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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.
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. He can be
reached in our Taylor, MI, office at 800.482.2864 or mparker@wadetrim.com.
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