Although the law in California applies only to people who have a medical need for marijuana, like glaucoma or cancer, the requirements for getting the card to buy the drug are notoriously lax. Doctors can recommend its use for ailments as common as sleeplessness and headaches. And marijuana in California has become almost as culturally accepted, and in some parts of the state nearly as widely used, as alcohol.

“Marijuana users are much more representative of the overall adult population in California than medical marijuana populations in other states,” said Amanda Reiman, the state policy director for the Drug Policy Alliance, an organization working toward the decriminalization of marijuana.

The percentage of California drivers with traces of marijuana in their systems, 7.4 percent, was slightly higher than the 7 percent of drivers found to have alcohol in their system during a spot check last year, according to a report from the California Office of Traffic Safety. The report found that 14 percent of those checked tested positive for some kind of drug that might impair driving.

In a broad study on the ramifications of legalizing recreational marijuana about to be published in The Journal of Policy Analysis and Management, two economics professors said a survey of evidence showed a correlation between increased marijuana use and less alcohol use for people ages 18 to 29.

The researchers, D. Mark Anderson of Montana State University and Daniel I. Rees of the University of Colorado, said that based on their study, they expected younger people in Colorado and Washington to use marijuana more and alcohol less.

“These states will experience a reduction in the social harms resulting from alcohol use: Reducing traffic injuries and fatalities is potentially one of the most important,” the professors said.

Mark A. R. Kleiman, a professor at the University of California, Los Angeles, and an expert on marijuana policy who was the chief adviser to Washington on its marijuana law, said the connection between alcohol and marijuana use, if borne out, would be a powerful argument in favor of decriminalization.