Travel Time Reliability Measures
What is the problem?
Everyday congestion is common in many U.S. cities and most travelers expect and plan for some delay, particularly during peak driving times. Some travelers and shippers adjust their schedules or budget extra time to allow for traffic delays. But what happens when traffic delays are much worse than expected?
It's the unexpected congestion on a day-to-day basis that troubles travelers the most. Travelers want travel time reliability—a consistency or dependability in travel times, as measured from day to day or across different times of day. Travelers want to know that a trip will take a half-hour today, a half-hour tomorrow, and so on.
What is travel time reliability?
In the past, traffic congestion has been communicated only in terms of simple averages. However, most travelers experience and remember something much different than a simple average throughout a year of commutes. Their travel times vary greatly from day to day, and they remember those few bad days they suffered through unexpected delays (Figure 1).
Why is travel time reliability important?
Travel time reliability measures the extent of this unexpected delay. A formal definition for travel time reliability is: the consistency or dependability in travel times, as measured from day-to-day and/or across different times of the day.
Travel time reliability is significant to many transportation system users, whether they are vehicle drivers, transit riders, freight shippers, or even air travelers. Personal and business travelers value reliability because it allows them to make better use of their own time. Shippers and freight carriers require predictable travel times to remain competitive. Reliability is a valuable service that can be provided on privately-financed or privately operated highways. Because reliability is so important for transportation system users, transportation planners and decision-makers should consider travel time reliability a key performance measure.
What should be done?
The Federal Highway Administration is encouraging agencies to adopt travel time reliability measures to better manage and operate their transportation system. Traffic professionals have come to recognize the importance of travel time reliability because it better quantifies the benefits of traffic management and operation activities than simple averages. For example, consider a typical before-and-after study that attempts to quantify the benefits of an incident management or ramp metering program. The improvement in average travel time may appear to be modest (left side of Figure 2). However, reliability measures will show a much greater improvement (right side of Figure 2) because they show the effect of improving the worst few days of unexpected delay.
To promote the adoption and use of travel time reliability measures, several guidance documents have been developed:
- Overview brochure (HTML, PDF 2.9MB) - provides a layperson's introduction.
- Guidance document (HTML) - provides implementation details for those agencies that may be considering adoption of reliability measures.
The FHWA Office of Operations has also sponsored numerous training workshops and provided technical assistance to state and local agencies. If you are interested or have questions about travel time reliability measures, please contact us.
PDF files can be viewed with the Acrobat® Reader®.