Miles Traveled in Congestion

How many miles do Bay Area drivers travel in congestion?

Miles Traveled in Congestion

Miles traveled in congestion reflects the share of miles traveled on regional freeways in congestion for a typical weekday; it is often referred to as the congested share of freeway miles driven (also referred to as vehicle miles traveled, or VMT). Congestion is defined as speeds less than 35 mph, approximately the speed at which freeway throughput is maximized.

As the Bay Area economy grows, so too does congestion on the region’s freeways. While Bay Area congestion has indeed grown in recent years as the economy has recovered, data indicates that the large majority of miles driven on Bay Area freeways are driven in free-flowing conditions. But slow-moving traffic imposes costly delays on drivers of private automobiles and commercial vehicles alike.

Regional Performance
While congestion has slowly grown over the past decade, it only affects a small fraction of miles driven on regional freeways.

Nearly six percent of freeway miles driven in the Bay Area are affected by traffic congestion. While a relatively small share of overall regional travel, this figure has grown in recent years. In fact, 2015 was the third year in a row that traffic congestion reached a record high. This growth in congestion has been driven by worsening conditions in San Francisco, Santa Clara and Alameda counties. Still, as this data makes clear, the effect of regional traffic congestion is much more limited than most drivers might think.

For the first time on record, San Francisco topped the list of the region’s counties with the greatest share of miles driven in congestion. At 8.6 percent in 2015, it surpassed Alameda County, which has historically been the most congested in the Bay Area. The latest data for these counties, when considered alongside those of neighboring counties, reflect slower freeway speeds and increasing congestion in the urban core. On the other hand, in the more northerly counties of Sonoma, Napa and Solano, drivers spend very little time in congested conditions. These counties have seen little-to-no growth in congestion over the past decade.

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Historical Trend for Share of Miles Traveled in Congestion


Metropolitan Transportation Commission/Iteris/Caltrans Performance Monitoring System (PeMS): Congested Mileage Analysis (2004-2015)

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Image: Flickr (Creative Commons license), Photographer: VCU CNS,

Methodology Notes: 

Congested VMT data only includes freeway facilities and relies upon PeMS traffic data. They reflect delay on a typical weekday, which is defined as Tuesday through Thursday during peak traffic months. Delay statistics emphasize recurring delay - i.e. consistent delay greater than 15 minutes on a specific freeway segment. Congested delay is defined as congestion occurring with speeds less than 35 mph and is commonly recognized as inefficient delay (meaning that the freeway corridor is operating at speeds low enough to reduce throughput - as opposed to speeds greater than 35 mph which increase throughput).