The Urban Congestion Report (UCR): Documentation and Definitions

The quarterly Urban Congestion Report is sponsored by the Federal Highway Administration's Office of Operations in Washington, DC to:

provide timely congestion and travel reliability information to state and local agencies;
demonstrate the use of archived traffic operations data for performance monitoring; and,
promote state and local performance monitoring to support transportation decisions.

The Urban Congestion Report is developed using archived traffic operations data. The Urban Congestion Report includes only those roadways that are instrumented with traffic sensors for the purposes of real-time traffic management and/or traveler information. In many cities, this typically includes the most congested parts of the freeway system. The Report currently does not include congestion information on arterial streets.

The congestion information presented in these reports may not be representative of the entire roadway system in any particular city. Construction may affect the roadways that are included in this report. The congestion and reliability trends are calculated by comparing the most recent three months this year to the same three months last year. Only instrumented roadways that provided data in both years are included in the Urban Congestion Report.

Several automated and manual quality assurance procedures are used in developing the Urban Congestion Report. The automated validity checks and other data processing details are described in Report # FHWA-HOP-05-018, "Monitoring Urban Freeways in 2003: Current Conditions and Trends from Archived Operations Data", http://tti.tamu.edu/documents/FHWA-HOP-05-018.pdf (PDF 866KB).

The following paragraphs describe the measures and indicators highlighted on each report. Detailed information on calculating these measures follows the descriptions.

Congestion and Reliability Indicators

Congested Hours: the average number of hours during specified time periods in which instrumented road sections are congested (speeds less than 45 mph). For this measure, congestion is defined to occur when link speeds are less than 45 mph. This measure is reported for weekdays (6a-10p).
Travel Time Index: ratio of the average peak period travel time as compared to a free-flow travel time. The free-flow travel time for each road section is the 15th percentile travel time during traditional off-peak times (weekdays between 9a-4p, 7p-10p; weekends between 6a-10p), not to exceed the travel time at the posted speed limit (or 60 mph where the posted speed is unknown). For example, a value of 1.20 means that average peak travel times are 20% longer than free-flow travel times. In this report, the AM peak period is 6a-9a and the PM peak period is 4p-7p on non-holiday weekdays. Averages across road sections and time periods are weighted by VMT.
Planning Time Index: a ratio of the total time needed to ensure 95% on-time arrival as compared to a free-flow travel time. This measure is computed for the AM peak period (6a-9a) and the PM peak period (4p-7p) for non-holiday weekdays. For example, a value of 40% means that a traveler should budget an additional 8 minute buffer for a 20-minute average peak trip time to ensure 95% on-time arrival. The planning time index is computed as the 95th-percentile travel time of the month divided by the free-flow travel time for each road section and time period. The free-flow travel time for each road section is the 15th percentile travel time during traditional off-peak times (weekdays between 9a-4p, 7p-10p; weekends between 6a-10p), not to exceed the travel time at the posted speed limit (or 60 mph where the posted speed is unknown). Averages across road sections and time periods are weighted by VMT.

Data Quality Measure

% Usable Data: the number of recorded data values divided by the number of total expected data values (given the number of instrumented road sections, active sensors, and time periods).

Congested Hours

The average length of time each day that roads in a particular city are congested.

Input data

5-minute section-level speeds and vehicle-miles of travel (VMT).

Procedures

For all sections, time periods (between 6 am and 10 pm), and all valid weekdays:

Flag the sections that have speeds slower than 45 mph - the VMT for these sections will be counted as congested VMT.
For each section and each valid weekday in the month, sum the number of congested 5-minute periods to compute the total daily duration of congestion. Again, only time periods between 6 am and 10 pm should be counted.
Compute the average number of congested hours per month over all valid weekdays.

Travel Time Index

The extra time spent in traffic during peak traffic times as compared to light traffic times. In mathematical terms, it is the peak travel time divided by the free-flow travel time. If using speed in calculations, it is the free-flow traffic speed divided by the peak traffic speed. Both calculations will yield identical travel time index values.

Input data

Average free-flow speeds for each section during off-peak times
5-minute section-level speeds and VMT for peak traffic periods (6-9a, 4-7p)

Procedures

Compute free-flow travel speed for each section as the 85th percentile speed during the previous 3 months during weekday off peak times (9a-4p, 7p-10p) and weekend/holiday times (6a-10p), not to exceed the travel time at the posted speed limit (or 60 mph where the posted speed is unknown).
Compute the travel time index for each section and 5-minute peak period (6-9a, 4-7p) of each valid weekday. Compute by dividing the free-flow traffic speed by the peak traffic speed. If the computed travel time index is less then 1.00, then round up to 1.00.
Compute the travel time index for the entire network for each day by computing a VMT-weighted average of all travel time indices during peak times.
Compute the monthly average travel time index by taking a simple (or VMT-weighted) average across all valid weekdays.

Planning Time Index

The extra time cushion needed during peak traffic periods to prevent being late. In mathematical terms, it is the near-worst case travel time (95th percentile) divided by the free-flow travel time. If using speeds in calculations, it is the free-flow traffic speed divided by the near-worst case traffic speed (5th percentile).

Input data

Average free-flow speeds for each section during off-peak times
5-minute section-level speeds and VMT for peak traffic periods (6-9a, 4-7p)

Procedures

Use free-flow travel speed as computed for travel time index.
Compute the 5th percentile travel speed for each section and 5-minute peak time period (6-9a, 4-7p) during ALL valid weekdays. That is, include all weekdays in the month in the calculation of 5th percentile speeds.
For each road section and 5-minute peak time period, compute the planning time index as the free-flow traffic speed divided by the 5th percentile traffic speed. If the planning time index is less than 1.00, round the index value up to 1.00.
Compute the average planning time index for each road section by taking VMT-weighted averages across all 5-minute index values.
Compute the average network planning time index by taking VMT-weighted averages across all road sections.

General Calculation Principles

Holidays are not included as valid weekdays. We use Federally-designated holidays plus one extra day each around Thanksgiving and Christmas.

A "bad days" list is compiled that includes dates, times, and road section that have failed a visual quality control check. "Bad days" are excluded from valid weekdays on a section-by-section basis.

When computing average index values across road sections or time periods, VMT is used to weight the calculations. VMT can also be used to weight the average travel time index calculation across all weekdays; however, in most cases, the differences are not significant (for weighted vs. non-weighted averages) when computing averages across weekdays.

Factoring is used throughout the calculations as a way to estimate VMT that may be missing for road sections, time periods or days. The factoring is performing by adjusting VMT values up by the percent of missing data values. The unfactored VMT estimates are also carried through the process in case this is concern about overestimation of VMT.

Questions?

The Urban Congestion Report is produced by the Texas Transportation Institute for the Federal Highway Administration. Please contact us if you have any questions.

PDF files can be viewed with the Acrobat® Reader®.

Search Performance Measurement: Home Contact Us Projects Urban Congestion Reports Documentation and Definitions Urban Congestion Trends and Related Reports Travel Time Reliability Measures Resources Performance Measures Fundamentals Example Programs Research Resources and Publications	The Urban Congestion Report (UCR): Documentation and Definitions Urban Congestion Reports The quarterly Urban Congestion Report is sponsored by the Federal Highway Administration's Office of Operations in Washington, DC to: provide timely congestion and travel reliability information to state and local agencies; demonstrate the use of archived traffic operations data for performance monitoring; and, promote state and local performance monitoring to support transportation decisions. The Urban Congestion Report is developed using archived traffic operations data. The Urban Congestion Report includes only those roadways that are instrumented with traffic sensors for the purposes of real-time traffic management and/or traveler information. In many cities, this typically includes the most congested parts of the freeway system. The Report currently does not include congestion information on arterial streets. The congestion information presented in these reports may not be representative of the entire roadway system in any particular city. Construction may affect the roadways that are included in this report. The congestion and reliability trends are calculated by comparing the most recent three months this year to the same three months last year. Only instrumented roadways that provided data in both years are included in the Urban Congestion Report. Several automated and manual quality assurance procedures are used in developing the Urban Congestion Report. The automated validity checks and other data processing details are described in Report # FHWA-HOP-05-018, "Monitoring Urban Freeways in 2003: Current Conditions and Trends from Archived Operations Data", http://tti.tamu.edu/documents/FHWA-HOP-05-018.pdf (PDF 866KB). The following paragraphs describe the measures and indicators highlighted on each report. Detailed information on calculating these measures follows the descriptions. Congestion and Reliability Indicators Congested Hours: the average number of hours during specified time periods in which instrumented road sections are congested (speeds less than 45 mph). For this measure, congestion is defined to occur when link speeds are less than 45 mph. This measure is reported for weekdays (6a-10p). Travel Time Index: ratio of the average peak period travel time as compared to a free-flow travel time. The free-flow travel time for each road section is the 15th percentile travel time during traditional off-peak times (weekdays between 9a-4p, 7p-10p; weekends between 6a-10p), not to exceed the travel time at the posted speed limit (or 60 mph where the posted speed is unknown). For example, a value of 1.20 means that average peak travel times are 20% longer than free-flow travel times. In this report, the AM peak period is 6a-9a and the PM peak period is 4p-7p on non-holiday weekdays. Averages across road sections and time periods are weighted by VMT. Planning Time Index: a ratio of the total time needed to ensure 95% on-time arrival as compared to a free-flow travel time. This measure is computed for the AM peak period (6a-9a) and the PM peak period (4p-7p) for non-holiday weekdays. For example, a value of 40% means that a traveler should budget an additional 8 minute buffer for a 20-minute average peak trip time to ensure 95% on-time arrival. The planning time index is computed as the 95th-percentile travel time of the month divided by the free-flow travel time for each road section and time period. The free-flow travel time for each road section is the 15th percentile travel time during traditional off-peak times (weekdays between 9a-4p, 7p-10p; weekends between 6a-10p), not to exceed the travel time at the posted speed limit (or 60 mph where the posted speed is unknown). Averages across road sections and time periods are weighted by VMT. Data Quality Measure % Usable Data: the number of recorded data values divided by the number of total expected data values (given the number of instrumented road sections, active sensors, and time periods). Congested Hours The average length of time each day that roads in a particular city are congested. Input data 5-minute section-level speeds and vehicle-miles of travel (VMT). Procedures For all sections, time periods (between 6 am and 10 pm), and all valid weekdays: Flag the sections that have speeds slower than 45 mph - the VMT for these sections will be counted as congested VMT. For each section and each valid weekday in the month, sum the number of congested 5-minute periods to compute the total daily duration of congestion. Again, only time periods between 6 am and 10 pm should be counted. Compute the average number of congested hours per month over all valid weekdays. Travel Time Index The extra time spent in traffic during peak traffic times as compared to light traffic times. In mathematical terms, it is the peak travel time divided by the free-flow travel time. If using speed in calculations, it is the free-flow traffic speed divided by the peak traffic speed. Both calculations will yield identical travel time index values. Input data Average free-flow speeds for each section during off-peak times 5-minute section-level speeds and VMT for peak traffic periods (6-9a, 4-7p) Procedures Compute free-flow travel speed for each section as the 85th percentile speed during the previous 3 months during weekday off peak times (9a-4p, 7p-10p) and weekend/holiday times (6a-10p), not to exceed the travel time at the posted speed limit (or 60 mph where the posted speed is unknown). Compute the travel time index for each section and 5-minute peak period (6-9a, 4-7p) of each valid weekday. Compute by dividing the free-flow traffic speed by the peak traffic speed. If the computed travel time index is less then 1.00, then round up to 1.00. Compute the travel time index for the entire network for each day by computing a VMT-weighted average of all travel time indices during peak times. Compute the monthly average travel time index by taking a simple (or VMT-weighted) average across all valid weekdays. Planning Time Index The extra time cushion needed during peak traffic periods to prevent being late. In mathematical terms, it is the near-worst case travel time (95th percentile) divided by the free-flow travel time. If using speeds in calculations, it is the free-flow traffic speed divided by the near-worst case traffic speed (5th percentile). Input data Average free-flow speeds for each section during off-peak times 5-minute section-level speeds and VMT for peak traffic periods (6-9a, 4-7p) Procedures Use free-flow travel speed as computed for travel time index. Compute the 5th percentile travel speed for each section and 5-minute peak time period (6-9a, 4-7p) during ALL valid weekdays. That is, include all weekdays in the month in the calculation of 5th percentile speeds. For each road section and 5-minute peak time period, compute the planning time index as the free-flow traffic speed divided by the 5th percentile traffic speed. If the planning time index is less than 1.00, round the index value up to 1.00. Compute the average planning time index for each road section by taking VMT-weighted averages across all 5-minute index values. Compute the average network planning time index by taking VMT-weighted averages across all road sections. General Calculation Principles Holidays are not included as valid weekdays. We use Federally-designated holidays plus one extra day each around Thanksgiving and Christmas. A "bad days" list is compiled that includes dates, times, and road section that have failed a visual quality control check. "Bad days" are excluded from valid weekdays on a section-by-section basis. When computing average index values across road sections or time periods, VMT is used to weight the calculations. VMT can also be used to weight the average travel time index calculation across all weekdays; however, in most cases, the differences are not significant (for weighted vs. non-weighted averages) when computing averages across weekdays. Factoring is used throughout the calculations as a way to estimate VMT that may be missing for road sections, time periods or days. The factoring is performing by adjusting VMT values up by the percent of missing data values. The unfactored VMT estimates are also carried through the process in case this is concern about overestimation of VMT. Questions? The Urban Congestion Report is produced by the Texas Transportation Institute for the Federal Highway Administration. Please contact us if you have any questions. PDF files can be viewed with the Acrobat® Reader®.
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