Mechanics for the nation's third-largest public transportation system went on strike Tuesday, forcing hundreds of thousands of commuters to scramble for alternate transportation for the second time in three years.

Metropolitan Transportation Authority (search) mechanics walked off the job after midnight, and bus drivers, train operators and other workers were honoring picket lines. The move — which came on the heels of an unrelated strike at the region's groceries — shut down buses and trains that an estimated 500,000 daily riders count on to get around Los Angeles County.

"I'm just stranded," commuter David Strattling, 59, said early Tuesday. "I won't be able to go to work today."

He had traveled from his home in Lancaster to downtown Los Angeles on a non-MTA bus, but found he couldn't transfer to the subway. Since no one from his office could come pick him up, he planned to go back home.

As other commuters took to their cars, the California Highway Patrol (search) reported traffic was packed on the region's freeways.

"The strike will continue indefinitely, until we get a contract," Neil Silver, president of the Amalgamated Transit Union (search), said early Tuesday by telephone. He was speaking from a picket line where he had joined about 50 members of the union, which represents some 2,200 MTA employees.

More than a dozen maintenance workers waived picket signs after midnight at an MTA center in West Hollywood where buses are cleaned and refueled. "We just dropped everything," said David Wilson, 26, of Los Angeles.

A walkout by bus drivers in 2000 shut down the system for 32 days.

Fernando Reyes, who depends on the bus to go from his Glendale home to his restaurant job in Santa Monica, said he had to take taxis to keep his job during the 2000 strike. The 32-year-old barely broke even, forking over his $35 in tips to taxis each night — something he wasn't willing to do this time around.

"I don't know what I'll do," Reyes said. "I guess I'll have to bike to work. I'll have to leave early in the morning."

The MTA carries about 500,000 riders a day, MTA spokesman Marc Littman said. The number accounts for between 75 percent and 80 percent of those in the county who use public transportation.

Up to 9,000 union workers could potentially honor the strike, Littman said. Sixteen other municipal bus lines in the region were expected to operate as usual, he said. Metrolink commuter trains also were to operate normally.

The decision to proceed with a walkout was made Sunday, after negotiations between the union and the MTA broke off with "absolutely no progress" despite intervention from a state mediator, Silver said.

The parties are at odds over the mechanics union's health fund, which is in dire financial shape. The union wants greater contributions from the MTA to cope with soaring medical costs. The MTA pays nearly $17 million every year into the fund, which is administered by the union and pays for the medical coverage of 2,000 employees and retirees.

The MTA hasn't increased its contribution to the fund in more than a decade and rising medical costs have forced the union to spend fund reserves to keep up, Silver said. "They were waiting for us to run bone dry," he said.

The transit agency accuses the union of mismanaging the health trust fund and cites an independent audit that found the union wasted millions of dollars.

In its latest offer, the MTA said it would give the union money to keep the health trust fund from going bankrupt but asked for temporary control to restore it to financial health.

"Union leaders basically ran the trust fund into the ground and now they want the taxpayers to bail them out," MTA CEO Roger Snoble said in a statement.

Veteran bus driver Arlene Mills, 41, said she was supporting the transit strike because of concerns about health care costs. The El Monte resident joined a picket line with mechanics early Tuesday in West Hollywood after working her day shift and spending two hours on the picket lines with grocery store clerks in their separate strike.

"Health care coverage is critical," Mills said, explaining complaints by workers underlying both job actions affecting Southern California. "It's not about wages anymore, it's about health care."

About 70,000 unionized workers at three supermarket chains in Southern California went on strike late Saturday, vowing not to return to work until they receive a contract with health benefits they can approve.