A new law makes possessing up to an ounce of marijuana in California no more serious than getting a speeding ticket — a development both sides battling over a marijuana legalization ballot measure hope to exploit with the vote just a month away.

The law signed late Thursday by Gov. Arnold Schwarzenegger reduces possession of an ounce or less of pot from a misdemeanor to an infraction with a maximum punishment of a $100 fine.

Even as a misdemeanor, possession of up to an ounce was still punishable only by a $100 fine and no jail time. But offenders also faced arrest, a possible court appearance and a criminal record.

Schwarzenegger reiterated his opposition to the ballot measure known as Proposition 19 when he signed Senate Bill 1449 but said the new law would save the state courts money.

"In this time of drastic budget cuts, prosecutors, defense attorneys, law enforcement and the courts cannot afford to expend limited resources prosecuting a crime that carries the same punishment as a traffic ticket," the governor said in a statement.

Other opponents of Proposition 19 said the new law helped their cause by negating the argument that legalizing pot would let police focus on more dangerous crimes, said Roger Salazar, a spokesman for the No on Prop 19 group.

"From our perspective it takes away the last reason anyone would have to vote for Proposition 19," Salazar said.

Meanwhile, backers of Prop 19 called the new law a step in the right direction but said the ballot measure was still needed.

"So long as there are any penalties on marijuana users, and so long as the production and sale of cannabis are illegal, we can't rest," Jeff Jones, a spokesman for the Yes on 19 campaign, said in an e-mail to supporters.

Proposition 19 would more or less make the new law irrelevant by legalizing possession of up to an ounce for personal use for adults 21 and older.

The ballot measure also would allow small marijuana gardens on private property and let local governments set rules for taxing and selling the drug.

The bill making possession an infraction was authored by San Francisco Democratic Sen. Mark Leno and supported by the Judicial Council of California — the official policymaking body of the court system — and by the California District Attorneys Association.

California's major police associations all opposed the legislation. Those groups said reducing possession to an infraction would discourage people cited for the offense from seeking state-funded drug treatment as provided for drug offenders under a ballot measure passed in 2000.

Other marijuana advocates praised the change made by the law as they pointed to the spike in misdemeanor marijuana arrests in the state in recent years.

"Californians increasingly recognize that the war on marijuana is a waste of law enforcement resources," said Dale Gieringer, director of the California chapter of the National Organization for the Reform of Marijuana Laws and co-author of the state's landmark medical marijuana law.

Authorities made more than 61,000 arrests for marijuana-related misdemeanors in 2008, the latest year for which data exists, California Department of Justice records show.

In the prior decade, such arrests averaged about 48,000 until 2006, when the figure exceeded 50,000 for the first time.