Traffic & Congestion
Updated: october 2018

Time Spent in Congestion

Definition: Time spent in traffic congestion - also known as congested delay - refers to the number of minutes weekday travelers spend in congested conditions in which freeway speeds drop below 35 mph. Total delay, a companion measure, includes both congested delay and all other delay in which speeds are below the posted speed limit.


Time spent in congestion increased 1.5 times faster than population growth since 2000


of all congested delay occurs in Alameda and Contra Costa counties


minutes of congested freeway delay is experienced each day by the average commuter


How much time do Bay Area drivers spend sitting in traffic?

Everyone has been there - cruising along at 65 mph only to see a sea of brake lights up ahead in the distance. Traffic congestion is frustrating to drivers, affecting mobility between cities and counties of the Bay Area. At the same time, congestion is usually a byproduct of favorable economic conditions, and many of the nation's most vibrant metropolitan areas struggle with congestion.


Congested delay occurs when speeds drop below 35 mph. When speeds fall below this threshold, the number of vehicles accommodated by the freeway corridor declines and the facility begins to operate below peak efficiency. While travelers are delayed when speeds are lower than the speed limit (e.g., 55 mph in a 65 mph zone), this is not defined as congested delay as the freeway is actually operating more efficiently at this speed.

Regional Performance

Traffic congestion is outpacing regional economic growth.

While there are more transportation alternatives than ever before, today's congestion is worse than what was experienced during previous economic booms. Since the peak of the dot-com boom in 2000, per-commuter congested delay increased by about 65 percent while population and jobs grew by 15 percent and 12 percent, respectively. Commuters are likely also experiencing delays on arterials and local roads, although there is insufficient data to quantify these trends.

Historical Trend for Time Spent in Highway Congestion

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Local Focus

Bay Area freeway congestion is worst in the evenings, with each of the top ten most congested corridors experiencing delay in the evening peak.

While congestion only generates a few minutes of delay for the average commuter, commuters in a handful of high-demand corridors experience far worse conditions. The top ten most congested segments constitute nearly half of all regional freeway congestion, with just the top two segments along the Bay Bridge corridor constituting 16 percent all Bay Area traffic congestion. While congestion on other corridors in the region mirrors typical commute patterns, the Bay Bridge is affected by congestion for long periods throughout the day.


Over three-quarters of all congested delay on the Bay Area's freeway network occurs in Alameda, Contra Costa and Santa Clara counties. Congestion in these counties remained stable between 2016 and 2017, with total hours of vehicle delay decreasing by about two percent in Contra Costa County and increasing at roughly the same rate in San Francisco.


US-101/I-80 from Cesar Chavez to Treasure Island ranks as the most-congested segment during the PM peak period

Time Spent in Highway Congestion by Highway Segment (2017)

Congested Freeway
Uncongested Freeway
Time of day - 9am

Select a congested segment on the map for more information

Top Congested Segments

      Sources & Methodology

      Methodology Notes

      Time spent in congestion measures the hours drivers are in congestion on freeway facilities based on traffic data. In recent years, data for the Bay Area comes from INRIX, a company that collects real-time traffic information from a variety of sources including mobile phone data and other GPS locator devices. The data provides traffic speed on the region's highways. Using historical INRIX data (and similar internal datasets for some of the earlier years), MTC calculates an annual time series for vehicle hours spent in congestion in the Bay Area. Time spent in congestion is defined as the average daily hours spent in congestion on Tuesdays, Wednesdays and Thursdays during peak traffic months on freeway facilities. This indicator focuses on weekdays given that traffic congestion is generally greater on these days; this indicator does not capture traffic congestion on local streets due to data unavailability.

      This congestion indicator emphasizes recurring delay (as opposed to also including non-recurring delay), capturing the extent of delay caused by routine traffic volumes (rather than congestion caused by unusual circumstances). Recurring delay is identified by setting a threshold of consistent delay greater than 15 minutes on a specific freeway segment from vehicle speeds less than 35 mph. This definition is consistent with longstanding practices by MTC, Caltrans, and the U.S. Department of Transportation as speeds less than 35 mph result in significantly less efficient traffic operations. 35 mph is the threshold at which vehicle throughput is greatest; speeds that are either greater than or less than 35 mph result in reduced vehicle throughput. This methodology focuses on the extra travel time experienced based on a differential between the congested speed and 35 mph, rather than the posted speed limit.

      To provide a mathematical example of how the indicator is calculated on a segment basis, when it comes to time spent in congestion, 1,000 vehicles traveling on a congested segment for a 1/4 hour (15 minutes) each, [1,000 vehicles x 1/4 hour congestion per vehicle= 250 hours congestion], is equivalent to 100 vehicles traveling on a congested segment for 2.5 hours each, [100 vehicles x 2.5 hour congestion per vehicle = 250 hours congestion]. In this way, the measure captures the impacts of both slow speeds and heavy traffic volumes.

      MTC calculates two measures of delay - congested delay, or delay that occurs when speeds are below 35 miles per hour, and total delay, or delay that occurs when speeds are below the posted speed limit. To illustrate, if 1,000 vehicles are traveling at 30 miles per hour on a one mile long segment, this would represent 4.76 vehicle hours of congested delay [(1,000 vehicles x 1 mile / 30 miles per hour) - (1,000 vehicles x 1 mile / 35 miles per hour) = 33.33 vehicle hours - 28.57 vehicle hours = 4.76 vehicle hours]. Considering that the posted speed limit on the segment is 60 miles per hour, total delay would be calculated as 16.67 vehicle hours [(1,000 vehicles x 1 mile / 30 miles per hour) - (1,000 vehicles x 1 mile / 60 miles per hour) = 33.33 vehicle hours - 16.67 vehicle hours = 16.67 vehicle hours].

      Data sources listed above were used to calculate per-capita and per-worker statistics. Top congested corridors are ranked by total vehicle hours of delay, meaning that the highlighted corridors reflect a combination of slow speeds and heavy traffic volumes (consistent with longstanding regional methodologies used to generate the €top 10" list of congested segments). Historical Bay Area data was estimated by MTC Operations staff using a combination of internal datasets to develop an approximate trend back to 1998.

      To explore how 2017 congestion trends compare to real-time congestion on the region's freeways, visit SF Bay 511.

      Data Sources

      Metropolitan Transportation Commission/Iteris: Congested Corridor Analysis (2017)
      No link available

      Metropolitan Transportation Commission: Historical Congestion Analysis (1998-2017)
      No link available

      California Department of Finance: Population and Housing Estimates
      Form E-8 - Historical Population and Housing Estimates (1998-2010)
      Form E-5 - Population and Housing Estimates (2011-2017)

      California Employment Development Department: Labor Market Information (1998-2017)

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