No. T6, July 1997

Traffic Congestion in the Central Puget Sound Region

Traffic congestion has been a fact of life in the central Puget Sound region almost as long as cars have been on the roads. Old movies in the City of Seattle archives show congested conditions during "rush hour" at the Fremont Bridge in the 1920s. What has changed is the extent of the congestion. In the 1990s some roads are congested 12 hours a day, and nearly all main roads are congested for several hours each weekday afternoon. This Puget Sound Trend shows the personal trips people are making by time of day and the effect these trips have on the vehicle volumes on some local freeways.

The data for Figure 1 come from the Puget Sound Transportation Panel, an on-going survey of approximately 1,700 households in the central Puget Sound region. Six waves of this survey have been done since the first wave in 1989. In each wave, each household completed a two-day travel diary of all week-day trips made by household members age 15 years and older. Since the distribution of trips by time of departure has not changed significantly over the years of the survey, the data from waves two through six (1990 through 1996) have been aggregated for this analysis.

In this analysis, the word trip refers to each segment of a chain of trips starting and ending at home, with one or more stops along the way. Only the first trip and the final return trip are connected to home. Other trips in the middle of the chain are "not home-based."

Because the average work trip is longer than the average of most other trip types, and shopping trips are a little shorter, the trip data from the Panel Survey have been weighted so that Figure 1 shows the regional distribution of miles traveled for each trip type, by time of day. The trips are categorized by the location of their start and end points. The categories are:

  • trips that start at home and go directly to work (example: morning commute);
  • trips that neither start nor end at home (examples: a mid-day trip from work or a store to lunch, a store for shopping, or a doctor's appointment);
  • trips that start at work and go directly to home (example: non-stop evening commute);
  • trips between home and a store for shopping (examples: from home to a grocery store, or the return);
  • trips between home and some other activity (examples: going to a friend's house, or returning from a movie).

Figure 1Graph: Regional Miles Traveled, Personal Trips

Figure 1 clearly shows:

  • most home-to-work trips are between 5 a.m. and 8 a.m.;
  • there are fewer direct work-to-home trips than home-to-work;
  • there is an increase in trips around noon;
  • the combination of afternoon commute and other types of trips produces the highest number of trips between 4 p.m. and 5:30 p.m.;
  • evening activities continue through 9 p.m.

Figure 2 shows average mid-week traffic volumes in the spring of 1995 at six different locations on the local freeway system. These counts have been obtained from the Washington State Department of Transportation's Surveillance, Control, and Driver Information system. This system uses continuous traffic counts from electronic loops embedded in the roadways to monitor the flow of traffic in order to detect slowdowns, control the flow of traffic at on-ramps, and provide information to drivers. The six locations represented in Figure 2 are on I-5 near Southcenter, north of downtown Seattle, and north of Northgate; on SR 520 east of the bridge; on I-405 south of I-90; and on I-90 at the middle of the floating bridge.

Figure 2Graph: Freeway Volumes by Time of Day

The shapes of these curves follow the shape of Figure 1 very closely. Some additional details emerge:

  • SR 520 and I-405 have modest peaks. This happens because both facilities operate at full capacity throughout the day. No additional capacity is available for the increased trips of the peak periods.
  • I-90 has the most prominent peaks, since it absorbs the peak traffic which would otherwise use the SR 520 bridge.
  • The afternoon peaks are similar in volume (due to capacity constraints) but longer in duration than the morning peaks.
  • The noon peak is not as prominent in the freeway volumes as in the trips of Figure 1 - but most of those mid-day trips are not the type which would use the freeway.
  • The freeway volumes reflect the continuation of evening activities through 9 p.m.

For more information, please contact Larry Blain, (206) 464-5402.

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