Two of San Francisco Bay Area Rapid Transit's largest unions have given notice that they plan to go on strike. The unions gave notice of a possible strike late Thursday night. It would start when their contracts expire at 11:59 p.m., Sunday. Union representatives gave the 72-hour notice as a courtesy to passengers so they could plan ahead for what would certainly be a difficult Monday morning commute. The notice does not guarantee that employees will strike. Negotiations are scheduled to continue through the weekend.