OAKLAND, Calif. — Commuters in the San Francisco Bay Area got up before dawn today and endured heavy traffic on roadways, as workers for the region’s largest transit system walked off the job for the second time in four months.

People were lined up well before 5 a.m. today at a Bay Area Rapid Transit train station in Walnut Creek for one of the charter buses BART was running into San Francisco. And traffic at the San Francisco-Oakland Bay Bridge toll plaza into San Francisco and the roads leading to it was backed up for miles.

About 400,000 riders take BART every weekday on the nation’s fifth-largest commuter rail system. The system carries passengers from the farthest reaches of the densely populated eastern suburbs to San Francisco International Airport across the bay.

SEIU said it was fighting to prevent BART from changing employees’ fixed work schedules. Some employees work four-day, 10-hour shifts while others work five-day, eight-hour shifts. Union officials said BART wanted to schedule people as it saw fit.