Communicating Changes in Horizontal Alignment (2006)

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16 CHAPTER 3 INITIAL GUIDELINES AND RECOMMENDATIONS FOR CHANGES TO THE MUTCD INTRODUCTION The focus of this project was to address the issue of con- sistency in the use of TCDs to communicate information on changes in horizontal alignment to the driver. As noted earlier in this report, there was a question as to whether the millennium edition (ME) of the MUTCD or the 2003 edition should be the “base condition.” Early work on the project was actually done before the 2003 edition was in wide use, and it was also clear that some practitioners did not even use the ME. When it came time to propose guidelines and changes, it was decided to be consistent with the early work and to use the millennium edition as the base version of the MUTCD; differences would be resolved post hoc. Thus, the guidelines that are discussed in the rest of this chapter use the millennium edition as a base. This is also true for the final practitioner survey discussed in the next chapter. Resolution of differ- ences between the millennium edition and the 2003 edition is done in the final chapter wherein final guidelines are presented and discussed. The MUTCD provides surprisingly little guidance on the use of individual TCDs for communicating with the driver and no guidance on combinations of TCDs for this purpose. The millennium edition of the MUTCD states the following: • The horizontal alignment turn (W1-1), Curve (W1-2), Reverse Turn (W1-3), Reverse Curve (W1-4), or Wind- ing Road (W1-5) sign may be used in advance of situa- tions where the roadway alignment changes. • A Large Arrow (W1-6) sign may be used on the outside of the turn or curve. • An advisory speed plaque (W13-1) may be used to indi- cate the speed for the change in horizontal alignment. • The Turn (W1-1) sign or the Curve (W1-2) sign may be combined with the Advisory Speed (W13-1) plaque to create a combination Horizontal Alignment/Advisory Speed (W1-9) sign. • The (W1-1) or (W1-2) sign may be combined with the Cross Road (W2-1) or the Side Road (W2-2 or W2-3) sign to create a combination Horizontal Alignment/Intersection (W1-10) sign that depicts the condition where an inter- section occurs within a turn or curve. • The Chevron Alignment (W1-8) sign may be used to provide additional emphasis and guidance for a change in horizontal alignment. • On roadways without continuous centerline markings, short sections may be marked with centerline pavement markings to control the position of traffic at specific locations, such as around curves. . . . • Delineators may be used on long continuous sections of highway or through short stretches where there are changes in horizontal alignment. The word “may,” while used extensively, is not defined per se in the millennium edition of the MUTCD, but is defined in the 1988 edition as “a permissive condition, NO requirement for design or application is intended.” “May” is typically used in the context of defining “options.” In the ME, it is noted (Introduction, p. I-3) in the definition of the term “option” that [An option is] a statement of practice that is a permissive condition and carries no requirement or recommendation. . . . The verb may is typically used. Necessary uniformity in the use of devices is defined in §1A.06: Uniformity of devices simplifies the task of drivers because it aids in recognition and understanding, thereby reducing the perception/reaction time. . . . Uniformity means treating sim- ilar situations in a similar way. In order to achieve this uniformity, the MUTCD relies on the application of engineering judgment by setting the standard that “when engineering judgment determines the need for a horizontal alignment sign, one of the W1-1 through W1-5 signs shall be used.” However, the MUTCD provides little guidance that would result in the use of engineering judgment leading to uniform application of TCDs to communicate information on the change in horizontal alignment. The definition of engineering judgment in the MUTCD is as follows: The evaluation of available pertinent information, and the application of appropriate principles, standards, guidance and practices as contained in this Manual and other sources for

17 not surprisingly, there is no attribution of the reasons for the inconsistencies. Practitioners were generally in favor of the concept of better guidelines for using TCDs that would lead to more consistent use, but there was an even stronger sentiment for maintaining sufficient flexibility for rendering judgments regarding which devices should be used when. However, practitioners expressed concern about prescriptive guidelines that could be interpreted as what “must” be done. In this context of varying views of what the content of guidelines could or should consist of, a set of recommendations was developed. In the paragraphs that follow several rela- tively modest changes to the MUTCD (the millennium edition version) are recommended based on the results of the research presented here. These changes were then presented to prac- titioners in a survey. The response to the survey is contained in the following section of this report. RECOMMENDED CHANGES TO THE MUTCD First Change Recommended Change The first recommended change is as follows (changes to the language of the MUTCD are in bold): 1. The first option in §2C.06 of the MUTCD should be changed to read horizontal alignment signs may be used in advance of situations where the roadway alignment changes and should be used when the alignment change would result in an advisory speed equal to or lower than the posted speed limit. The Winding Road (W1-5) sign should be used where there is a series of turns or curves that requires driving caution and where curve or turn signs would be too numerous to be effective. Where any of the curves has an advisory speed that is (X) mph or more below that of the first curve, then a curve or turn warning sign and an advisory speed plaque should be used. Bases for the Recommendation The bases for the first recommendation are as follows: • The state MUTCD review showed that at least 10 states have already adopted this language. • The practitioner survey indicated that horizontal align- ment signs are already widely used except on winding roads and low-volume roads, which are covered in sep- arate sections of the MUTCD. • This change would increase uniformity at a relatively low cost because most agencies already sign those curves the purpose of deciding upon the applicability, design, oper- ation, or installation of a traffic control device. Engineering judgment shall be exercised by an engineer, or by an indi- vidual working under the supervision of an engineer, through the application of procedures and criteria established by the engineer. Documentation of engineering judgment is not required. It is not surprising that there are inconsistencies in the use of TCDs to communicate information on changes in hori- zontal alignment to the driver because the MUTCD does not specify or even suggest factors to be considered when apply- ing engineering judgment in this instance. FINDINGS FROM THIS STUDY RELATED TO UNIFORMITY The driver survey respondents and the driver focus group participants in this study both reported a perception of non- uniform treatment of horizontal alignment changes across different jurisdictions. The practitioner survey results included the finding that in many jurisdictions the decisions on which, if any, TCDs to use to inform the driver of a change in horizontal alignment are made by non-engineers. The participants in the practitioner focus groups identified the need for better guidance on when to use supplemental TCDs at horizontal curves. The practitioner survey revealed a wide variation in the use of chevrons: from using them on all curves when an advisory speed plaque is used to only using them when there is a 6° or greater degree of curvature. The DPM study was inconclusive with respect to using chevrons because they did not seem to change driver performance when negotiating curves. The practitioner survey also revealed a wide variation about the decision of when to use advisory speed plaques: some jurisdictions use them on all curves when the advisory speed is equal to or less than the posted speed limit but other jurisdic- tions use them on curves where the advisory speed is 20 mph or more below the posted speed limit. This variation could be problematic because the driver survey respondents and the driver focus-group participants both stated that they use the advisory speed as a guideline in selecting the speed when entering a horizontal curve. The latter is in spite of the fact that they do not necessarily slow to the posted advisory speed. Overall, the TCD options provided in the MUTCD are per- ceived to be adequate—there does not appear to be an unmet demand among practitioners for a range of new and different options. Likewise, drivers seem reasonably satisfied with the TCDs that they encounter and, in fact, place TCD changes relatively far down a list of changes they would like to see at horizontal curves. In a similar vein, most practitioners stated that they use TCDs consistently although they are not so sure about their counterparts in other jurisdictions. Practitioners were likely to attribute inconsistency in the use of TCDs to differences in operating budget and other resources and not to interpretation of the manual. Drivers also perceive some inconsistency from one jurisdiction to another although,

18 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 following: • 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 rel- atively closely spaced curves. Bases for the Recommendation The bases for the third recommendation are as follows: • The MUTCD provides no guidance on the use of sup- plemental TCDs. • The practitioner survey respondents identified many of these factors as characteristic of curves that are difficult to sign appropriately. They expressed the need for addi- tional guidance on when to use supplemental TCDs. • The practitioner focus group defined the use of supple- mental TCDs as the major inconsistency in communicat- ing with the driver at horizontal curves. • Ninety-five percent of the respondents to the driver survey said arrows and chevrons helped them to successfully negotiate horizontal curves. • The driver survey respondents identified visibility and unexpected events as factors that cause them problems and where additional guidance would be beneficial. • The driver focus group participants identified edgelines, delineation, and chevrons as devices that assist them in negotiating curves. • The DPM results showed that curves with complicating factors such as intersections in the curves were more problematic for drivers. This recommendation concerns the definition of the term “engineering study,” principally, the addition of an explicit list of seven factors to be considered. Regarding the explicit language in the 2003 MUTCD, the following language is from §1A.13, Item 26: Engineering Study—the comprehensive analysis and evalu- ation of available pertinent information, and the application of appropriate principles, Standards, Guidance, and practices as contained in this Manual and other sources, for the pur- when the advisory speed would be lower than the posted speed limit. • The use of “should” (as opposed to “shall”) allows for engineering judgment to be used when there is a good reason to not place a sign at a curve or a set of curves. Second Change Recommended Change The second recommended change is as follows. 2. §2C.42 of the MUTCD should be changed to read: 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. Bases for the Recommendation The bases for the second recommendation are as follows: • The practitioner survey showed a wide variation in deter- mining when to use advisory speed plaques, with X rang- ing from 0 to 20 mph. Selecting a single value of X could lead to more uniform signing. • The driver survey respondents and the driver focus-group participants both stated that they use the advisory speed as a guide in selecting the speed they choose when entering a horizontal curve. With the wide variance in practice, this guidance is obviously inconsistent across jurisdictions. • The DPM analysis showed that drivers routinely exceed the advisory speed on curves. This phenomenon was determined to be independent of the use of other TCDs at the curves. While drivers do slow down more for lower advisories, they exceed those lower advisories by an increasingly greater amount—that is, the lower the advi- sory, the lower the observed speed, and the greater the differential between the observed and posted advisory speeds. This indicates that drivers use the advisory speed plaque as their primary basis for selecting an approach speed to a change in horizontal alignment. Third Change Recommended Change The third recommended change is as follows. 3. Add a section to the MUTCD (similar to §2B.11 for set- ting 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:

pose of deciding upon the applicability, design, operation, or installation of a traffic control device. An engineering study shall be performed by an engineer, or by an individual work- ing under the supervision of an engineer, through the appli- cation of procedures and criteria established by the engineer. An engineering study shall be documented. It is argued here that the definition is too general for the pur- poses of placing TCDs on horizontal curves; it is thus recom- mended that a more explicit list of factors to be considered be included in the definition. The definition cited does little to help practicing engineers know what to look at when under- taking such a study. Indeed, there are other instances in the MUTCD where additional factors to be considered in other sit- uations are explicitly listed. For example, in §2B.13 regarding speed limit signs on p. 2B-11, six factors are listed including road characteristics, pace speed, roadside development and environment, parking practices and pedestrian activity, and crash experience. Signal and other warrants are also, in essence, lists of factors to be considered in engineering studies. 19 OTHER RECOMMENDATION In addition to the changes to the MUTCD noted above, a fourth related recommendation is provided: The NCHRP should consider funding a project to incor- porate 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. The bases for this last recommendation are as follows: • The practitioner focus-group participants indicated a need for guidance in the use of supplemental TCDs. • The literature review demonstrates that no single variable is effective in defining “problem” curves, but a combi- nation of factors can be associated with problem curves. This is a scenario where expert systems are most useful. In the next chapters, practitioner responses to these pro- posals are outlined and final recommendations are presented.