The idea of controlling vehicle speed evolved from the assumption that reducing speed also reduces crashes. Speed limits are selected to balance travel efficiency versus safety. It can be argued that a rational speed limit is one that is safe, that most people consider appropriate, that will protect the public, and that can be enforced. Previous work has shown that better methods are needed to identify appropriate speed limits especially on urban roads having higher traffic volumes, a mix of road users, and more roadside activity. The objective of this project was to develop a knowledge-based expert system for recommending speed limits in speed zones that are considered to be credible and enforceable. The expert system (hereafter referred to as USLIMITS2) was developed based on results from previous research, responses from practitioners to hypothetical case studies as part of two web-based surveys, input from experts from three panel meetings, and lessons learned from the current USLIMITS program developed by the Australian Road Research Board for FHWA.

The expert system developed as a result of this research is accessed through the Internet and has been designed to address the establishment of speed limits in speed zones on all types of roadways, from rural two-lane segments to urban freeway segments. The types of speed limits not addressed by the system include statutory limits such as maximum limits set by State legislatures for Interstates and other roadways, temporary or part-time speed limits such as limits posted in work zones and school zones, and variable speed limits that are raised or lowered based on traffic, weather, and other conditions. Based on input from the user, the expert system employs a decision algorithm to advise the user of the speed limit for the specific road section of interest. Appropriate warnings are also provided in a summary report that may suggest that additional information and/or action is necessary to address areas of concern. The system is meant to assist the user in making the speed limit decision for a road segment, but will not make the decision for him or her. This report documents the research effort that was used to develop the expert system. For those primarily interested in applying the expert system to assist in setting speed limits on roads and streets in their area, a User Guide is provided in Appendix L, and flow charts illustrating the decision rules are provided in Appendix K.