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Incorporating Travel-Time Reliability into the Congestion Management Process (CMP): Informational Brief

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United States Department of Transportation Federal Highway Administration

U.S. Department of Transportation
Federal Highway Administration
Office of Operations
1200 New Jersey Avenue, SE
Washington, DC 20590
www.ops.fhwa.dot.gov

Publication #: FHWA-HOP-14-035

February 2015

What is Travel-Time Reliability?

Travel-time reliability is the variation in travel time for the same trip from day to day.

Figure 1. Illustration. Causes of Congestion. This figure is a pie chart that illustrates the various causes of congestion. They are divided up into ones that cause recurring congestion versus those that cause non-recurring congestion. Causes of non-recurring congestion include, special events, work zones, inclement weather, and incidents. Causes of recurring congestion include bottlenecks or capacity constraints. The pie chart indicates that non-recurring congestion account for slightly more overall delay to travelers than recurring congestion.
Source: Federal Highway Administration

Why Does Reliability Matter?

Data collected from across the United States indicate that non-recurring congestion accounts for more delay than recurring congestion due to capacity constraints. Highly variable or unpredictable travel times make it difficult for travelers and shippers to plan trips, leading to ineffective or counterproductive travel decisions that waste time and money.

What Affects Reliability?

Variable travel times tend to originate from disruptions on the transportation system. These disruptions occur as a result of any number or combination of factors such as poor signal timing, inclement weather, crashes, construction work zones, arena concerts, or major sporting events. In turn, they may cause unexpected congestion even in areas that don't normally experience congestion.

Why Should Reliability be Incorporated into CMPs?

The CMP is one of the most effective ways to implement reliability performance measures that more closely address the concerns of the traveling public. Congestion management plans tend to be updated frequently and are focused on monitoring conditions and developing strategies for alleviating congestion. Three major factors are driving the current spotlight on travel-time reliability:

Photographs (Top and Middle) courtesy of Kittelson & Associates, Inc., Photograph (Bottom) courtesy of iStock (B. Escov)

Examples of Practices

The effects of non-recurring congestion on travel-time reliability can be minimized if the causes are identified and plans for addressing the disruptions are developed and implemented. For example, incident management plans typically include protocols for the efficient clearance of roadways or rail lines and for the provision of dynamic messaging signs to inform and advise travelers. A sample of some regions that have begun to address travel-time reliability in their CMPs is highlighted below.

Photograph (Top) courtesy of iStock (R. Yasick), Photograph (Bottom) courtesy of Kittelson & Associates, Inc.

Performance Measures for Travel-Time Reliability

Reliability can be estimated by looking at travel-time variability. If variability is great, the travel time is considered to be unreliable, because it is difficult to generate consistent and accurate estimates for it. If there is little or no variation, the travel time is considered to be reliable. The Planning Time Index (95th percentile compared to free-flow travel time) is a common performance measure, among others, that use indices shown in the Measures of Reliability figure.

The PTI is indentified in The Highway Capacity Manual as one of the most intuitive and consistent measures for reliability. A PTI of 1.5, for example, means that a traveler should plan for 50% more time for their trip compared to light traffic conditions for a 95% probability of arriving on time.

Measures of Reliability

[Figure 3]. Graph. The Travel-Time Distribution and Measures of Reliability. This two-dimensional line graph has number of trips (generic) on the x-axis and travel time (generic) on the y-axis. The line graph shows a sample distribution of this relationship and key travel times along the line graph such as free-flow, mean, 85th percentile, and 95th percentile.
Source: Strategic Highway Research Program, Phase 2

Getting Started

Metropolitan planning organization (MPO) staff who would like to incorporate travel-time reliability into their CMPs might consider the following steps for creating a robust plan:

  1. Update Goals, Objectives, and Performance Measures
  2. Assess Data Needs and Availability
  3. Diagnose Causes of Unreliability for Select Facilities
  4. Evaluate and Prioritize Reliability Strategies
  5. Prepare CMP Update

Resources for MPOs and States

FHWA-sponsored training workshops and the Primer, Incorporating Travel-Time Reliability into the Congestion Management Process, are useful resources that demonstrate how reliability can be integrated into the following CMP actions:

  1. Developing Regional Objectives
  2. Develop Multi-modal Performance Measures
  3. Analyze Congestion Problems and Needs
  4. Identify and Address Strategies
  5. Evaluate Strategy Effectiveness
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