Freeway Performance

Average Congestion (by time and location): HOV Lanes (draft 8 November 2005)

How and Where Average Congestion Was Measured

To better understand how traffic conditions change as vehicles travel along the freeway network, the average or “routine” freeway travel conditions a motorist could expect to see on any given weekday were analyzed.  The estimates were derived from the volume and lane occupancy data collected by WSDOT as part of its automated ramp metering system. 

Data are presented separately for HOV lanes and general purpose (GP) lanes.  All the weekday data for each corridor were combined to provide an image of the “routine” traffic conditions along each corridor during an average 24-hour weekday.  For the general purpose lanes, data for lanes are combined at each particular location.  As a result, the images usually do not show congestion in specific lanes of travel (like exit lanes) when the freeway as a whole is not experiencing congestion. 

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 Average Congestion Maps

The average congestion maps provide a clear understanding of how traffic conditions change as vehicles travel from one location to another on the freeway network.  The images show the congestion patterns at different points (mileposts) along each corridor during an average 24-hour weekday. (Therefore, on some days conditions are much better than those illustrated, and on others they can be much worse.) Each map is presented in a contour format similar to that of a topographic or elevation map, using colors that indicate relative levels of congestion 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.

Figure 3.1. Traffic Profile: Southbound I-5 Ship Canal Bridge

The example shows a slice of a typical traffic congestion 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 a 24-hour day, from midnight to midnight. This example traffic profile represents average weekday traffic conditions based on data collected every 5 minutes during approximately 261 weekdays in 1997.

The colors on the profile represent congestion as follows:

• green means that traffic generally moves at or near the speed limit under uncongested, free-flow conditions

• yellow means that travelers encounter borderline traffic conditions with more restricted movements (for example, lane changing difficulties), but still travel near the speed limit

• red is more heavily congested traffic traveling perhaps between 45 and 55 mph

• blue is very congested, unstable traffic that ranges from stop and go to as fast as 45 mph (although it would travel at 45 mph for only a brief distance before slowing dramatically again).

A study of this profile of the Ship Canal Bridge (southbound) shows that, on average, from midnight to about 6:30 AM traffic flows freely. This is followed by a brief period of rapidly increasing congestion (yellow to red to blue), so that by 7:00 AM traffic is very congested and may well be nearly stopped.

This congestion is a reflection of the increasing number of cars approaching the Ship Canal Bridge from farther up the freeway, and cars merging onto the freeway from the NE 50th and NE 45th Street on-ramps just north of the bridge. Notice that the worst traffic congestion (blue) is at the north end of the bridge near the NE 45th Street onramp, whereas at the south end of the bridge, which offers an exit to SR 520, traffic moves more freely. (As SR 520 traffic merges with I-5 southbound traffic, just south of where the example picture ends, I-5 becomes more congested again.)

The high congestion level lasts until about 8:45 AM, after which traffic slowly clears out until about 10:00 AM. There is a brief period of freely flowing traffic, but the congestion build-up and slow-downs begin again about 11:30 AM. This congestion pattern continues until about 7:00 PM. By 8:00 PM traffic is free flowing once again.

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.