In a ritual nearly as familiar as Santa Claus and crowded stores, Valley police agencies have again stepped up enforcement of drunken-driving laws this holiday season.

Backed by state grants, police are setting up more sobriety checkpoints and putting more officers on the street -- not only to catch intoxicated drivers but also to educate the public. Studies have found sobriety checkpoints reduce alcohol-related crashes because they create awareness about the risk of arrest.

But some public-safety officials say that message might be lost on the group most at risk -- young drivers. Trying to elude arrest for drunken driving, young people use technology to keep each other informed about the location of sobriety checkpoints, said Sgt. Dave Gibeault, head of the Fresno Police Department's traffic unit.

Tools include Twitter, text messages and an iPhone application specifically designed to identify checkpoints, Gibeault said.

Statistics provide a cause for concern: The number of people killed or injured in alcohol-related accidents during the holiday season in Fresno County has hovered around 34 for the last four years, according to state figures -- stubbornly resisting high-profile enforcement campaigns.

Gibeault thinks young people are drinking more, in part because clubs aimed at that population have proliferated. And young drinkers can check their cell phones to find out where police are stopping motorists, he said.

His own daughter often sends him text messages about where she's heard he's running checkpoints.

On Twitter, a free electronic message service that runs on both cell phones and computers, drivers can warn each other with "tweets" listing intersections where police have set up checkpoints.

Fresno attorney Brian Andritch sees nothing wrong with efforts to spread the word about checkpoints.

Andritch, who used to prosecute drunken drivers when he worked in the Fresno County District Attorney's Office, now defends them -- and warns others about sobriety checkpoints on Twitter.

"I don't see how it's any different than what police are doing in promoting checkpoints," he said in an interview.

Gibeault said it's one thing to spread the word about checkpoints in general, which police want. It's quite another to provide information that might encourage people to drive drunk, he said.

Wayne Ziese, a spokesman for the California Office of Traffic Safety, said he's heard a lot of stories about young people using technology to avoid drunken driving arrests.

"Young people continue to be the most dangerous drivers," he said. "They will continue to drink and drive until they have families and realize they have something to lose."

Ziese said law enforcement hasn't figured out how to respond to the more immediate and precise information about checkpoints circulating on the Web and via cell phones. The Office of Traffic Safety provides funding to help with such enforcement, including more than $5 million to Fresno County agencies in the last five years.

"New technology brings us new challenges, whether it's warfare or DUI," Ziese said.

Fresno police are known for their innovative approaches to DUI enforcement, and will likely figure out an appropriate response, Ziese said.

But changing the checkpoints can be a problem, Gibeault said. Police can't easily move them once their location has been broadcast, because of legal requirements and the large number of officers and equipment involved, he said.

In any case, the purpose of the checkpoints isn't to take drunken drivers off the road, Gibeault said. The point is preventing them from getting in the car in the first place.