The union representing train operators and station agents for the San Francisco Bay area's commuter rail system says it will go on strike Monday.

The move involving the nation's fifth largest commuter rail service would leave hundreds of thousands of Bay Area Rapid Transit passengers stranded on both sides of the bay and clog highways and bridges with traffic.

Jesse Hunt, head of Amalgamated Transit Union Local 1555, announced the intention to strike at a news conference Thursday after talks broke down with BART management.

The union rejected a contract proposal earlier this week that would have frozen salaries, capped health benefits and imposed work rules intended to cut down on overtime.

The union returned to the bargaining table Wednesday, but BART management declared an impasse. The union decided to strike after BART's board voted unanimously Thursday to impose terms on members that Hunt said would amount to a 7 percent pay cut.

"We believe that management has not bargained in good faith throughout this," Hunt said.

Asked whether negotiations were still an option, he said: "Anything's possible at this point."

BART is facing a projected $310 million deficit over the next four years and wants to reduce its labor costs by $100 million.

"This is completely in the union's court," said BART spokesman Linton Johnson. "It's their choosing to throw our riders out in the street."

Johnson said the union would have to "offer something" to break the impasse.

San Francisco Mayor Gavin Newsom called on BART and the union to return to the bargaining table, saying a strike would affect businesses across the region.

BART has more than 100 miles of rail line and serves about 340,000 commuters a day. The agency planned to set up a bus service in case of a strike, Johnson said.

The California Department of Transportation said it would have more people on traffic duty to monitor stop lights and clear obstructions, said Lauren Wonder, a department spokeswoman. The agency also plans on having more toll booth workers during the morning rush hour and delay construction work until after peak hours.

"The idea is to keep traffic flowing as much as possible," she said.

Wonder said commuters should try to carpool, use public transportation, telecommute or even delay their trips.

Other transportation agencies, including AC Transit, which provides bus service to communities east of San Francisco Bay, plan to increase service in case of a strike.

Two other BART unions, including the agency's largest, have approved contracts. But their leaders say they would honor Local 1555's decision to strike and not cross the picket line.