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Methodology

This section provides an overview of the methodology in INRIX Traffic Scorecard. In 2007, the initial INRIX Traffic Scorecard was the first analysis to deliver detailed road segment level across an entire country – the United States. In 2010, INRIX was again the first company to deliver detailed analysis across six European countries. Modest but important changes in the methodology used in previous Scorecards will now enable INRIX to publish updates to the Scorecard shortly after the conclusion of every month – on a worldwide basis as INRIX raises the bar again.

Source Data & Analysis

The INRIX Traffic Data Archive is the source of “Big Data” (typically several years of historical traffic information) used in the Scorecard. For this INRIX Traffic Scorecard, urban areas in North America and Europe are analyzed.

INRIX has developed efficient methods for interpreting its real-time traffic data to establish monthly and annual averages of travel patterns in all major cities. These same methods can aggregate data over periods of time to provide reliable information on speeds and congestion levels for segments of roads.

Analysis Time Period

The Scorecard contains detailed information from January 2010 through the current completed month. In some cases, urban areas have been added since January 2010 – in those cases, data will begin in the first month in which quality data is available.


Metropolitan Area & Roads/Segments Analyzed

One of the difficulties in analyzing and comparing metropolitan area congestion is defining what a metropolitan area is. Across North America and Europe, INRIX has strived to take standard definitions of metropolitan areas rather than creating our own.

In North America, we have adopted the Census Bureau definition of Core-Based Statistical Areas (CBSA). For a current map of CBSAs by state from the Census Bureau, go to http://www.census.gov/population/metro/files/metro_micro_Feb2013.pdf.

In Europe, INRIX has chosen to leverage the broad Urban Audit project created by the Eurostat and their definition of Larger Urban Zones (LUZ). At present the Urban Audit includes 321 cities from the 27 European Union Member States, 26 Turkish cities, six Norwegian cities and four Swiss cities. For more information and maps of LUZs, go to http://epp.eurostat.ec.europa.eu/statistics_explained/index.php/European_cities_-_spatial_dimension.

In each metropolitan area (CBSA or LUZ), INRIX’s current “Core” reporting network – major motorways and arterials – are analyzed. INRIX utilizes a common industry convention known as “TMC location codes” developed and maintained by the leading electronic map databases vendors to uniquely define road segments. The typical road segment is the interchange and the portion of linear road leading up to the interchange across all lanes in a single direction of travel. The length of a segment will depend upon the length of the distance between interchanges/junctions/intersections.


Road Segment Data

There are two key building blocks for the different analyses included in this report:
  • Reference Speed (RS): An uncongested “free flow” speed is determined for each road segment using the INRIX Traffic Archive.
  • Calculated Speed (CS): All archived speeds for each 15 minute period each day for each road segment is calculated for each month (e.g. Monday from 06:00 to 06:15 for April 2012) and a “calculated speed” for each time slot is established for each road segment. Thus, each segment has 672 corresponding calculated speed values – representing four 15 minute time windows for all 24 hours of each day times the seven days in a week.

Overall Congestion by Metropolitan Area

To assess congestion across a metropolitan area, INRIX utilizes and adapts several concepts that have been used in similar studies and previous Scorecards.

INRIX Index (II): The INRIX Index represents the barometer of congestion intensity. For a road segment with no congestion, the INRIX Index would be zero. Each additional point in the INRIX Index represents a percentage point increase in the average travel time of a commute above free-flow conditions during peak hours. An INRIX Index of 30, for example, indicates a 20-minute free-flow trip will take 26 minutes during the peak travel time periods with a 6-minute (30 percent) increase over free-flow.

For each road segment, an INRIX Index is calculated for each 15 minute period of the week, using the formula INRIX Index = (RS/CS) – 1.

“Drive Time" Congestion: To assess and compare congestion levels year to year and between metropolitan areas, only “peak hours” are analyzed. Consistent with similar studies, peak hours are defined as the hours from 06:00 to 10:00 and 15:00 to 19:00, Monday through Friday – 40 of the 168 hours of a week.

For each Metropolitan Area, an overall level of congestion is determined for each of the 40 peak hours by determining the extent and amount of average congestion on the analyzed road network. This is easy to compute once INRIX Indices are calculated for each segment:

  • STEP 1: For each of the 40 peak hours, all road segments analyzed in the CBSA or LUZ are checked. Each segment where the INRIX Index > 0 is contributing congestion, and it is analyzed further.
  • STEP 2: For each segment contributing congestion, the amount the INRIX Index is greater than 1 is multiplied by the length of the segment, resulting in a congestion factor.
  • STEP 3: For each 15 minute period, the overall metropolitan congestion factor is the sum of the congestion factors calculated in STEP 2.
  • STEP 4: To establish the Metropolitan INRIX Index for a given 15 minute period, the metropolitan congestion factor from STEP 3 is divided by the number of road miles analyzed.
  • STEP 5: A peak period INRIX Index is determined by averaging the 15 minute indices from STEP 4.

Wasted Time (Hours/Minutes) in Congestion

To convert delay from a typical commute trip into monthly and annual delay totals – “Hours Wasted in Congestion” – requires an estimate of typical commute trip length (in time) and the number commute trips the typical commuter takes in a month/year.

In the United States, the Census Bureau’s American Community Survey, published annually, include details regarding commuting trip times by Core-based Statistical Areas. The Census Bureau’s “American Fact Finder” (http://factfinder2.census.gov) contains vast quantities of commuting information. Dataset ID “GCT0801” contains the information used in this report, specifically the 2010 American Community Survey 1-Year estimates, with the specific table located here: http://tinyurl.com/acs2010cbsatriptimes. Note the average metropolitan area commute trip time in the United States was 25.3 minutes in 2010.

The assumed number of annual commute trips is assumed at 440 – equivalent to traveling to and from work 5 days a week for 44 weeks. “Wasted Hour” Estimates are annualized and to create a monthly estimate of wasted hours, the annual result is divided by 12.

In Europe and Canada, government published trip time estimates are used where credible and aligning with the metropolitan areas being analyzed. Otherwise a 30 minute trip time is used.

Congested Corridors (United States Only)

In the United States, we analyze specific road segment on an annual basis to understand within metropolitan areas, where the most congested corridors are. Note that this data unlike the rest of the information in the INRIX Traffic Scorecard is available only in the United States and is only updated annually (not monthly).

The following approach is used to determine and then rank corridors. 2010 data was used to identify the most congested corridors, using the following criteria (for comparison purposes, the same corridors identified in 2010 are used in 2011):

  • The corridor must be comprised of multiple road segments (i.e., TMCs).
  • The corridor must have at least one segment that is congested ten hours a week or more on average
  • All road segments in the corridor must have at least four hours a week of congestion on average.
  • To prevent inadvertently breaking up logical corridors, in cases where one or two short segments do not meet the four hour minimum, exceptions are made. However, they must be in the middle of a corridor, not at the start or end.
  • Once the corridors were identified (341 total identified with 167 of them listed as having significant delays), another analysis determined several different travel time statistics that are used to describe and rank each corridor. The following steps were used to analyze and rank the corridors:
For each corridor:
  • The uncongested/free flow travel time is calculated (from the RS of each road segment in a corridor).
  • Average travel times for both peak periods (AM and PM) are determined.
  • The highest peak period travel time is compared to the uncongested/free flow travel time, resulting in both an average peak period delay and peak period INRIX Index.
  • To illustrate how bad a corridor is at its most congested, the worst hour delay and INRIX Index is computed.
  • To rank corridors:
    • A corridor congestion factor is determined for each corridor by multiplying average delay by the INRIX Index for the worse of the AM or PM peak periods.
    • Each corridor’s congestion factor can be compared to and ranked against others in a metropolitan area and against all corridors.