot initiative that would legalize, regulate, and tax marijuana for adults over 21 years of age. Bishop Ron Allen writes,

[B]ecause marijuana stays in the body so long, police officers will have virtually no way to prove if someone just ingested marijuana 10 minutes ago or 10 hours ago. Unlike with alcohol, there is no current test to show the level of marijuana intoxication. All authorities can currently do is test for the presence of marijuana. If this initiative passes, it is perfectly fine to have marijuana in your system at any time -- even while driving a school bus, taxi or light-rail train. I would never again feel safe sending any member of my family into a vehicle where I cannot be assured that the driver is not under the influence of marijuana, plain and simple.

It’s fascinating, and a little frightening, to see such trust in the law. To hear him tell it, the law would appear to be the only thing keeping Allen safe. Of course, Allen has no especially good guarantee that a driver today is not under the influence of marijuana. All he has are these very same unreliable drug tests, and it's not like the existence of a law makes them work any better.

We may like to imagine that the law protects us from all possible danger of intoxicated drivers -- even to the point where we don't need particularly good drug tests -- but clearly, that's not the case. Marijuana is California's biggest cash crop, and I guarantee you it's not all for export. People do sometimes get high, in California and elsewhere, and ninety years of prohibition has done nothing to stop it.

But even if Prop 19 passes, employers will still be allowed to test for drugs. In fact, it's guaranteed in the text of the initiative, and employers certainly should continue to test whenever public safety is at stake. Proposition 19 will do nothing to change matters here, except that in time it might lead to safer and more accurate drug tests. That's because it's extraordinarily difficult to do research on a controlled substance, so many labs today either can't or don't bother researching them.

The other benefits of Prop 19 are even bigger. Fewer people will go to California's overcrowded jails. They will pay taxes on the pot they purchase, which can only be sold licitly to adults; this will tend to keep it out of the hands of kids. Like any law, it won't work perfectly, but it will certainly help -- when all pot is illegal, you might as well hit the kiddie market, too. But when adult demand is satisfied in a legal market, the illegal sources, currently all of them, will shrivel.

Pot will also be controlled for purity, quantity, and quality, protecting users against adulterated products or short weights. If there is a dispute about a commercial transaction, it can be settled peacefully in the court system and need not ever lead to private violence. Street gangs will lose a source of revenue, and the state of California will gain one. As will private businesses.

Now, on the margin, this will likely mean that more people will use marijuana. Trends from other countries, including the Netherlands and Portugal, suggest that this effect will be minimal, however, and in any case, marijuana remains a non-toxic drug whose addiction potential is very low, even when compared to currently legal drugs like alcohol and tobacco.

Given the many benefits of the policy, I’d say that a few more marijuana users would be an acceptable trade-off. In return, we get fewer drug-related violent crimes, fewer prison sentences, fewer and weaker gangs, and a government that's able to concentrate on what really matters. All for possibly a few more evenings sitting on the couch, eating Doritos and listening to Pink Floyd.

Like all laws, Prop 19 won't work perfectly. But I'm having a hard time finding much downside.