Methodology

Source Data

The raw data comes from the historical traffic data warehouse of the INRIX Smart Dust Network. Since 2006, INRIX has acquired tens of billions of discrete “GPS-enabled probe vehicle” reports from vehicles traveling the nation’s roads – including taxis, airport shuttles, service delivery vans, long haul trucks, and consumer vehicles.

INRIX has developed efficient methods for interpreting probe vehicle reports that are provided in real-time to establish a current estimate of travel patterns in all major cities in the United States. These same methods can aggregate data over periods of time (annually in this report) to provide reliable information on speeds and congestion levels for segments of roads. With the nation’s largest probe vehicle network, INRIX has the ability to generate the most comprehensive congestion analysis to date, now covering major roads throughout the nation.

Metropolitan Area

The US Census Bureau definition of Core Based Statistical Areas is used to define metropolitan areas. This report uses the latest 2007 census estimates to identify the top 100 areas. 2008 Population Estimates are now available and will be used in the 2009 Annual Report to be published in early 2010. To maintain consistency, this update continues to use the 2007 estimates and rankings.

Roads/Segments Analyzed

This report focuses on the major limited access roads in the United States. In all of its products, INRIX utilizes an emerging industry convention known as “TMC location codes” developed and maintained by the nation’s leading electronic map databases vendors, including Tele Atlas, 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. For this report, over 110,000 road miles in over 48,000 discrete road segments have been analyzed.

Analysis Time Period

Road Segment Data

There are two key building blocks for the different analyses included in this report:

Reference speed (RS): For each road segment, all probe vehicle reports obtained in overnight hours (where congestion is usually unlikely) in 2008 are analyzed.
Hourly average speed (HS): All probe vehicle reports for each road segment are grouped by hour of day, day of week (e.g. Monday from 3 to 4pm) and an “average speed” for each time slot is established for each road segment. Thus, each segment has 168 corresponding hourly average speed values – representing 24 hours of each day times the seven days in a week.

Overall Congestion by Metropolitan Area

To assess congestion over a metropolitan area, INRIX utilizes several concepts that have been used in studies.

Travel Time Index (TTI): TTI is the ratio of peak period travel time to free flow travel time. The TTI expresses the average amount of extra time it takes to travel in the peak relative to free-flow travel. A TTI of 1.3, for example, indicates a 20-minute free-flow trip will take 26 minutes during the peak travel time periods, 6-minute (30 percent) travel time penalty. For each road segment, a TTI is calculated for each hour of the week, using the formula TTI = RS/HS.
“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 6 to 10 am and 3 to 7 pm, 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 TTI’s are calculated for each segment:
- STEP 1: For each of the 40 peak hours, all road segments analyzed in the CBSA are checked. Each segment where the TTI > 1 is contributing congestion, and it is analyzed further.
- STEP 2: For each segment contributing congestion, the amount the TTI is greater than 1 is multiplied by the length of the segment, resulting in a congestion factor.
- STEP 3: For a given hour, the overall metropolitan congestion factor is the sum of the congestion factors calculated in STEP 2.
- STEP 4: To establish the Metropolitan Travel Time Index for a given hour, the metropolitan congestion factor from STEP 3 is divided by the number of road miles analyzed.
- STEP 5: A peak period Metropolitan Travel Time Index is determined by averaging the hourly Metropolitan Travel Time Indices from STEP 4.

New for this mid-year update, monthly Travel Time Index values have been calculated for each CBSA and nationally as well and are included in previous sections.

Bottlenecks

Each road segment’s bottleneck factor can be compared with others in a metropolitan area and against all bottlenecks nationally. It can also be compared year-to-year, as we have in this Scorecard.

Congestion – and how to measure it – can be in the eye of the beholder. Is congestion defined as how bad a road segment is at its worst or is it how often the segment gets “congested” (and what is the threshold for “congestion” anyways – tapping the brakes, stop and go conditions, etc.)? INRIX has developed a method that combines both the amount of time a road segment is congested with the intensity of congestion during those periods.

The process used to analyze each of the road segments is as follows:

The same RS and HS values are utilized as in the overall congestion by metropolitan area portion of the study;
All 168 hours of the week are considered, not just the 40 “peak hours.” As will be evident in the data, severe Bottlenecks aren’t just limited to peak hours;
For each hour of the week that the average speed is less than 50% of the reference speed, the hour is considered “congested;”
For all “congested” hours, the average intensity of the congestion is determined by establishing an average travel time ratio;
The total Bottleneck factor equals the number of hours of congested by the average travel time ratio.
Each road segment’s Bottleneck factor can be compared with others in a metropolitan area and against all Bottlenecks nationally. It can also be compared year-to-year, as we will do going forward.