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29 It has also been recommended that these changes should CONCLUDING REMARKS be in the MUTCD as recommended because not all engineers and other persons determining which TCDs to use at a hori- Initially, the outcomes of this project had been envisioned zontal curve necessarily have ready and immediate access to to include a comprehensive set of guidelines for using various other references such as the TCD Handbook. Nonetheless, the TCDs, including advisory speed plaques, for changes in hor- expanded list of factors to be included should also be included izontal alignment on different components of the road system. in the TCD Handbook along with a recommended procedure Over time, the emphasis changed to two-lane, two-way rural for collecting the various data. roads. Moreover, as the literature was reviewed and practi- tioners consulted, it became clear that it would be impossible with limited resources to develop comprehensive guidelines FOURTH RECOMMENDATION for use of different TCDs in the myriad combinations of cir- cumstances that exist in the field, for example: Under what Initial Proposed Statement explicit circumstances should chevrons be used? What would the expected result be? Finally, most practitioners felt that The initial proposed statement is as follows: existing guidelines and engineering judgment were adequate NCHRP should consider funding a project to incorporate these for determining when different devices should be used. factors in an expert system similar to the U.S. Limits system An interesting conundrum was presented by the responses being developed to provide guidance on the speed limit to be from road users, from practitioners, and even from the NCHRP posted in speed zones. panel. While there is general agreement that improving con- sistency in the application of the various TCDs available to The response of the practitioners to the proposed expert provide information and guidance to drivers encountering a system was not as positive as had been expected. However, change in horizontal alignment is desirable, there is significant based on some respondent comments, at least some of the reluctance to support language or guidelines that would result opposition can be attributed to the respondents not knowing in this improved consistency. Moreover, as just noted, it is not enough about such a system. Moreover, it is neither intended clear that specific guidelines applicable to each of the myriad that development of such a system nor use of it be linked combinations of factors related to horizontal curves could ever directly to the MUTCD. A problem for which there is variation be realized. among the opinions of different professionals and for which The MUTCD already contains language recognizing that there is no precise set of rules or algorithm that can be used there may be an exception to any standard, guidance, or option to produce a "right" answer is a problem for which an expert when engineering judgment determines that the situation can system is expected to have application. be made safer or can operate more efficiently if an exception There is also value in linking development of an expert is used (i.e., §1A.09 in the 2003 edition). However, the for- system to outcomes and experience gained through implemen- mulation of standards and guidance can be effective tools tation of the third recommendation--that is, if studies for in achieving uniform application of TCDs. Since horizontal horizontal curves become more the norm, valuable information curves are so numerous and varied, some guidance can none- should be obtained by practitioners that ultimately assists them theless be an effective means of reducing the inconsistent in making more informed decisions regarding TCD treatments use of these devices, an inconsistency that is recognized by and specifying speed advisories for those locations. This the engineering community. experience will also be helpful in developing and refining If more explicit standards or guidance are not acceptable expert systems. to many practitioners and/or if it is impossible to develop an exhaustive set of guidelines, the alternative should be to Final Proposed Statement improve consistency in the exercise of engineering judgment by identifying the factors that should be considered in deter- The recommendation for development of such an expert mining the appropriate treatment. If all engineers consider the system is still advanced although the proposed statement is same factors in making a decision, there is an increased prob- modified: ability that they will reach the same decision for a given set of conditions, thus leading to greater uniformity. Conversely, NCHRP should consider funding a project to develop an expert uniformity will not be achieved by allowing engineers and system that would incorporate the factors listed in the third recommendation and that would be used for guidance for non-engineers to continue to select the TCDs to be used on traffic control device deployments (including advisory speeds) horizontal curves without guidance or even the identification for horizontal curves. Information and experience obtained of factors that should be used in making these decisions. as a result of the implementation of the third recommendation Thus, the suggested changes in guidelines became more should also be incorporated into the system. This system should be similar in concept to the U.S. Limits system being general, and even those were subject to debate. For example, developed to provide guidance on the speed limit to be posted while something as straightforward as changing from "may" in speed zones. to "should" for the use of warning signs was supported by a
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30 significant majority of practitioners, there was still substantial The recommendations for additional research come in two opposition. It is unlikely that widespread support for more areas. The first area is the need for the investigation of devel- explicit sign-specific guidelines would be well received. Thus, opment of an expert system. Because of the combinations of the suggested changes to the MUTCD were modest in some conditions encountered in the field, an expert system would respects. However, they were all directed to providing more seem to represent a viable tool to be used by engineers in consistent information to the motorist. Nonetheless, from other assessing the need for different TCDs at specific locations. perspectives even these changes were cause for consternation. Used properly, it would help engineers identify the options that Finally, there was a comment regarding whether the existing "best practices" might indicate for TCDs at an explicit site. arsenal of TCDs available for horizontal curves is "working." The second area is further research into motorist reaction The United States has a crash rate that is among the lowest to wholesale changes in determining the appropriateness of advisory speeds using a ball-bank reading of 16°. Based on in the world, and the crash rate has decreased substantially feedback from both motorists and practitioners, it is important over time. This decrease is due to good traffic engineering to ascertain how motorists will respond to what will generally practice, among other things, and points to how well the sys- be higher advisory speed values on many curves. If motorists, tem does work, not that it couldn't improve. If anything, this and especially "unfamiliar" ones, continue to routinely exceed research project demonstrated that the TCD arsenal readily the new speed advisories by the same margins as they do now, available to practitioners is actually working well. In addition, dangerous situations could easily exist. Moreover, it is expected most of the practitioners that participated in the project were that "conversion" to a new method would lead to considerable satisfied with the array of available TCDs and the "instructions" inconsistency in the use of advisory speed plaques for several for using them. For the most part, and including recommen- years. This would occur, if for no other reason, because of the dations made here, changes that are required are "tinkering apparent disparities in budgets of different county road com- at the margin." If the expectation was for radical changes to the missions, resulting in some counties, states, or parts of states MUTCD, then the outcomes here are disappointing. However, "converting" their advisory speed plaques immediately while if the desire was to obtain a reasonable assessment of whether others lag for several years until the advisory speed plaque and things needed to be changed, then that has been achieved-- other curve-related TCDs might otherwise be updated as a and the required changes are minimal. part of normal maintenance.