"In almost every respect imaginable, Prohibition was a failure," former New York Times public editor Daniel Okrent concluded at the close of his new book, "Last Call: The Rise and Fall of Prohibition." "It encouraged criminality and institutionalized hypocrisy. It deprived the government of revenue, stripped the gears of the political system, and proposed profound limitations on individual rights."

America's laws against marijuana have had similar effect. About 40 percent of Americans have tried the weed. In March, the Partnership for a Drug Free America reported that 38 percent of ninth- through 12th-graders studied in 2009 reported consuming marijuana in the past month.

The last three presidents opposed legalizing marijuana, even though President Obama says he smoked marijuana, George W. Bush hinted that he did and Bill Clinton said he did not inhale. Gov. Arnold Schwarzenegger inhaled on camera - and the most he'll say now is that it is "time for a debate" on Proposition 19, the November ballot measure that would legalize marijuana under state (but not federal) law.

In 2005, Harvard economist Jeffrey A. Miron looked at the cost of marijuana prohibition. He estimated that legalizing and taxing marijuana would yield $6.2 billion in annual tax revenue nationally - assuming that governments levied taxes comparable to alcohol and tobacco taxes. In addition, the federal government would save $2.4 billion, while state and local governments would save $5.3 billion on enforcement.

Miron has argued that usage rates would not necessarily rise if marijuana is legal. I think usage will go up; even proponents admit that Prop. 19's passage probably would lower the cost. There is no way to sugarcoat the possibility that, despite bill language that legalizes possession only for adults 21 years old or older, some teens may find it easier to get pot. And that is not a good thing.

On the other hand, it's not as if prohibition has put a dent in teen usage. The same survey that found that found 38 percent of high school students had used marijuana found that 39 percent consumed alcohol in the past month.

Okrent believes that legalizing and regulating marijuana could make it harder for young teens to get. The repeal of Prohibition - with closing hours, age limits and government's ability to shutter violators - "made it harder, not easier, to get a drink."

Pleasant Hill Police Chief Pete Dunbar told The Chronicle Editorial Board that the violence associated with the marijuana trade makes it "the most dangerous drug" of all. Hence his opposition to Prop. 19.

But the group Law Enforcement Against Prohibition includes a growing number of former cops and prosecutors who support Prop. 19 because they want to starve criminal enterprises.

Stephen Downing, a former Los Angeles Police Department deputy chief, likened drug gangs to a starfish - cut off one limb, and they grow another. "If you take away 60 percent of the cartels' traffic, it will have a real impact on their profits," Downing told me.

"California's No. 1 cash crop is marijuana," he added. California growers, under regulation and paying taxes, could squeeze Mexican cartels out of the trade.

Downing told me he sees it as his "patriotic duty" to fight for Prop. 19.

Dunbar called the measure "too loosey-goosey." Prop. 19 leaves it to local governments to decide if they want to regulate and tax the production and sale of marijuana - and that means different laws for different locales.

But as attorney James Wheaton, who wrote the measure, explained, "Oakland is going to have completely different issues than Humboldt County." Communities that want to ban the sale of marijuana will be free to do so.

When I was younger, I knew kids who started using drugs and never reached their full potential.

Today, I have a lot of successful friends who used marijuana when they were younger, are glad they never were arrested, but say they will vote against Prop. 19 because they don't want to send the wrong message. In part, I think, they want the government to do their parenting for them. But it's wrong to criminalize behavior - possession of up to an ounce of (nonmedical) marijuana remains a misdemeanor in California - to send a message. You criminalize behavior that threatens public safety. While marijuana use can threaten public safety, in every way, laws against marijuana enrich criminal cartels.

What is the benefit? To decrease the chance of kids using drugs - by what, 1 percent? - the public for years has backed laws that fuel criminal practices.

Two years before repeal of Prohibition, smart people were convinced that Prohibition would never be overturned. Its author proclaimed that there was as much chance of repealing the 18th Amendment as there was for a hummingbird to fly to Mars "with the Washington Monument tied to its tail."

Okrent told me he didn't know he was for Prop. 19 until he started promoting his book. "People are going to consume this stuff," he told me.

It's just that simple. That's why the law doesn't work.