Traffic & Congestion
Updated: september 2017

Miles Traveled in Congestion

Definition: 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, the level at which freeway throughput is maximized.


San Francisco County ranks #1 for the share of miles driven in congestion


of miles driven were affected by congested conditions in 2016


The share of miles driven in congested conditions has nearly doubled since 2004


How many miles do Bay Area drivers travel in congestion?

As the Bay Area economy expands, so too does congestion on the region's freeways. While Bay Area congestion has increased in recent years as the economy has grown since the Great Recession, data indicates that the large majority of miles traveled on Bay Area freeways are driven in uncongested conditions. Still, slow-moving traffic - wherever it occurs - imposes costly delays on drivers of private automobiles and commercial vehicles alike.

Regional Performance

While congestion has doubled over the past decade, it affects only a fraction of miles driven on regional freeways.

Congestion remained at record-high levels in 2016, with six percent of freeway miles driven affected by traffic congestion. This trend has been driven by worsening conditions at long-standing regional bottlenecks on freeways crisscrossing San Francisco, the East Bay and the South Bay. While past years have seen significant annual growth in congested miles traveled, traffic congestion did not notably get worse in 2016 compared to the previous year, despite a booming economy and corresponding growth in jobs and population. And, as this data makes clear, the effects of regional traffic congestion are more limited in extent than some drivers might think: 94 percent of miles are driven in uncongested conditions.


For the second year in a row, San Francisco topped the list of the region's counties with the greatest share of miles driven in congestion. At 9.5 percent in 2016, it continues to exceed congestion levels in nearby 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.


of miles driven in Sonoma, Solano and Napa counties were affected by congestion in 2016

Historical Trend for Share of Highway Miles Traveled in Congestion

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Sources & Methodology

Methodology Notes

Miles traveled in congestion measures vehicle miles traveled (VMT) on congested freeway facilities based on Caltrans Performance Monitoring System (PeMS) traffic data. For over 10 years, PeMS has collected data on traffic volumes and speeds on California's freeways from a network of individual traffic detectors. Using this historical data, MTC calculates an annual time series for congested VMT in the Bay Area. The share of weekday VMT in congestion is the average daily share of congested VMT on Tuesdays, Wednesdays and Thursdays during peak traffic months on freeway facilities for the entire 24-hour period. 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. Consistent with the indicator for Time Spent in Congestion, this indicator defines traffic congestion as freeway travel speeds being 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. The share of congested travel is calculated by dividing congested VMT by total VMT. For example, when it comes to congested VMT, Vehicle A traveling in congested conditions for 1 mile for a total trip length of 2 miles, [1 Congested VMT/ 2 Total VMT = 50% Congested VMT], has a higher level of miles traveled in congestion than Vehicle B traveling in congested conditions for 4 miles for a total freeway trip length of 16 miles, [4 Congested VMT/ 16 Total VMT = 25% Congested VMT]. In that way, the difference in share of congested VMT and in nominal congested VMT are not parallel. The passengers in Vehicle A experience less congested VMT than passengers in Vehicle B (1 mile < 4 miles), but the share of congested miles for Vehicle A is more than that of Vehicle B (50% Congested VMT > 25% Congested VMT). To calculate the indicator, all the region's (or county's) congested VMT is summed and divided by that same geography's total VMT, meaning that the longer-distance traveler (Vehicle B) would be weighted more heavily. To calculate this indicator, miles traveled are either considered to be occurring in congestion or not in congestion based on vehicle speed (using the 35 mph threshold). Under these conditions, the indicator would not measure year-over-year increase in delay due to congestion on a segment if average speeds fall from 30 mph to 20 mph. Refer to the Time Spent in Congestion indicator for an alternative measure of traffic congestion and delay.

Data Sources

Metropolitan Transportation Commission/Caltrans Performance Monitoring System (PeMS): Congested Mileage Analysis (2004-2016)
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