Freeway Performance

Congestion Frequency (by time and location) (draft 8 November 2005)


How and Where Congestion Frequency Was Measured

To better understand the likelihood of encountering significant traffic congestion as vehicles travel from one location to another on the freeway network, the frequency with which congestion occurs during different times of the day at different locations along each of the freeway corridors in the Puget Sound Region was analyzed.  The estimates were derived from volume and lane occupancy data collected by WSDOT with the inductance loop detectors that are located in the pavement of the freeway and that provide the data used to operate the WSDOT’s ramp metering system.  The data presented describe a composite condition for all mainline, general purpose freeway lanes at each particular location for a given direction.  Therefore, the resulting images usually do not show congestion on an exit lane caused by a queue from an off-ramp, unless that congestion causes all freeway lanes at that location to congest.

The graphs illustrate the frequency with which “bad” traffic conditions occur on weekdays. “Bad” traffic is defined as Level of Service F, or unstable traffic flow conditions.  Congestion frequency refers to the likelihood that significantly congested traffic will occur at a particular location and time of day based on data from the entire year.  This information is then combined into a summary image for each corridor during a 24-hour weekday.

Corridor performance was measured along all the corridors in the study area. This includes I-5 from south of S 272nd Street in Federal Way to SR 526 in Everett; I-405 from its southern terminus in Tukwila to its northern terminus at the Swamp Creek Interchange; SR 520 from Seattle to Marymoor Park; SR 167 from Auburn to Renton; and I-90 from Seattle to Issaquah.  (Note that data coverage on I-5 was extended in 2003, so that congestion maps for years prior to 2003 end at S. 184 St near the top of the Southcenter Hill.)

How to Read the Congestion Frequency Maps

The congestion frequency corridor maps show general purpose lane congestion frequency patterns at different points (mileposts) along the corridor during a 24-hour weekday. Each map is presented in a contour format similar to that of a topographic or elevation map, except that in this case the colors indicate the relative frequency of significant congestion (rather than average congestion conditions) as a function of time of day and location (milepost) along a freeway corridor. Alongside each graph is a map of the freeway corridor with the approximate locations of major cross-streets.

The example [embed Figure 3.9] shows a slice of a  congestion frequency map for the southbound general purpose lanes on I-5 at the Ship Canal Bridge just north of downtown Seattle (mileposts 169 to 168). Vertically, the graph represents the length of the bridge. Horizontally, the graph shows part of a 24-hour day, from 6:00 AM to 8:00 PM. This example traffic profile represents average weekday traffic conditions, based on data collected every 5 minutes during 261 weekdays in 1999.

The colors on the profile represent the likelihood of encountering heavy congestion as follows:

  • light gray means that traffic at that time and location is significantly congested less than 20 percent of the time (i.e., no more than one weekday per week, on average)

  • dark gray means that traffic at that time and location is significantly congested 20 to 40 percent of the time (i.e., one to two weekdays per week, on average)

  • light blue means that traffic at that time and location is significantly congested 40 to 60 percent of the time (i.e., two to three weekdays per week, on average)

  • dark blue means that traffic at that time and location is significantly congested 60 to 80 percent of the time (i.e., three to four weekdays per week, on average)

  • black means that traffic at that time and location is significantly congested 80 to 100 percent of the time (i.e., four to five weekdays per week, on average).

In this example of the Ship Canal Bridge, heavy congestion is not likely to be encountered until about 6:30 AM, as indicated by the light gray color. By about 7:00 AM, however, the likelihood of encountering significant congestion in the north part of the bridge has increased to a 60 to 80 percent chance (three to four weekdays per week), as indicated by the dark blue. The likelihood of heavy congestion stays high until about 8:30 AM on the north part of the bridge, and moderate (two to three weekdays per week, or light blue) on the south part of the bridge. This higher likelihood of congestion at the north end of the bridge reflects the combination of increasing volumes of vehicles arriving at the Ship Canal from the north, and vehicles entering the freeway at the NE 45th and NE 50th street on-ramps, which are just upstream from the Ship Canal bridge. Congestion on the south end of the bridge is affected by traffic at the SR 520 interchange as well as the geographic extension of congestion approaching the downtown Seattle core.

From around 9:30 AM to 11:30 AM, heavy congestion is infrequent; however, another period of increasing congestion frequency begins to build starting around 11:30 AM. By 2:30 PM, the likelihood of encountering congestion on the bridge is moderate to high again and stays that way for several hours; the chances of traveling in significant congestion remain moderate (a 40 to 60 percent chance) the rest of the afternoon until around 5:30 PM, then gradually decrease until about 7:00 PM. After 7:00 PM the likelihood of significant congestion on the bridge returns to no more than 20 percent.


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 23-Nov-2005