Overview
The purpose of this guide is to provide traffic engineers, highway designers, transportation planners, and decision makers with a catalog of highway design treatments that can be used to reduce nonrecurrent congestion and improve the reliability of urban and rural freeways. Nonrecurrent congestion is congestion outside of what is normally expected during daily traffic peaks. It can be caused by crashes, noncrash incidents, special events, weather, work zones, demand surges, or traffic control devices. Roadways that experience several of these events often have low reliability, and the drivers who use the facility have difficulty accurately estimating the amount of time their trip will take. The concepts of nonrecurrent congestion and reliability are discussed in Chapter 1 of this guide.
Although many agencies are familiar with the potential benefits of deploying realtime traveler information systems and roadside assistance programs to reduce the impacts of incidents, special events, and inclement weather, little consideration has been given to how the design of the roadway and roadside can play an important role in mitigating the congestion caused by these events. This guide considers the impact of both standard features (such as the provision of full-depth shoulders) and innovative features (such as queue-jump lanes) and explores the benefits they may provide during nonrecurrent congestion. Some treatments are used primarily to address nonrecurrent congestion, but others may be implemented for another reason, such as increased capacity during periods of nonrecurring congestion, and may have additional benefits during nonrecurrent congestion events. The full range of treatments included in this guide is presented in Chapter 2.
Chapter 2 also provides a process for narrowing the full list of treatments to those that may provide the most benefit in various situations. A decision tree is presented that walks the reader through the process of considering the main cause of nonrecurrent congestion and the means by which that nonrecurrent congestion might be mitigated. Ideally, the reader will gain a handful of treatments from Chapter 2 that will be investigated further as candidates for possible implementation.
Chapter 3 of the guide catalogs the treatments and provides a range of information on each one, including the following:
- Description and primary objective of the treatment
- Typical applications of the treatment
- Design criteria or guidelines, if available, in design manuals or from research
- How the treatment can be used to address nonrecurrent congestion
- Factors that affect the treatment’s effectiveness
- Cost considerations
- Resources for additional information
The reader may choose to browse through this catalog to learn more about several treatments or go directly to the descriptions of the treatments identified in Chapter 3 as potential candidates for implementation.
Chapter 3 provides information on how each treatment can be used to reduce nonrecurrent congestion and the factors that may make it more or less successful in doing so, but it does not provide guidance on quantifying the benefit of the treatment in terms of nonrecurrent congestion reduced or reliability improved. However, these benefits are real, and they can be monetized to allow for a benefit–cost calculation to be performed. To assist the reader in carrying the treatments identified in this guide through the next step of consideration for implementation, an Analysis Tool has been developed as a sister product to this guide. The tool allows the user to compare the benefit–cost ratio of several treatments, providing valuable information to design decision makers. The final report documenting the research project that developed these products provides detailed information on the principles and calculations that drive the Analysis Tool. These products are available on the SHRP 2 website.
Chapter 4 of this guide is a catalog of secondary treatments that complements the treatments in Chapter 3. Finally, Chapter 5 provides information about existing installations of several of the treatments included here, allowing the reader to identify agencies that have experience with various treatments and that may be able to provide insight and guidance to other agencies considering the treatment.
This guide can be used as a resource during all stages of the design process, but it will perhaps be the most beneficial before or during early project planning phases. Although some of the treatments presented can be easily retrofitted into existing facilities, others will be most cost-effective when incorporated into a planned construction or maintenance project. In addition, some treatments are most effective at reducing nonrecurrent congestion when incorporated with specific geometric or operational elements, and coordination of all aspects of the project in the early phases can help maximize benefits. Once potential treatments are identified, and their benefits to nonrecurrent congestion are understood, the Analysis Tool can be used to quantify those benefits. A benefit–cost analysis that captures delay reduction and reliability benefits can help justify the implementation of treatments that may otherwise have not been considered for a project.