Experienced Bay Area drivers know it’s a good idea to build in some extra time as a cushion against heavier-than-normal traffic or other unforeseen events. This cushion is known as a buffer time index, or BTI. Transportation planners represent BTI as a multiplier for the extra time required. A driver with a 20-minute typical commute that features a BTI of 0.5, for example, would need to budget an extra 10 minutes each day to be confident of an on-time arrival.
In 2016, travel time reliability on Bay Area highways shifted slightly, with the morning peak period becoming a bit more reliable while the evening peak period became less so. This is due in part to rising congestion, especially in the evening peak, which results in less consistent travel times. Despite these diverging trends, however, travel time reliability as measured by buffer time index has remained similar since 2010. During peak periods over the past half-decade, a driver with a 30-minute typical commute would need to leave approximately 11 minutes early to be confident of an on-time arrival. Reliability has remained largely stable even as traffic congestion has increased (up 60 percent since 2000, as measured by congested delay per commuter). In many parts of the region, heavily traveled corridors continue to be “reliably congested,” providing consistent travel times even in near-gridlock conditions.
Daily travel time reliability patterns for individual freeway segments stay surprisingly consistent year after year. State Route 242, for instance, has ranked as the Bay Area’s most unpredictable freeway corridor during the morning peak period every year since 2010. This means if your morning commute takes you along southbound State Route 242 in eastern Contra Costa County, you should expect the unexpected. With a buffer time index of 1.16 in 2016, drivers should plan eight extra minutes to travel the short 3.4-mile segment from State Route 4 to Interstate 680. Similar conditions exist on eastbound State Route 92 in San Mateo County during the evening peak. Unpredictable traffic backups from the San Mateo-Hayward Bridge and the US-101 interchange make this freeway segment the most unreliable drive in the Bay Area on weekday evenings.