Congestion Measures
​Several performance measures are used to measure and track congestion, but they all relate to what travelers find important—travel time and the day-to-day variation in travel time. The Texas Transportation Institute (TTI) is a leader in developing measurements for determining congestion.  Congestion Measures can be subdivided into Mobility Measures and Reliability Measures.

Mobility Measures
These are the five most common measures for mobility: 
  • Volume-to-Capacity Ratio (V/C Ratio): the volume divided by capacity.  For the Level of Service (LOS) calculations, volume is often taken to be the 30th yearly highest. 
  • The Level of Service (LOS): a grade from A to F – with A being free flow and F being very congested – that indicates how well the roadway or intersection is serving its intended traffic.  LOS is based on a volume to capacity (v/c) ratio and has long been used as the primary measure of congestion for planning purposes.  See the Highway Capacity Manual for more details.
  • Travel Time Index: ratio of average peak travel time to an off-peak (free-flow) standard, in this case 60 mph for freeways. For example, a value of 1.20 means that average peak travel times are 20% longer than off-peak travel times.
  • Travel Delay: the amount of extra time spent traveling due to congestion.
  • Percent of Congested Travel: the congested vehicle-miles of travel divided by total vehicle-miles of travel. A relative measure of the amount of travel affected by congestion.

Reliability Measures
These are the two most common measures for reliability: 

  • Buffer Index: the extra time (buffer) needed to ensure on-time arrival for most trips. 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.
  • Planning Time Index: statistically defined as the 95th percentile Travel Time Index, this measure also represents the extra time most travelers include when planning peak period trips. For example, a value of 1.60 means that travelers plan for an additional 60% travel time above the off-peak travel times to ensure 95% on-time arrival 
References
TTI Urban Mobility Study
The 2011 Urban Mobility Report builds on previous Urban Mobility Reports with an improved methodology and expanded coverage of the nation’s urban congestion problem and solutions. 
TTI Urban Mobility Study

TTI Keys to Estimating Mobility
There are several keys to developing and applying mobility measures that are technically useful and generally understandable. Travel time measures are relatively easy to comprehend, but they have not always been used because of data concerns, mandated reporting practices, and other issues.
TTI Keys to Estimating Mobility

FHWA's Operations Performance Measurement
Performance measurement is the use of evidence to determine progress toward specific defined organizational objectives. This includes both quantitative evidence (such as the measurement of customer travel times) and qualitative evidence (such as the measurement of customer satisfaction and customer perceptions). Operations performance measurement, therefore, measures progress toward meeting the objectives of transportation system management and operations.
FHWA's Operations Performance Measurement

Highway Capacity Manual
The fifth edition of the Highway Capacity Manual (HCM2010), which updates HCM2000, will significantly enhance how engineers and planners assess the traffic and environmental effects of highway projects. 
Highway Capacity Manual

Washington State DOT 2007 Congestion Report
As Washington's robust economy continues to grow, so does congestion. More people want to travel on the same roads at the same time. Between 2004 and 2006, the central Puget Sound population and economy continued to flourish. In 2007, Forbes Magazine ranked Washington state as the 5th best state for business.
2007 Congestion Report WSDOT.pdf2007 Congestion Report WSDOT.pdf

Chicago Freeway Performance Report
This report examines household travel within the northeastern Illinois region and attempts to determine how trip making may have changed since 1990. The main source of data for this analysis is the 2008 Travel Tracker household travel survey which was conducted for the northeastern Illinois region during 2007 and 2008.
Chicago Freeway Performance Report

Performance Measurement for Highway Decision Making
This report documents the work completed for the Strategic Highway Research Program (CHRP 2) Project C02 A Systems-Based Performance Measurement Framework for Highway Capacity Decision Making. The goal of CHRP 2 C02 is to create a state-of-the-art performance measures framework that individual transportation agencies and other public agencies can adapt to support the needs of both agencies and stakeholders in the decision-making process for major transportation capacity projects.
Performance Measurement for Highway Decision Making