OAKLAND, Calif. — Commuters braced for traffic snarls this morning as two of San Francisco Bay Area Rapid Transit’s largest unions went on strike.

The walkout promised to derail the more than 400,000 riders who use the nation’s fifth-largest rail system. Officials say another 60,000 vehicles could be on the road.

The strike was called after an 11th-hour effort to resume negotiations failed to produce a new contract by midnight Sunday. Both the unions and management said they were far apart on salary, pensions, health care and safety.

The unions, which represent nearly 2,400 train operators, station agents, mechanics, maintenance workers and professional staff, were asking for a 5 percent raise each year over the next three years. BART said that train operators and station agents in the unions average about $71,000 in base salary and $11,000 in overtime annually.