Travel Time Reliability

How predictable are travel times on our freeways?

Travel Time Reliability

Definition: 
Transportation planners quantify the travel time reliability of a given route by means of a buffer time index (BTI). BTI is a measure of the amount of time, over and above the average travel time, that a driver would need to budget to ensure on-time arrival at the desired destination, with a 95 percent confidence rate. BTI is expressed as a fraction of the average travel time – the lower the BTI, the more reliable the trip. This measure focuses solely on the regional freeway system, as no comparable data is available on the local street network or transit network.

Experienced drivers know that, when planning a trip on Bay Area freeways, it’s a good idea to build in some extra time as a cushion against the effects of heavier-than-normal traffic or other unforeseen events. Transportation planners have a term for this cushion: buffer time index, or BTI. BTI represents a multiplier for the additional time required; for example, a driver with a 20-minute typical commute that features a BTI of 0.5 would need to leave 10 extra minutes 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.

As measured by buffer time index, travel time reliability for AM and PM commutes has barely fluctuated 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. In contrast to reliability, traffic congestion has increased by 70 percent over the last five years, as measured by congested delay per commuter. In many parts of the region, heavily traveled corridors are truly “reliably congested,” providing consistent travel times even in gridlock conditions.

 

Historical Trend for Travel Time Reliability - Bay Area

 
Local Focus
Morning commuters on State Route 242 and evening commuters on U.S. Route 101 experience the most unreliable travel times in the region.

If your morning commute takes you along southbound State Route 242 in eastern Contra Costa County, be prepared for the unexpected. With a buffer time index of 1.10 in 2015, drivers should plan eight extra minutes to travel the short 3.4-mile freeway segment. Similar conditions existed on eastbound State Route 92 in San Mateo County in the evening peak period during 2015. Unpredictable traffic backups from the San Mateo-Hayward Bridge and the US-101 interchange made this freeway segment the most unreliable drive in the Bay Area most weekday evenings.

 

2015 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. US-101 northbound from SR-85 to I-280/I-680
3. I-680 southbound from SR-4 to SR-24
4. I-880 southbound from SR-92 to SR-84
5. I-80 westbound from MacArthur Maze to US-101

Least reliable segments for PM peak

1. US-101 northbound from SR-84 to SR-92
2. SR-13 northbound from SR-24 to I-580
3. I-280 southbound from SR-85 to US-101
4. I-880 southbound from I-80 to SR-92
5. I-80 eastbound from MacArthur Maze to I-580
Sources: 

Metropolitan Transportation Commission/INRIX: Freeway Reliability Analysis (2010-2015)

No link available

California Department of Transportation: Annual Traffic Volume Reports (2010-2015)

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.