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1、Research Results Digest 318 May 2007 SUMMARY This digest describes research conducted to develop a knowledge-based expert system decision-support tool for recommending speed limits in speed zones on highways and local roads that are considered credible and enforceable. The tool is intended to assist
2、 responsible authorities in setting speed- zone limits to enhance traffic safety and op- erating efficiency. The system has been designed to be useful for all types of primary roadways, from rural two-lane segments to urban freeway segments. The system does not address statutory limits such as maxi-
3、 mum limits set by legislatures for Inter- states and other major classes of roadways, temporary or part-time speed limits such as those posted in work zones and school zones, or variable speed limits that change as a function of traffic, weather, and other conditions. The expert system is designed
4、to be implemented as a web-based software application. The digest is based primarily on the fi nal report for NCHRP Project 3-67, “Ex- pert System for Recommending Speed Limits in Speed Zones” (available from the project description page of the TRB website: http:/www.trb.org/TRBNet/Proj ectDisplay.a
5、sp?ProjectID=821). The project reviewed current literature on guidelines, criteria, and procedures used for setting speed limits in speed zones in the United States and experience with use of XLIMITS, USLIMITS, and other existing speed-limit expert systems. A group of subject-matter experts engaged
6、in setting and enforcing speed limits was convened to provide un- derlying decision rules for the expert system. The software application was developed with consideration of user needs and re- quirements for long-term management and maintenance of the expert system. (The application can be accessed
7、through the Internet at http:/www2.uslimits.org and is available for download and installation on an Internet server from the TRB website at http:/www.trb.org/news/blurb_detail.asp? id=7568.) This digest is organized into three sec- tions and an appendix. The first section describes the motivation f
8、or the research and the scope of NCHRP Project 3-67. The second section describes the decision rules embedded in the expert system and how AN EXPERT SYSTEM FOR RECOMMENDING SPEED LIMITS IN SPEED ZONES This digest presents the results of NCHRP Project 3-67, “Expert System for Recommending Speed Limit
9、s in Speed Zones.” The study was conducted by a team led by the University of North Carolina Highway Safety Research Center with Wade Trim Associates, Inc. and PB Farradyne, Inc. Raghavan Srinivasan, Senior Transportation Research Engineer at the Highway Safety Research Center, was the Principal Inv
10、estigator. Subject Area: IVA Highway Operations, Capacity, and Traffic ControlResponsible Senior Program Officer: Andrew C. Lemer NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM C O N T E N T S Summary, 1 Research Scope and Motivation, 2 Expert System Decision Rules and Their Derivation, 4 The Softwar
11、e Application and Its Use, 5 Appendix: Expert System Decision Rules and Logic for USLIMITS2, 6 these rules were derived. The third section de- scribes the software application and its use. The appendix contains diagrams of the decision rules and logic of their application. RESEARCH SCOPE AND MOTIVAT
12、ION The control of traffic speed is generally acknowl- edged to be a significant factor influencing road safety and operating efficiency. The setting of speed limits and their role in controlling traffic speed are nevertheless controversial. Engineers, public safety officials, and others involved in
13、 setting and enforc- ing speed limits may disagree on the appropriate bal- ance between safety and road-user convenience that should prevail on particular road segments, consid- ering conditions of topography, weather, adjacent activities, and traffic. Motorists, other road users, and roadway neighb
14、ors have their own perspectives on this balance and may or may not abide by the profes- sionals judgments. The inherent complexity of fac- tors infl uencing traffic behavior and crash experience as well as the difficulties of observing and measur- ing these factors make it difficult to draw definiti
15、ve and generalizable conclusions from analysis of ob- servations at specifi c locations. Most jurisdictions have adopted laws defining absolute speed limits; traveling at a speed above the absolute limit is by defi nition illegal and presumably imprudent and unreasonable. Responsible authori- ties a
16、re empowered to lower or raise speed limits on a particular road segment if they judge that these al- tered limits are reasonable and safe under conditions found to exist at that location. Engineering and traf- fi c studies typically provide the basis for making such speed-zone judgments. These stud
17、ies generally con- sider such factors as the physical features of the roadway, crash experience, traffic characteristics and control (for example, signals and other control de- vices), and the length of the roadway segment under consideration (speed-limit changes should not be too frequent or applie
18、d to very short road segments). One of the factors that many experts consider very important for setting a speed limit is the prevailing vehicle speed. The Manual on Uniform Traffic Con- trol Devices (MUTCD) is quite explicit, stating that “when a speed limit is to be posted, it should be within 10
19、km/h or 5 mph of the 85th percentile speed of free-fl owing traffic” (MUTCD 2003). However, the manual also indicates other factors that may also infl uence the appropriate speed limit, including road- way characteristics such as shoulder condition, grade alignment, and sight distance; pace speed; r
20、oadside development and environment; parking practices and pedestrian activity; and reported crash experience for at least a 12-month period. Neither the MUTCD nor other sources offer spe- cific guidance and procedures for setting limits in speed zones, requiring engineers or other officials to rely
21、 on their experience and judgment in weighing various factors to decide on appropriate speed limits. The situation can sometimes result in inconsistencies in how speed limits are set from one jurisdiction to another and can be confusing to drivers. If the regu- lation of speed is to be effective, th
22、ese experts sug- gest, the posted limit must be generally consistent with speeds that drivers feel are safe and proper. While too high a speed limit can reasonably be an- ticipated to increase the likelihood and severity of crashes, inappropriately low speed limits can lead to poor compliance and la
23、rge variations in speed within the traffic stream, also increasing crash risk. Enforcement is widely recognized to be crucial to the success of speed limits as a means for making roads safer. If law enforcement officers and the courts are confi dent that speed limits have been developed on areasonab
24、le basis, their enforcement of the limits will be more effective. Some professionals have ar- gued that speed limits should be set generally at levels that are self-enforcing so that law enforcement officials can concentrate their efforts on the worst offenders. In 1998, TRB published TRB Special Re
25、port 254 (Managing Speed: Review of Current Practices for Setting and Enforcing Speed Limits), which reported the results of a studyrequested and funded by NHTSA, FHWA, and the Centers for Disease Con- trol and Preventionthat reviewed current practice for setting and enforcing speed limits on all ty
26、pes of roads. TRB Special Report 254 notes that the prac- tice of setting speed limits at the 85th percentile or some other statistic derived from prevailing traffic reflects an assumption that most drivers are capa- ble of judging the speed at which they can safely travel. However, this assumption
27、raises the ques- tion of why speed limits are then necessary at all. The report suggests at least three reasons: Drivers impose signifi cant risks on others. For example, a driver with poor understanding of 2 risk or higher tolerance for risk may decide to drive faster than might be considered by ot
28、h- ers to be appropriate for roadway conditions. The higher speed increases the probability of a crash involving property damage and possi- bly injury or death for the driver and others. Even if the speeding driver is traveling alone and is involved in a single-vehicle crash, the medical and propert
29、y damage costs typically are not fully paid by the driver, but are distrib- uted among other insured drivers, government agencies, and a larger community. Some drivers are unable to judge correctly the capabilities of their vehicles (e.g., stopping dis- tances) and to anticipate road conditions suff
30、i- ciently to determine appropriate driving speeds. Inexperienced drivers or experienced drivers operating in unfamiliar surroundings may be more susceptible to this problem, underesti- mating risk and making inappropriate speed choices. Drivers may underestimate the effects of speed on crash probab
31、ility and severity, even if they understand in principle the risk of a crash. Young and inexperienced drivers are most often prone to such misjudgments. The speed limits set in speed zones reflect a balance of several considerations specific to the roadway segment to be regulated. Traffic engineers
32、normally conduct an engineering study of the seg- ment and apply principles of current engineering practice to recommend what the speed limit should be. Law enforcement officials may be involved in the development of a recommendation, taking into consideration the issues likely to be associated with
33、 enforcement of the limit; TRB Special Report 254 suggests that such involvement should be standard practice. Citizens groups and elected officials also may be involved when community concerns about traffic speed have been raised. The resulting limit set in a speed zone is then often the product of
34、com- plex negotiation and may differ from the speed that any single group or individual might have consid- ered most reasonable. A tool that could be used by traffic engineers, en- forcement officials, and others to set speed-zone lim- its, embodying best current practices and taking into account th
35、e myriad factors infl uencing determination of an appropriate limit, could be quite valuable. Such a tool could enhance the consistency and quality of both the process for setting limits on a particular road segment and the result, thereby improving road safety. By increasing users confi dence that
36、speed limits are being reasonably determined, such a tool could also improve the effectiveness of enforcement efforts and further improve safety. Such reasoning led several states in Australia to undertake development of such a tool. The decisions and judgments made to set a speed-zone limit were th
37、ought to be particularly well suited to an expert sys- tem approach. An expert system is a computational al- gorithm, generally computerized, that seeks to mimic the thought process of an expert. Such systems are based on rules and representative judgments derived from knowledgeable people, the “exp
38、erts.” Such a system for providing advice on speed limits for speed zones was developed in the 1980s by the Australian Road Research Board (ARRB) for the state of Victoria. That initial tool was developed further for use in other Australian states and New Zealand. The original system, VLIMITS, was a
39、 DOS- based program that prompted users to respond to a series of questions about the road segment and then recommended a speed limit. The systems logic was “hard coded,” meaning that the system did not learn with experience, as some expert systems do. The most recent versions of the program use a m
40、ultistep process to recommend a speed limit with warnings about specifi c factors that should be studied further before the limit is imposed. The ARRB, under contract to FHWA, adapted the logic of these earlier programs to develop USLIMITS specifi cally for application in the United States. One of t
41、he changes made to suit conditions in this coun- try was to force the recommended speed limits to be within the 50th- to 85th-percentile range. The system is considered proprietary, and its logic and decision rules are not available to the user. Hence, users can- not be certain which variables influ
42、ence the final recommendation or nature of that infl uence. Although many USLIMITS users surveyed as part of the NCHRP research thought that the speed limits rec- ommended by the system are reasonable, they also thought that more information should be made avail- able regarding the decision rules an
43、d the factors used or not used in developing the fi nal recommendation. Also, USLIMITS is available through the Internet (), but a username and password are required. 3 NCHRP Project 3-67 was designed to produce an expert system to succeed USLIMITS. The new system would be made available with comple
44、te information about the systems logic and factors in- fl uencing speed-limit recommendations. That new system, the product of this research, is USLIMITS2. The following sections of this digest describe the decision rules embodied in the expert system and their bases, and what hardware and software
45、are required to use USLIMITS2. More complete de- scriptions of the application and its development, including a users guide for the expert system, are contained in the final project report (available from the project description page of the TRB web- site: http:/www.trb.org/TRBNet/ProjectDisplay. asp
46、?ProjectID=821). EXPERT SYSTEM DECISION RULES AND THEIR DERIVATION The core of USLIMITS2 is a set of decision rules developed with the help of two selected groups of ex- perts: an expert panel that participated in meetings and conferences and a larger expanded panel that re- sponded to questionnaire
47、s and surveys. These groups included traffic engineers; law enforcement officials; other road safety professionals; and other experienced officials familiar with the setting, enforcement, and adjudication of speed limits for speed zones. Alto- gether, 17 individuals participated in the expert panel,
48、 but the decision rules for each of the several cate- gories of roadway in the expert system are based on judgments made by 7 to 12 individuals. The expanded panel included a total of 148 individuals. Members of the expert panels participated in several meetings and teleconferences to discuss what f
49、actors are important in setting speed-zone limits and to consider how these factors would in- fluence their judgments of the appropriate speed limit for particular road segments. In the fi rst meet- ing, the expert panel members were invited to re- view a set of photographs and accompanying text and statistics (for example, operating speeds and crash experience), referred to by the researchers as “case studies” and “scenarios.” The combined set of case studies and scenarios was selected by the re- search team to compose a standardized sample of typical situatio