Washington's marijuana legalization initiative, which takes effect on December 6, is broadly similar to Colorado's: Both initiatives eliminate penalties for possession of up to an ounce by adults 21 or older, and both call for state-licensed pot shops, in Washington's case to be regulated by the state liquor control board, which is supposed to adopt regulations by December 2013 and start issuing licenses in 2014. But Washington's Intitiative 502 is considerably more restrictive and prescriptive than Colorado's Amendment 64. Here are some of the significant differences:

Home cultivation. Unlike Amendment 64, I-502, does not allow people to grow their own pot. Alison Holcomb, director of the Yes on I-502 campaign, says pre-drafting research indicated that voters were not receptive to home cultivation. Since "marijuana isn't quite yet out of the black market, and won't come out of the black market, even under I-502, until other states make it a legal product and the federal prohibition goes away," she says, "there is still concern among parents, families living in neighborhoods, saying 'I'm not quite ready for my next-door neighbor to start growing marijuana in their basement.'"

DUI standards. I-502 establishes a per se standard for driving under the influence of marijuana: five nanograms of THC per milliliter of blood. Amendment 64 does not address the issue, except to say that "driving under the influence of marijuana will remain illegal." Under current Washington law (which is similar to Colorado's), Holcomb says, an arrest requires evidence of impairment, and a conviction requires evidence of consumption, but neither is tied to a particular THC level. The more specific standard can help or hurt defendants, depending on the situation. Pro-legalization critics of I-502 worry that regular consumers, such as patients who use marijuana for symptom relief every day, may hit the five-nanogram limit even when their driving ability is not impaired. Holcomb emphasizes that blood cannot be drawn for a test without reasonable suspicion that a driver is impaired, and she argues that jurors "might be skeptical that the person was actually impaired [if] the results were not at that threshold level." But she concedes that any per se rule, whether for marijuana or alcohol, carries a risk of punishing people who do not in fact pose a public safety threat.

Retail restrictions. While Amendment 64 leaves the details of regulating pot shops to the Colorado Department of Revenue, I-502 includes specific rules aimed at making the stores as unobtrusive and uncontroversial as possible: no consumption on the premises, no products visible from the street, no identification beyond a single plain sign of specified size, and no outdoor ads on publicly owned property (including mass transit systems) or "within one thousand feet of the perimeter of a school grounds, playground, recreation center or facility, child care center, public park, or library, or any game arcade admission to which is not restricted to persons aged twenty-one years or older." That same location restriction applies to the stores themselves. The idea, says Holcomb, is to make the privately operated pot shops resemble old-style state-run liquor stores—"very bland, very boring," and inconspicuous, so "you have to specifically seek it out to go there and buy marijuana….We don't want to treat people like criminals for making the choice to use marijuana, and at the same time we don't want to promote marijuana use."

Taxes. Amendment 64 tells the state legislature to impose an excise tax "at a rate not to exceed fifteen percent" on sales of marijuana to retailers. That cap expires on January 1, 2017. But since a 1992 amendment to Colorado's constitution requires that all tax increases be approved by voters, it is not clear whether this levy actually will be imposed. Colorado Attorney General John Suthers says Amendment 64 did not comply with the constitution's requirements for new taxes, so a second vote will be necessary before the state can collect any revenue from marijuana sales. Washington has no such requirement, and the tax imposed by I-502 is much heavier: 25 percent at each of three levels (grower to processor, processor to retailer, and retailer to customer). As a result, prices paid by consumers, taking into account wholesaler and retailer markups (which add to the taxable price), will be more than double what they would be without taxes. Then again, production costs and markups should be much lower in a legal market, even with the threat of federal prosecution. A 2010 RAND Corporation analysis of marijuana legalization in California suggested pretax retail prices might be one-tenth the black-market price, in which case even an effective tax in the range of 100 percent to 200 percent may not seem very onerous, although it will create a strong incentive for evasion.

There is at least one way in which Washington's initiative is more permissive than Colorado's: Unlike Amendment 64, it does not let local governments ban marijuana stores within their jurisdictions. In fact, it charges the Washington Liquor Control Board with ensuring "adequate access to licensed sources of useable marijuana and marijuana-infused products to discourage purchases from the illegal market." Holcomb says allowing local bans would undermine that goal. Furthermore, Washington is taking a bigger leap than Colorado, which already has state-regulated medical marijuana dispensaries, many of which may decide to get into the recreational market. Washington's medical marijuana law, by contrast, does not explicitly authorize dispensaries, although as in California they operate in more tolerant jurisdictions (such as Seattle) as patient-caregiver collectives. Another significant difference that could eventually make Washington's new law either stricter or more liberal than it is now: Unlike Amendment 64, which can be changed only by another constitutional amendment, I-502 can be amended by the state legislature. For the first two years after passage, amending an initiative requires a two-thirds majority, but after that a simple majority suffices.