Web-based Tool Helps Set Safe and Efficient Speed Limits

By Martin Parker, PE

USLIMITS2 is a free tool that recommends appropriate speed limits by using expert advice and artificial intelligence to analyze a variety of factors.

Agencies and officials can get free expert advice over the Web the next time they set the legal speed limit for a roadway. The National Cooperative Highway Research Program (NCHRP) completed a second generation, knowledge-based, expert system for determining the appropriate speed limit for a variety of roadway types nationwide. The system, called USLIMITS2, replicates the decision-making process of seasoned traffic experts and recommends a speed limit for a section of road based on road function, crash frequency, operating speeds, road characteristics and several other factors. By considering all the variables that impact speed zoning decisions, the expert system offers consistency in setting safe and operationally efficient limits. Three years in development, the knowledge base for USLIMITS2 included a review and analysis of published research and operational studies. The literature was summarized in an interim report that highlighted the impacts of speed limit changes, factors that affect speed zoning decisions, and current methods used to set speed limits. In addition, speed zoning experts were selected from a nationwide pool and gathered in Washington DC. Expert panel members participated in workshops, information exchange sessions and case study evaluations to define the decision process they use to set maximum speed limits for specific road conditions. The decision process was used by computer programmers to build the expert system’s logic. The prototype developed underwent verification, validation and evaluation to ensure the expert system replicated the decisions of human experts accurately. After beta testing, the prototype was refined and implemented.

Teamed with the University of North Carolina, Wade Trim assisted with a review of relevant literature, recruited expert panel members, identified potential critical variables for setting speed limits and led the development and presentation of case studies to the expert panel.

The NCHRP has made the expert system publicly accessible for free on the Web at www2.uslimits.org. After setting up an account, users can review and edit previous results for a roadway or determine the speed limit for a roadway. Screen results can be printed, exported to Microsoft Word or Excel, or saved for viewing and editing. The system also warns users of issues that may require additional information or action. For example, if crash data was not entered for the roadway, the system emphasizes the importance of crash data when setting speed limits and suggests the process be repeated after crash data is available.

Like other expert systems, USLIMITS2 uses artificial intelligence to replicate human intelligence through the use of computer programs. The system is intended to assist users in making speed limit decisions; it does not make the decision for them. The system will assist small communities and agencies that lack experienced traffic engineers. For experienced traffic engineers, the system can provide a second opinion and increase their confidence in speed zoning decisions.

The expert system has limitations. It does not address statutory limits such as maximum limits set by State legislatures for Interstates and other roadways, temporary or part-time limits such as in work and school zones, and variable limits that are raised or lowered based on traffic, weather, and other conditions. The expert system is also limited in its use of artificial intelligence since it was not programmed to learn from past experience. It is anticipated that future enhancements will enable the system to learn from previous experiences and to offer new solutions, just as human experts do.

Martin Parker, Jr., PE, was the Senior Transportation Engineer for development of the USLIMITS2 Expert System. Since completion, he has presented its capabilities to various groups including the Institute of Transportation Engineers, Michigan Section. Martin provides traffic engineering and safety management services to governmental agencies and private sector clients. He can be reached in our Taylor, MI, office at 800.482.2864 or mparker@wadetrim.com.

Originally published in Innerview Vol. 21, No. 1, 2007

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