LOS ANGELES -- Train and bus mechanics for the nation's third-largest mass-transit system walked off the job Tuesday, stranding hundreds of thousands of Southern California commuters already burdened by a supermarket strike and sporadic sickouts by sheriff's deputies.

The labor disputes snarled traffic, inconvenienced grocery shoppers and threatened to disrupt the operation of county jails and courts.

"I'm just stranded," said commuter David Strattling, 59, who made it to Union Station on one of the buses not affected by the strike before realizing he could not go any farther. "I won't be able to go to work today."

On Tuesday, 70,000 Southern California grocery clerks from three chains began their third day on the picket lines with no sign of a new contract. Grocery clerks in four other states -- Missouri, West Virginia, Ohio and Kentucky -- are also on strike.

And Los Angeles County sheriff's deputies continued to call in sick, as they have in spurts over the past three weeks, in a protest over stalled labor talks. A court hearing was set for Tuesday afternoon on a request from the Los Angeles County Board of Supervisors for a court order barring union leadership from encouraging deputies to call in sick.

The strikes could deal a serious blow to the ailing California economy. Jack Kyser, chief economist for the Los Angeles Economic Development Corp., estimated the transit strike could cost $4 million a day and the supermarket strike $6.3 million per day in lost wages.

Traffic worse than usual

The transit strike caused the most headaches Tuesday, as commuters scrambled for other ways of getting to work. The freeways were even more jammed than usual.

"Most of the people affected by this strike are finding alternate means, and those alternate means are getting into the car that's in the driveway," California Highway Patrol officer John Seumanutafa said.

Waiting for one of the final buses just before midnight, 18-year-old Allia McCoy shook her head and racked her brain for some other way to get from her home in Hollywood to her job at a Beverly Hills store and to classes at Los Angeles City College.

"I have no idea what I'm going to do," she said.

The strike by 2,000 mechanics brought most of the Metropolitan Transportation Authority's bus and train service to a halt as thousands of drivers and other union workers honored the picket lines.

The MTA carries about 500,000 riders a day, or about 80 percent of those in Los Angeles County who use public transportation, spokesman Ed Scannell said.

More than a dozen non-MTA transit lines, including Metrolink commuter rail and various regional bus lines, were operating as scheduled.

It was the second time in three years that a strike halted the county's transit system.

A 2000 walkout shut down the MTA's bus, subway and commuter train service for 32 days.

Grocery clerks are seeking higher health benefits. The strike affects hundreds of stores owned by Kroger, Safeway and Albertsons in Southern California.

Store managers and replacement workers filled in for the strikers, driving supply trucks, restocking shelves and ringing up purchases to keep the nearly 900 stores open. The strike forced some stores to scale back their hours.

Sheriff's deputies, who have worked without a contract since January, are seeking a pay raise and want the county to offset a rise in health care costs.

Transit mechanics are also striking over health care. The union-administered health fund is in dire financial shape, and the union wants greater contributions from the transit agency. The MTA pays nearly $17 million a year into the fund, which pays for the medical coverage of 2,000 employees and retirees.

The MTA has not increased its contribution in more than a decade and rising medical costs have forced the union to spend fund reserves to keep up, said Neil Silver, president of the Amalgamated Transit Union.

In its latest offer, the transit authority said it would give money to keep the fund from going bankrupt but asked for temporary control to restore it to financial health. The MTA also wants increased contributions from mechanics.