Freeway Performance

Volume, Speed, Reliability Profiles (draft 23 November 2005)


How and Where Volume, Speed, and Reliability Profiles were Measured

Volume, speed, and reliability were analyzed for four central freeway locations in the Seattle area “rectangle” bounded by I-5, SR 520, I-405, and I-90, plus one location on SR 167. The selected locations were downtown Seattle on I-5 at University Street, downtown Bellevue on I-405 at NE 14th Street, SR 520 at 84th Avenue NE, I-90 on the floating bridge, and SR 167 a mile south of the I-405 interchange.  While these five sites are not representative of all freeway sections, they provide considerable insight into the region’s freeway system performance.

Each grapc shows three measures of system performance for one direction and type of lane at the selected freeway location. These measures include the following:

  • Average weekday traffic volume profile at a location, by time of day
  • Average weekday speed at a location, by time of day
  • Frequency of significant congestion at a location, by time of day for weekdays

Average Weekday Traffic Volume Profile at a Location, by Time of Day. These data come from loop detectors placed by WSDOT in the mainline freeway lanes to support the Department’s ramp metering and incident response systems.  Loop detectors report vehicle volumes and the percentage of time that a vehicle is present over that loop.  These statistics are reported every 20 seconds continuously throughout the year.  The graphs can be converted into measures of traffic volume and speed and summarized over time.  

Because both the magnitude and timing of volumes during the day are important considerations in trying to understand freeway performance, an average 24-hour weekday traffic profile was estimated at selected freeway sites. Vehicle volumes were estimated at 5-minute intervals over an average 24-hour weekday at each site; these volumes were then adjusted to an equivalent per-lane hourly rate (vehicles per lane per hour, or vplph) to allow direct comparisons between sites with different numbers of lanes.

Average Speed at a Location, by Time of Day. Because traffic volumes are affected by the speed that vehicles can travel (and vice versa), the average speed observed at each time of day was also estimated throughout the day for the selected sites.  Average speeds are displayed by color coding of the vehicle volume line.

Frequency of Significant Congestion at a Location, by Time of Day. Average conditions do not represent the condition that always occurs; on some days traffic conditions will be worse than average, and on other days conditions will be better. In fact, freeways tend to operate in one of three conditions,

  1. free flowing
  2. heavy traffic moving, but moving below the speed limit
  3. “badly congested” or unstable (stop and go) flow

A good measure of how motorists perceive traffic is how often a freeway exhibits “bad” or unstable flow.  Congestion frequency is measured by the likelihood that significantly congested traffic will be encountered at a location at a given time of day. For example, a congestion frequency of 75 percent at 5:00 PM indicates that there is a 75 percent chance of encountering congested conditions (stop and go, or unstable flow) at a particular location in a given direction of travel at 5:00 PM.


I-5 north
of the Seattle CBD

I-5 @ 112th SE – Everett (near Everett)
I-5 @ NE 145th St (near Northgate)


I-5 South
of the Seattle CBD

I-5 @ Albro Place (south of Seattle Downtown)
I-5 @ S 184th St (south of Southcenter)
I-405 North of I-90

I-405 @ NE 85th St (near Kirkland)
I-405 @ SE 52nd S. (near Newcastle)

I-405 South of I-90
I-405 @ Tukwila Parkway (near Southcenter)
  I-90 I-90 @ Midspan (Floating Bridge)
I-90 @ W Lk Sammamish Pkway (near Issaquah)
  SR 520 SR 520 @ 84th Ave NE (near Medina)
  SR 167 SR 167 @ S 204th Ave S (near Kent)

How to Read the Average Weekday Volume, Speed, and Reliability Conditions Graphs

Site-specific HOV lane performance is illustrated with a graph that combines HOV lane volume by time of day, and the frequency with which the HOV lane at that location fails to average 45 mph.
 [Figure 3-18. (Example) Average Volumes, Speed, and Speed Reliability Conditions Graph: Southbound on I-5 NE 137th St.]

The example [embed fig. 3-18] illustrates performance for the southbound I-5 HOV lane at NE 137th St. near Northgate. It shows average volumes and travel speed conditions from 4:00 AM to 12:00 midnight. The horizontal axis represents time of day. (Usually from midnight to midnight, but for this example only twenty hours are actually shown.) The line shows the expected traffic volume, and is measured with the left vertical axis in units of vehicles per lane per hour. The color of the line indicates the expected speed of vehicles in the HOV lane on the average day:

  • Gray indicates that traffic moves at or faster than 45 mph
  • Black represents traffic traveling slower than 45 mph

The column graph is read from the right vertical axis, which shows the frequency of congestion (speeds below 45 mph) at this location. In this example, HOV volumes exceeds 1,400+ VPLPH during the AM peak period. Travelers can expect to travel faster than 45 mph throughout the day, except between 7:00 AM and 8:00 AM, when they have approximately a 30 percent chance of encountering speeds of less than 45 mph.


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