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Yvonne Williams, president of ATU Local 192, talks to a fellow union member after meeting with media about the 72-hour strike notice submitted on behalf of AC Transit workers inside the office in Oakland, Calif., on Monday, Aug. 5, 2013. (Laura A. Oda/Bay Area News Group)

OAKLAND -- East Bay commuters already on edge from the on-again, off-again BART strike threats learned Monday afternoon that AC Transit buses might sit idle starting Wednesday.

The union representing AC Transit drivers, mechanics, dispatchers and clerical workers notified the transit district that its workers could go on strike at 12:01 a.m. Wednesday if contract negotiations did not improve.

"We did not make this decision lightly," ATU Local 192 union president Yvonne Williams said. "We want (management) to know that we are extremely serious."

But, unlike BART negotiations in which management and workers can't seem to get along, Williams said negotiations between her union and AC Transit management continue and the working relationship is respectful.

"We have a reasonable respect for each other," Williams said.

The strike notice caught management by surprise.

"We are a little unsure why that has come about," AC Transit spokesman Clarence Johnson said of the strike notice. "There are constant discussions between the groups, we have been making progress, and we are not at an impasse."

AC Transit said it has offered wage increases of 9 percent over three years. In exchange for the pay increase, the district proposed that workers pick up 10 percent of the premiums for their medical insurance -- to be phased in over three years.

Johnson said the union and management is only 1.2 percent apart on a wage proposal and that there was "substantial progress" made over the weekend on other issues.

But Williams said her union is upset that the district has not moved on issues related to health and safety, a concern among drivers, who say that restroom and lunch breaks are being shortened. The union also wants to ensure that future health care costs do not rise dramatically.

During the last few years, Williams said, the time drivers have to use the bathroom and take a break has been reduced, causing health concerns for drivers.

"Fundamental human rights are at stake," Williams said. "Our rate of injury is skyrocketing."

Should negotiations fail, Williams said, the 1,800 workers represented by the union will walk out at 12:01 a.m. Wednesday leaving over 100,000 AC Transit commuters stranded. A prolonged strike coupled by a possible BART strike next week would cause an unparalleled transportation nightmare for East Bay residents.

"I just hope AC Transit doesn't go on strike because if it does, we're out of options," said Monica Martinez, a student at Merritt College who rides the bus to school.

Johnson said AC Transit has no backup plan should workers strike.

"If our operators go on strike, we would have no personnel," he said. "We will be without bus service."

Williams said her union and management continue to bargain and she said she was hopeful a deal will be in place before a strike is needed.

"We're close, very close," she said.

Johnson also said he believed a deal could be reached before a strike.

"We believe we can reach some kind of agreement," he said. "We have no reason to believe there will be a disruption of service."

Staff writers Brittny Mejia and Denis Cuff contributed to this report.