The National Academies

NCHRP 03-83 [Final]

Low-Cost Improvements for Recurring Freeway Bottlenecks

  Project Data
Funds: $260,000
Research Agency: Dowling Associates
Principal Investigator: Alexander Skabardonis
Effective Date: 8/10/2005
Completion Date: 11/30/2012


The objective of this project was to develop a technical guide for identifying existing and future recurring freeway bottlenecks and determining appropriate low-cost geometric and operational improvements to mitigate them.


The contractor's final deliverables are available below.


A freeway bottleneck is the critical point of congestion with queues upstream and freely flowing traffic downstream. Recurring bottlenecks (those not caused by atypical conditions such as incidents) can occur for many reasons, including high volumes of entering and merging traffic, lane drops between ramps or at off ramps, heavy weaving sections, horizontal or vertical curves, and long upgrades. Proper identification of freeway bottlenecks and their causes is the key to formulating plans for reducing congestion.

Major construction projects often address bottlenecks but these types of projects are expensive and take several years to plan, design, and construct. Relatively low-cost geometric and operational improvements (e.g., auxiliary, shoulder, narrow, high-occupancy vehicle, reversible, and contraflow lane designs; ramp metering; truck restrictions) can often mitigate the effects of a bottleneck. The benefits of a low-cost improvement may not be as extensive or long-lasting as those of a major reconstruction project, but the improved system performance can easily justify its use.

Determining the best improvement for a particular bottleneck can be difficult. The freeway congestion due to a bottleneck can spread several miles upstream and impact the arterial street system. Improving a bottleneck may result in the congestion moving downstream to a new bottleneck that was not apparent previously, greatly reducing the expected benefits. Analysis of the entire network is necessary to accurately estimate the benefits and effects of different improvements.



Task 1. Review domestic and international literature on processes and analysis tools for identifying recurrent freeway bottlenecks and for determining appropriate low-cost geometric and operational improvements. Collect consultant and agency in-house studies of freeway bottlenecks and review them to identify best practices.

Task 2. Identify state departments of transportation and metropolitan planning organizations that have established procedures for identifying recurrent freeway bottlenecks and determining low-cost improvements. Interview four to six agencies with promising procedures to identify strengths and weaknesses of their approach.

Task 3. Develop a working definition of freeway bottlenecks and describe their principal causes. Characterize types of bottlenecks and low-cost geometric and operational improvements suitable for each type. Discuss how bottlenecks and low-cost improvements affect the operation of the freeway and arterial network.

Task 4. Develop a systematic framework for identifying bottlenecks and determining appropriate low-cost improvements. The framework should include feedback loops, if appropriate. Assess institutional issues that may arise from implementation of the framework.

Task 5. Within 5 months of the contract start, submit an interim report on the information developed in Tasks 1 through 4. The report should include a detailed outline of the guide that will be developed in Task 8 and identify case studies from Tasks 1 and 2 for the guide that demonstrate the effectiveness of representative low-cost improvements or highlight successful practices.


Task 6. Based on the Task 4 framework, develop and document a systematic process for identifying recurrent bottlenecks and determining appropriate low-cost geometric and operational improvements. The process description should show how it fits within a typical planning and project scoping process and identify candidate analysis tools and data needs for each step of the process. The process should be capable of using real-time and projected data.

Task 7. For the primary analysis tools identified in Task 6, assess their suitability for identifying recurrent bottlenecks, estimating their network-wide impacts, and projecting the benefits and service life of potential improvements. The assessment should include the resources, level of effort, and data required to use the tools.

Task 8. Develop a technical guide that presents the process, illustrates its use through case studies, and aids the selection of analysis tools. The guide should discuss the composition of the project development team, the proper documentation of design exceptions, and other institutional issues associated with implementing the process.

Task 9. Submit a final report that documents the entire research effort and includes the Task 8 guide as a stand-alone document.

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