The National Academies

NCHRP 03-59 [Completed]

Assessment of Variable Speed Limit Implementation Issues

  Project Data
Funds: $550,000 (NCHRP--$300,000; FHWA--$250,000)
Research Agency: Northwestern University
Principal Investigator: Alexander Weiss
Effective Date: 11/9/2000
Completion Date: 11/9/2005


The objectives of this project were to (1) assess the impacts of and the implementation issues associated with deployment of variable speed limits for a limited number of driving situations and (2) develop operational test plans for the most promising applications.


A pilot application in a Pennsylvania work zone in Fall 2003 showed the difficulties inherent in the implementation of variable speed limits. The contractor has submitted the final report and a research results digest is being developed for publication in 2010. 

Product Availability: 

NCHRP Legal Research Digest 47 is available.


Variable speed limits are intended to allow reasonable and realistic speeds based on time of day, traffic conditions, weather conditions, construction or maintenance activities, and other factors. With the exception of school zones, use of variable speed limits in the United States has been limited, although many transportation agencies have expressed interest in them. Use has probably not been more widespread due to concerns over their legal basis, the level and type of enforcement required, and the lack of information on proven benefits.

Some static speed limits in dynamic environments have low levels of compliance, and speed limits that are responsive to the situation should be more credible and may result in improved compliance. In other situations, the speed limit may be too high for the conditions, and a variable speed limit could provide additional information that may be beneficial to the driver.


The research consisted of the following tasks:

Task 1.  Analyze, describe, and critique pertinent domestic and international literature and practice that can be used to assess the effectiveness of variable speed limits.

Task 2.  Recommend a limited number of driving situations that would potentially benefit from variable speed limits (e.g., work zones, adverse weather, traffic congestion).

Task 3.  For the driving situations identified in Task 2, identify and describe alternative approaches for implementing and enforcing variable speed limits. For implementation approaches, the description should include the method of determining the speed displayed, the types of signs or other traffic control devices used to notify the driver of the speed limit, the location and frequency of these traffic control devices, and how fail-safe operation will be ensured. For enforcement approaches, the description should include the level of effort for enforcement officers and other personnel, technology (including automated enforcement) used to support speed enforcement, the means of documenting the speed limit in effect at the time of a violation, and the method for issuing and processing citations. For each of the approaches, assess the life-cycle costs, safety and operational benefits, and obstacles to implementation. Recommend approaches for study in Tasks 5 and 6 that are practical and likely to improve speed limit compliance for each of the Task 2 driving situations.

Task 4.  Submit an interim report documenting Tasks 1 through 3 and presenting a detailed work plan for Tasks 5, 6, and 9. The interim report shall also include an outline of the project final report.

Task 5.  Conduct a detailed analysis of implementation issues for the recommended implementation and enforcement approaches. Describe how they could best be addressed. Some implementation issues that should be considered are political, judicial, enforcement, and public support; development, implementation, operating costs, and resource requirements; crashworthiness of devices in the right-of-way; compatibility with the local intelligent transportation system architecture; uniformity of application; compliance with or needed changes to the MUTCD; advisory or regulatory nature of the limits; applicability for minimum as well as maximum limits; and applicability to different vehicles and roadway geometries. It is not necessary to assess legal and judicial issues (beyond the need for legislators and judges to understand and support variable speed limits), as they are being studied in a separate project being conducted as part of
NCHRP Project 20-6.

Task 6.  For each driving situation, recommend combinations of compatible implementation and enforcement approaches. For each of these combinations, analyze and estimate the impacts on safety, speed, speed variance, speed limit compliance, and capacity. (7) Develop operational test plans for the most promising driving situations and approaches. Details should include the type and geometry of road, length of road, number of sites to be tested, environmental conditions, traffic conditions, system specifications, functional requirements, criteria for speed selection, measures of effectiveness, evaluation plan, and budgeting information.

Task 8.  Prepare a final report documenting the entire research effort and summarizing the NCHRP 20-6 legal review. The report should include an executive summary targeting elected officials and the public.

Task 9.  In order to help identify implementation issues and benefits, develop, test, and evaluate a work zone variable speed limit application. The contractor will be principally responsible for finding a partner transportation agency, designing the system in cooperation with the agency, assisting with necessary approvals, and collecting and analyzing the data. The results of this task should be summarized in a research results digest. This task should begin as soon as is practical after acceptance of the Task 4 interim report.

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