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Transportation: Notes & Sources


Commuting mode of transport:  Public transportation includes travel by bus, street car, trolley, subway, railroad, and ferry; people traveling to work by taxicab, motorcycle, or other means are not included in the graph.  The data reflect transportation modes used to commute to work by workers age 16 and older. If an individual uses multiple modes to commute to work, the one used for the greatest distance is reported.  Mode reported is only for commuting to work and may not match transportation data that include travel for other purposes.

Confidence interval (also called "error bar") is therange of values that includes the true value 95% of the time. If the confidence intervals of two groups do not overlap, the difference between groups is statistically significant (meaning that chance or random variation is unlikely to explain the difference).   

Congestion cost:  the value of travel delay for 2011 ($16.79 per hour of “person travel” and $86.81 per hour of “truck time,” plus the cost of excess gasoline and diesel, using state average cost per gallon). This computation of total congestion cost incorporates the increased transportation costs for groceries and other goods that are passed on to all consumers, regardless of their personal transportation choices.

Freeway planning time index:  The number by which one should multiply the multiplier for the amount of time one should plan for a trip with the expectation that you’ll only be late one day per month (the 95th percentile multiplier).  In 2011, this number was 3.99.  Using the 80th percentile, you can expect to be late once a week; in Everett/Seattle/Tacoma corridor, this index is 1.33.

King County regions: With Communities Count survey data, geographic boundaries of the four King County sub-regions (North, Seattle, East, and South) are defined by the aggregation of ZIP codes.  Washington State Department of Health data on hospitalizations can only be coded to zip codes, while deaths are coded to census geographies. As a result, the boundary definitions for King County and the four regions are slightly different for hospitalizations than for deaths.

Race/Ethnicity: Federal standards mandate that race and ethnicity (Hispanic origin) are distinct concepts requiring 2 separate questions when collecting data from an individual. "Hispanic origin" is meant to capture the heritage, nationality group, lineage, or country of birth of an individual (or his/her parents) before arriving in the United States. Persons of Hispanic ethnicity can be of any race. Communities Count's terms for racial/ethnic groups are derived from those used by the U.S Census Bureau in 2010.

  • Communities Count terms:  Hispanic, Non-Hispanic, White Non-Hispanic, Black, American Indian/Alaska Native (AIAN), Asian, Native Hawaiian/Pacific Islander (NHPI), White, and Multiple Race (Multiple). Persons of Hispanic ethnicity can be of any race and are included in other racial categories. Racial/ethnic groups are sometimes combined when sample sizes are too small for valid statistical comparisons of more discrete groups. 
  • 2010 Census terms: Hispanic or Latino, Not Hispanic or Latino, White alone (Not Hispanic or Latino),  Black or African American, American Indian or Alaska Native, Asian, Native Hawaiian or Other Pacific Islander, White, Some Other Race, and Two or More Races.

Serious injury:  Any injury other than a fatal injury that prevents the injured person from walking, driving, or normally continuing the activities the person was capable of performing before the injury occurred.

Super commuter:  “A person who works in the central county of a given metropolitan area, but lives beyond the boundaries of that metropolitan area, commuting long distance by air, rail, car, bus, or a combination of modes.” (from Moss & Qing, 2012)

Travel time index:  Measure of congestion using the ratio of travel time in peak traffic to travel time when traffic is flowing freely.  For example, a travel time index of 1.30 means that a 20-minute free-flow trip, on average will take 26 minutes in peak travel time. The 2011 travel time index for the Seattle/Tacoma/Everett area was 1.26.   


Collision and fatality + injury rates per 1,000 population can be viewed as descriptors of a community’s traffic environment, with no implications about the residence of those involved in collisions.  Please see data table for King County cities below:

Commuting Trends by Mode of Transportation

  •  During the time displayed, some cities annexed unincorporated areas, which could account for rate fluctuations.  For example, Bellevue, Bothell, Burien, Issaquah, Kent, Kirkland, and Renton all annexed during that time, with many populations increasing by 10,000 or more.

Statistical significance: Unless otherwise noted, any difference mentioned in the text is statistically significant (unlikely to have occurred by chance).

Washington State Department of Health data present an incomplete picture of traffic safety in King County.

  • Death and hospitalization data are coded by residence of the victim not the place where the crash occurred.
  • Victims who are injured or killed in a vehicular crash in King County but reside outside the county are excluded, while King County residents are included even if they are injured or killed in another county or state. 
  • Hospitalization discharge data do not capture minor injuries treated at the scene or in an emergency department without admission to the hospital. 

Washington State Department of Transportation data are based on all crashes that occur in King County, and include residents of King County and other counties.

Data Sources

Collision and injury + fatality data for cities come from the Washington State Department of Transportation’s Collision and Data Analysis Branch.  http://www.wsdot.wa.gov/mapsdata/collision/collisionannual.htm

Communities Count Survey (2011):  Respondents came from a random sample of all King County households.  Due to the limitations of surveys that rely exclusively on landline telephones, Communities Count used a mixed-mode survey involving both random-digit-dial phone contact and address-based sampling for mailed questionnaires, with an internet response option as well. Phone interviews were conducted in English, Spanish, and, upon request, a few additional languages. Possible limitations of this kind of survey include: (a) people who do not have a telephone or a permanent address are missed; (b) people who do not speak English or Spanish may not participate; (c) people who have less education and lower incomes tend to be under-represented.  For some indicators, 2011 results are compared to results of 2004 and 2007 versions of the same survey (both of which sampled exclusively by random digit dial).

Commuting Mode of Transport: American Community Survey, 2005-2014. For more King County information, see the Census Bureau’s detailed table: http://factfinder.census.gov/bkmk/table/1.0/en/ACS/14_5YR/S0801/0500000US53033.

Death and hospitalization data are from the Washington State Department of Health. Hospitalizations can only be coded to zip codes, while deaths are coded to census geographies. As a result, the boundary definitions for King County and the four regions are slightly different for hospitalizations than for deaths. All rates are age-adjusted to the 2000 U.S. population. 

Additional data on motor vehicle deaths, deaths involving driver impairment, drinking, and speeding are from the National Highway Traffic Safety Administration, Fatal Accident Reporting System, Washington State Department of Transportation, and Washington Traffic Safety Commission.  http://www.wtsc.wa.gov/

Seat belt usage data are from the Washington Traffic Safety Commission. http://www.wtsc.wa.gov/

Traffic congestion trends: Data on annual traffic delay and congestion costs were gathered from the 2012 Annual Urban Mobility Report (December 2012), produced annually by the Texas A&M Transportation Institute and using data  (http://mobility.tamu.edu/ums/).  Full report at http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/mobility-report-2012-wappx.pdf.  Seattle area data at http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/ums/congestion-data/seatt.pdf.   Detailed 2011 data on specific congested corridors in all 4 regions of King County can be found at http://mobility.tamu.edu/files/2011/11/seatt-ccr.pdf.

Washington State Traffic Serious Injuries per 100 million VMT come from WSDOT Collision Database, prepared by WTSC on 9/17/2011. http://www-stage.wtsc.wa.gov/wp-content/uploads/downloads/2012/04/Traffic-Deaths-Serious-Injuries-All-Causes.pdf

Other Sources

King County data on leading contributing causes of traffic fatalities and serious injuries are from Fatality Analysis Reporting System (FARS) as of February 15, 2012, Washington Traffic Safety Commission, Traffic Research & Data Center.  Some of these data can be accessed at http://www-stage.wtsc.wa.gov/wp-content/uploads/downloads/2012/04/Traffic-Safety-Priorities-and-Crash-Statistics-for-Local-Jurisdictions-2006-2010.pdf.

King County Metro Transit 2012 Strategic Plan Progress Report (2013) provides updated information on the public transit system in King County, including ridership, accessibility, energy use, costs, and customer satisfaction.  The report can be accessed at http://metro.kingcounty.gov/am/reports/2012/metro-2012-strategic-plan-progress-report.pdf.

King County “super-commuter” rates are from Mitchell L. Moss & Carson Qing. The Emergence of the “Super-Commuter” (February, 2012). Rudin Center for Transportation, New York University Wagner School of Public Service.  http://wagner.nyu.edu/rudincenter/publications/supercommuter_report.pdf

Long-distance commutes to and from King County:  Census Bureau reports 240,000 workers commute into King County, WA, each day. U.S. Census Bureau News Release, March 5, 2013 at http://www.census.gov/newsroom/releases/archives/american_community_survey_acs/cb13-r12.html.

Maps of King County covering a wide range of topics can be accessed at http://www.kingcounty.gov/operations/GIS/Maps.aspx#PH.  Maps most immediately relevant to Communities Count are under the headings of Community data & demographics, Public health, and Environment & natural resources, but other maps should be useful as well (farmers markets, transit routes, walking and biking routes, parks, traffic counts, etc.).

Map of transit service and facilities in King County with locations of minority and low-income census tracts. Some Sound Transit routes operating in King County are also shown here.

Modes of transportation to work in King County:  For more information, see the Census Bureau’s detailed table   http://factfinder2.census.gov/bkmk/table/1.0/en/ACS/11_1YR/S0802/0500000US53033.

National and state comparisons of traffic fatality and injury rates for 2010 can be found in 2010 Motor Vehicle Crashes: Overview (February 2012 revision), Traffic Safety Facts, U.S. Department of Transportation, National Highway Traffic Safety Administration, DOT HS 811 552 athttp://www-nrd.nhtsa.dot.gov/Pubs/811552.pdf

Serious injury definition from Washington State’s “Target Zero” Strategic Highway Safety Plan 2010 at http://www.wsdot.wa.gov/partners/targetzero/PDF/TargetZeroPlan.pdf

Sightline.org provides informative blogs and news about transportation in the Pacific Northwest at http://daily.sightline.org/category/land-use-transportation/

Travel time to work in King County: data from U.S. Census Bureau’s American Fact Finder, Commuting Characteristics by Sex, 2011 (S0801):  http://factfinder2.census.gov/faces/tableservices/jsf/pages/productview.xhtml?src=bkmk. Also see State & County QuickFacts at http://quickfacts.census.gov/qfd/states/53/53033.html.

Walk Scores (by address, neighborhood, city, etc.) and Walkability Heat Maps: http://www.walkscore.com/

Notes, Sources & Related Links