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22 TABLE 3 Support for the second recommendation Second recommendation: MUTCD §2C.42 should be changed to read as follows: An Advisory Speed (W13-1) plaque should be used to indicate the advisory speed for a change in horizontal alignment when the advisory speed is X mph or more below the applicable speed limit. Employer Overall Guideline Questions Answer national county road state highway committee commission department members (n=61) (n=66) (n=9) Do you believe the change from yes 63.9* 77.3 55.6 may to should in the second recommendation should be adopted? no 36.1 22.7 44.4 (n=57) (n=61) (n=5) 0 1.8 3.3 0.0 5 21.1 34.4 50.0 If the second recommendation is adopted, what value of X 10 45.6 45.9 37.5 should be specified? >10 26.3 14.8 12.5 other 5.2 1.6 0.0 * All entries are percentages of total respondents. defined in the specific context of horizontal curves. This is in half thought that the list was adequate "as is" with the rest contrast to the provisions in other sections of the MUTCD, desiring to modify the list in some way--primarily by adding which are explicit. One example is §2B.13 of the 2003 factors. MUTCD, which provides factors to consider when setting Table 6 is the distribution of responses to a follow-up speed limits. To that end, the third recommendation was in- question regarding the factors that should be included in an tended to increase uniformity in the use of an engineering engineering study to determine the appropriate TCDs for study by defining the term, at least in the context of horizon- horizontal curves. As noted in Table 6, there is some dis- tal curves. Basic results are shown in Table 4. agreement in terms of what an appropriate study might in- Both state and local practitioners supported this recommen- clude. The factors listed are interesting from several perspec- dation (just greater than 60% of the respondents) although not tives. While some such as consideration of the accident history quite as strongly as they did the first two recommendations. A make sense, the relative number of respondents who single majority of the respondents from the national committee did out 85th-percentile speeds to be disregarded or at least dis- not support the third recommendation, although the sample counted is somewhat alarming, especially in conjunction with size is small. Respondents were also asked to indicate which the number who believe that continued emphasis should be factors should be added or eliminated, if any. placed on the ball-bank indicator. The latter was without any While comments per se were not solicited on the basic reference to interpreting the readings. Several respondents question, about 10% of the respondents made one anyway. also mentioned the need for research in using the ball-bank The comments were generally negative: "too cumbersome, indicator. confusing, and nebulous"; "engineers [already] know what to consider"; "not appropriate for MUTCD but perhaps the Fourth Recommendation (Expert System) TCD Handbook"; and "didn't like the list presented." Table 5 is an overview of what the respondents thought The fourth (or "other") recommendation was to develop an about the adequacy of the list of study factors. Just less than "expert system" that could be used to provide guidance on
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23 TABLE 4 Support for the third recommendation Third recommendation: Add a section to the MUTCD (similar to §2B.11 for setting speed limits in speed zones) to define the factors to be considered when conducting an engineering study to establish the appropriate TCDs when there is a change in the horizontal alignment of the highway. The language should read as follows: Standard: After an engineering study has been made in accordance with established traffic engineering practice or where engineering judgment determines the need for horizontal alignment signs, advisory speed plaques, and/or supplemental guidance, these TCDs shall be used. Guidance: The factors that should be considered in determining the system of TCDs to be displayed when there is a change in the horizontal alignment of the highway include · The difference in the posted speed limit and the 85th-percentile speed of free-flowing traffic; · The approach sight distance to the beginning of the curve; · The visibility around the curve; · Unexpected geometric features within the curve, such as an intersection or a change in the curve radius; and · The position of the most critical curve in a sequence of relatively closely spaced curves. Employer national county road state highway committee Overall Guideline Question Answer commission department members (n=60) (n=65) (n=8) Do you believe the third recommendation defining yes 61.7* 61.5 37.5 elements to be included in an engineering study for TCD application at horizontal curves should be adopted? no 38.3 38.5 62.5 * All entries are percentages of total respondents. TABLE 5 Adequacy of study factors listed in the third recommendation If answered "yes" regarding adoption of a recommendation for an engineering study for TCD applications at horizontal curves, is the list of factors sufficient? Employer Possible Responses national county road state highway committee commission department members (n=37) (n=42) (n=4) list in recommendation is sufficient 51.4 * 42.9 25.0 add factors 32.4 38.1 50.0 eliminate factors 5.4 7.1 25.0 both add and eliminate factors 10.8 11.9 0.0 * All entries are percentages of total respondents answering "yes" or providing suggestions.
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24 TABLE 6 Factors suggested for inclusion in horizontal curve study Number of Respondents Concurring Factors 5 factors in add to 5 eliminate Guideline 3 factors from 5 factors sufficient the difference in the posted speed limit and the 85th-percentile speed of free-flowing traffic 37* -- 2 85th-percentile (part of above, but listed separately as several wanted to eliminate -- -- 5 reference to this speed) the approach sight distance to the beginning of the curve 37 -- 3 the visibility around the curve 37 -- 3 unexpected geometric features within the curve, such as an intersection or a change in the curve 37 -- 1 radius the position of the most critical curve in a sequence of relatively closely spaced curves 37 -- 2 shoulder width -- 2 -- shoulder type -- 1 -- obstruction close to pavement -- 1 -- gradient -- 2 -- ball-bank study -- 5 -- ball-bank study using 10° -- 1 -- electronic ball-bank study -- 1 -- 85th-percentile speed of vehicles in curve -- 1 -- accident history -- 7 -- superelevation -- 1 -- roadway volume or average daily traffic -- 3 -- signing practices in area -- 1 -- degree of curvature -- 1 -- advisory speeds. As shown in Table 7, support for this rec- becomes too prescriptive or that the discretion encompassed ommendation fell below a majority for all respondents. in the term "engineering judgment" is compromised. A typi- There may be some clues for the lack of support in the (12) cal comment was that "engineers should determine sign instal- comments that respondents offered. First, some respondents lations." Still others, who made comments such as "not as are not aware of the capabilities of an expert system or how part of MUTCD," did not understand its relationship to the one works. This is clear from comments such as "do not MUTCD. Obviously, if such a system is developed, it will understand what an `expert system' would be or what it have to be accompanied by extensive "marketing" so that end would achieve" and "ball-bank indicators are already avail- users know what it is and how to use it. However, the danger able and have been used extensively for this purpose." Other- exists for such systems to be used as a "black box" by un- wise, some respondents worried that use of such a system sophisticated users.
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25 TABLE 6 (Continued) length of curve -- 1 -- correlation to no-passing zone -- 1 -- vertical alignment -- 2 -- curve advisory speed -- 1 -- overall alignment characteristics of segment -- 1 -- surface conditions -- 2 -- road/lane width -- 2 -- guardrail -- 1 -- curve widening -- 1 -- roadside hazards -- 1 -- number of respondents not supportive of the third 48 recommendation * Note that 77 respondents were in favor of Guideline 3. Of those, 37 responded that the list of factors was sufficient; the rest suggested one or more additions and/or deletions from the list. TABLE 7 Support for the fourth recommendation Fourth Recommendation: NCHRP should consider funding a project to incorporate these factors in an expert system similar to the U.S. Limits system being developed to provide guidance on the speed limit to be posted in speed zones. Employer Overall Guideline Questions Answer national county road state highway committee commission department members (n=58) (n=62) (n=9) Do you believe the fourth yes 44.8 * 46.8 22.2 recommendation regarding the development of an expert system should be pursued? no 55.2 53.2 77.8 * All entries are percentages of total respondents.