About Puget Sound Freeway and HOV Performance Statistics

This website describes the performance of the Puget Sound region's freeways, with additional attention paid to the region’s high occupancy vehicle (HOV) lane system. Data from before 2003 are from a period when HOV lanes had restricted access 24 hours a day, seven days a week. Beginning in July of 2003, the operational policies of WSDOT changed: Today, some of the HOV lanes on the eastside of Lake Washington have no restrictions between the hours of 7:00 PM and 5:00 AM.  These facilities include all of I-405 and SR 167, and the portions of I-90 and SR 520 that are east of I-405.



This website presents an overview of the level of usage and performance on the principal urban freeways in the central Puget Sound. The freeways included in this report are managed by the Washington State Department of Transportation (WSDOT) through operation of its FLOW system, a coordinated network of traffic monitoring, measuring, information dissemination, and control devices that operates on urban state and interstate highways in the central Puget Sound region.  This report is a product of a WSDOT-sponsored project whose purpose is twofold: (1) to enhance the Department’s ability to monitor and thus improve the effects of its traffic management efforts on Seattle-area highways, and (2) to provide useful information to the public and other decision makers about the status of traffic performance in the region

Several considerations should be kept in mind when the results in this website are interpreted.  First, this is summary material intended to provide an overview of the freeway system’s usage and performance based on information collected at selected locations. Generalizing to other locations in the freeway network requires caution, as performance can vary significantly even among closely spaced sites.  (Note, though, that the data analysis procedures used for this project were designed to be general and can be employed at locations other than those included in this report, provided that the appropriate data have been collected.)

Second, the analysis in this report is dependent on the availability and quality of traffic data for central Puget Sound freeways.  Although the regional traffic data used for this website have been generally detailed and comprehensive, data for some locations and time periods have occasionally been unavailable or of variable quality because the measurement process was affected by construction activity, lack of sensor installations, or equipment problems.  The analysis methods used for this report were designed to compensate for extended segments of unavailable or incomplete data as much as was practicable; nevertheless, some of the results are considered tentative because of the nature of the input data upon which they are based.

Third, the measures reported in this document are usually average values based on many days of traffic data; they do not represent a particular day of traffic performance but rather a “typical” day of representative performance.   In addition, measures such as speed, congestion, and travel time values are estimates based on approximate formulas; such measures are best treated as relative, rather than absolute, values, and are best used in a comparative way. Further information about data quality issues and the constraints and caveats of the analysis in this report are provided in the FLOW Evaluation Design Technical Report:


Geographic Scope

This website summarizes freeway usage and performance on I-5, I-405, SR 520, SR 167, and  I-90 in an area approximately bounded by Puget Sound to the west, Redmond and Issaquah to the east, Sea-Tac and Auburn to the south, and Everett to the north.  The results reflect the combined effects of all WSDOT traffic management efforts in the region. The information presented reflects the “state of the system,” and as such, does not evaluate the individual contributions of specific traffic management system components, although the effects of some components may be apparent in these aggregate results.


HOV Lanes

HOV lanes, also known as carpool lanes and diamond lanes, are designated for use by carpoolers, transit riders, ride-sharers, and motorcyclists who meet the occupancy requirement. By restricting access in this way, the HOV lane benefits users by allowing them to travel the freeway system at a faster speed, thus saving time and experiencing greater travel time reliability in comparison to motorists on general purpose (GP) lanes. As indicated in the 1992 Washington State Freeway HOV System Policy report, the objectives of the HOV facilities are threefold:

  • Improve the capability of congested freeway corridors to move more people by increasing the number of people per vehicle.
  • Provide travel time savings and a more reliable trip time to high occupancy vehicles that use the facilities.
  • Provide safe travel options for high occupancy vehicles without unduly affecting the safety of freeway general-purpose mainlines.

To ensure that these incentives for HOV users provide benefit, a state policy related to speed and reliability standards has been established. It states that any HOV facility "should maintain or exceed an average speed of 45 mph or greater at least 90 percent of the time" during the peak hour. To accurately evaluate the system's effectiveness, the policy also requires an annual HOV system report to document system performance, examining the HOV lanes' person-carrying capability, travel time savings, and trip reliability benefits in comparison to adjacent GP lanes, as well as the lanes' violation rates.


Disclaimer. The contents of this Web page reflect the views of the authors/researchers, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Washington State Transportation Commission, Department of Transportation, or the Federal Highway Administration. The information presented does not constitute a standard, specification, or regulation.

Revised April 9, 2010