Traffic Congestion and Reliability:
Trends and Advanced Strategies for Congestion Mitigation
This report is the second in an annual series developed by the FHWA's Office of Operations. This series is meant to highlight recent trends in congestion across the Nation and to highlight the activities that Federal, state, and local transportation agencies have initiated to control congestion. The series pays particular attention to the concept of travel time reliability—how consistent travel conditions are from day-to-day—and strategies aimed at improving reliability.
Mitigating congestion is a high priority for the FHWA, which has established congestion mitigation as a key focus area.4 This report supports this effort by providing a review of congestion issues and solutions in the United States. The emphasis of the report is on measuring trends in travel time reliability and making travel more reliable through Transportation System Management and Operations (TSM&O) initiatives. The topic of congestion is clearly much broader than this focus. While the broader context of congestion is discussed, the report spends most its effort on defining and measuring travel time reliability, and highlighting TSM&O strategies to address it.
One of the key principles that the FHWA has promoted in congestion measurement is that the metrics used to track congestion should be based on the travel time experienced by users of the highway system. While the transportation profession has used many other types of metrics to measure congestion (such as "level of service"), travel time is a more direct measure of how congestion affects users. Travel time is understood by a wide variety of audiences—both technical and nontechnical—as a way to describe the performance of the highway system. All of the congestion metrics used in the report are based on this concept.
This year's report covers several topics, many of which are recurring themes in the series. However, when the same basic topics as previous years' reports are covered, new information that has come to light is used. The topics covered in this report are:
- The characteristics of congestion and travel reliability. Congestion results from the interplay of traffic demand, physical and operational characteristics of highways, and traffic-influencing "events" (such as traffic incidents and bad weather). These ingredients are present in different proportions on different highways. Understanding how they contribute to total congestion is the starting point for crafting congestion strategies. Current efforts to measure congestion are also highlighted.
- The significance of reliability to travelers. One of the consequences of the interplay of traffic, highway characteristics, and events is that congestion does not occur in the same way every day. Because traffic demand and events are variable—and can show up at different points on the highway system—congestion also varies from day-to-day. This variability—or "unreliability"—in travel conditions impacts highway users, especially those who must plan around a tight schedule.
- Recent trends in congestion, especially reliability. National estimates of congestion trends are reviewed and discussed. These estimates are derived using models that calculate congestion statistics from basic traffic and highway characteristics. On a national basis, we do not yet have a system in place to directly measure congestion everywhere it needs to be monitored. However, data that allow for direct and continuous monitoring of travel conditions are becoming more widespread in metropolitan areas. Sufficient history now exists in several urban areas to allow trend analysis for the last five years.
- What works in dealing with congestion, particularly approaches that combine several strategies. Several transportation agencies from around the country have been successful in applying congestion treatments. These success stories are highlighted as a way of showing what aggressive application can do against congestion. Moreover, past experience has taught us that while single strategies can target specific pieces of congestion, combining multiple strategies into "congestion packages" is more effective. The report examines areas where the "package" approach has successfully been used.
- New tools and initiatives for dealing with congestion. Information and vehicle technologies are becoming more sophisticated and more available to the average consumer. These offer the promise of greatly improving how we manage our transportation system for both congestion and safety. The report discusses the advances in technology and what FHWA is doing to promote their deployment.