SAN FRANCISCO -- Thousands of people are expected to descend on San Francisco's Golden Gate Park to smoke pot for the annual 4/20 celebration, the unofficial marijuana holiday that some say has its roots in the Bay Area.

There will be more police, park rangers, fire and transportation officials on scene this year to make sure the cannabis holiday is a safe experience for the 15,000 revelers expected to flood to the park's Hippie Hill Wednesday, Board of Supervisors President London Breed said.

"There's no sponsor to hold accountable for the activities that exist on 4/20. But because we as a city welcome folks from all over the world, we are doing everything we can within our capacity to keep the community as safe and as clean as possible," Breed told the San Francisco Examiner.

The unsanctioned event costs the city between $80,000 and $100,000 per year because additional help has to be requested from city agencies to ensure safety, control heavy traffic and collect trash. Crews have cleaned up more than 5 tons of trash in previous years, Breed said.

The origins of the number 420 as a code for marijuana are murky. Some say 420 was once used by Southern California police to denote marijuana use. But others say the number became a code in the 1970s among high school students in San Rafael, north of San Francisco, who used it as a meeting time to gather to smoke marijuana after school.

Fans of the drug have long marked April 20 as a day to enjoy pot -- especially at 4:20 p.m. -- and call for increased legal access to it.

This could be the last year recreational marijuana users who gather to smoke the plant or eat it in cookies and brownies have to call for pot to be legal in California, where medical marijuana use was approved by voters in 1996.

A pot legalization initiative is likely to be on the November ballot in California. Voters in Nevada, Arizona and Massachusetts are also expected to consider marijuana legalization measures. Vermont's legislature is discussing a proposal to legalize the possession of up to one ounce of marijuana.

Recreational use is now legal in Colorado, Washington, Oregon and Alaska.