Travel Time Reliability

How predictable are travel times on our freeways?

Travel Time Reliability

Transportation planners use what’s known as a buffer time index, or BTI, to gauge the reliability of travel times along a given route. BTI is a measure of how much time (over and above the average travel time) a driver would have to budget to be 95 percent sure of arriving on time at his or her destination. BTI is expressed as a fraction of the average travel time: the lower the BTI, the more reliable the trip. This measure covers freeways only, as no comparable data are available for local streets or the transit network.

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.

Regional Performance
Despite increased congestion in the region, travel time reliability on Bay Area freeways remains remarkably consistent.

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.


Historical Trend for Travel Time Reliability - Bay Area

Local Focus
Morning commuters on State Route 242 and evening commuters on State Route 92 experience the most unreliable travel times in the region.

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.

Shifts in the annual buffer time indices for specific freeway segments do reflect some of the changing traffic problems in our region. For example, the 2016 evening commute on eastbound Interstate 80 and eastbound State Route 4 had markedly more unreliable travel times. These segments linking eastern Contra Costa County, Solano County and the Sacramento region to Bay Area job centers grew more unreliable due to the combination of increased traffic volumes and ongoing highway construction projects.

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2016 Travel Time Reliability Along Freeway Segments

Time of day - 9am

Select a segment on the map for more information.

Least reliable segments for AM peak

1. SR-242 Southbound from SR-4 to I-680
2. SR-85 Northbound from US-101 to SR-17
3. US-101 Northbound from SR-85 to I-280/I-680
4. I-680 Southbound from SR-4 to SR-24
5. I-238 Northbound from I-880 to I-580

Least reliable segments for PM peak

1. SR-92 Eastbound from SR-280 to US-101
2. I-80 Westbound from I-580 to MacArthur Maze
3. SR-242 Northbound from I-680 to SR-4
4. I-238 Northbound from I-880 to I-580
5. I-80 Westbound from MacArthur Maze to US-101

Metropolitan Transportation Commission/INRIX: Freeway Reliability Analysis (2010-2016)

No link available

California Department of Transportation: Annual Traffic Volume Reports (2010-2016)

Image: MTC Library, Photographer: Noah Berger (ID# ahz_046)

Methodology Notes: 

Buffer time index was calculated based on the average reliability of each freeway segment over the course of one-hour time windows. Peak periods were defined as 6 AM to 10 AM and 3 PM to 7 PM. Regional BTI was calculated using traffic volumes and length on each segment and weighting BTI accordingly across the network.