Appropriately set and enforced speed limits are essential for managing speeds and improving highway safety. In a recent study of current practice for setting speed limits, a TRB special committee concluded that setting speed limits primarily on the basis of the 85th-percentile speed may not be appropriate on roads in built-up areas with a mix of road users and high traffic volumes and roadside activity. There are many factors bearing on reasonable and safe speed, and there is little agreement about their relative importance among residents, drivers, public officials, enforcement officers, and engineers. This absence of consensus has contributed to unrealistic and inconsistent speed limits. TRB Special Report 254: Managing Speed: Review of Current Practices for Setting and Enforcing Speed Limits recommends that an expert system approach be developed to advise and assist practitioners in setting speed limits in speed zones.

The expert system developed in this project will be used by traffic engineers to analyze specific segments of all types of roads, from rural local roads to urban freeways (statutory limits that apply to a category of roads within a jurisdiction are not included). Part time and temporary speed limits (e.g., school zones, work zones) are outside the scope of the project. The speeds recommended by the system should enhance safety and efficiency. The system's rationale for the recommended speed must enhance the engineer's ability to justify it to the body responsible for setting speed limits and the public.

Enforcement is crucial to the success of speed limits. If law enforcement officers and the courts are confident that the system and its outputs are reasonable, their enforcement of the limit will be more effective. It is expected that use of the system will reduce the number of unrealistically low speed limits that draw on enforcement resources but do not commensurately improve safety and may, in fact, degrade safety by widening the distribution of speeds. Use of the system will allow enforcement agencies to better target the truly egregious drivers that pose a greater risk.

This project was intended to develop a new knowledge-based expert system for recommending enforceable, credible speed limits in speed zones.   The project scope entailed the following tasks. (1) Review current literature on guidelines, criteria, and procedures used for setting speed limits in speed zones in the United States. Review experience with XLIMITS, USLIMITS, and other existing speed-limit expert systems and assess their applicability to this project. (2) Establish an appropriate group of subject-matter experts on setting and enforcing speed limits. (3) Identify user needs and establish user requirements for the expert system. These should include, but not be limited to, computer hardware and software requirements, interface requirements (including accessibility), system output requirements, and appropriate method(s) for distributing the system. (4) Identify appropriate variables and factors and document criteria used in determining speed limits. Develop critical decision rules and logic flow for the expert system, including how missing data will be handled. (5) Identify and describe promising long-term management alternatives for administering and maintaining the expert system after the project is complete, including the benefits and drawbacks of each. (6) Submit an interim report documenting Tasks 1 through 5. The report will include an updated, detailed work plan with milestones for the remaining tasks, a software life-cycle graphic, and information on the development tool that will be used in Task 7. (7) Build the working expert system. The system should be user-friendly and include context-sensitive help. Output of the system will include the recommended speed limit and the basis for the recommendation. (8) Verify and validate the system in an independent manner. Evaluate the system, including beta-testing by the project panel and a cross-section of practitioners. (9) Modify the expert system and deliver it in a form suitable for implementation. The final report shall be readily accessible from the system. (10) Submit a final report documenting the entire research effort, including a summary of the logic of the expert system.

Product Availability: NCHRP Research Results Digest 318 is available.  The researchers' final report, including a users' guide for the expert system web application, is available here.  Installation files for the web application are available here.