Communicating Changes in Horizontal Alignment (2006)

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NCHRP Project 3-61 examined the use of traffic control devices (TCDs) for com- municating changes in the horizontal alignment of roadways. Although the initial scope had been fairly broad, the emphasis that emerged was on two-lane, two-way rural roads. The objective was to develop guidelines and/or recommendations for the consistent and uniform use of TCDs for horizontal curves. Activities included a liter- ature review; focus group exercises involving practitioners in Michigan, North Car- olina, and Indiana with some supplemental interviews of other practitioners in North Carolina and Tennessee; a nationwide survey of practitioners; focus groups with drivers in Michigan; a survey of crash-involved and typical drivers in Michigan and North Carolina; a field observation study of 40 drivers in Michigan, including three “expert” drivers from the Michigan State Police; and a final nationwide survey of practitioners to assess their response to proposed changes to the Manual on Uniform Traffic Control Devices (MUTCD). Initially, the outcomes of this project had been envisioned to include a comprehen- sive set of guidelines for using various TCDs—including advisory speed plaques—for changes in horizontal alignment on different components of the road system. Over time, the emphasis changed to two-lane, two-way rural roads. Moreover, as the literature was reviewed and practitioners consulted, it became clear that with limited resources, it would be impossible to develop comprehensive guidelines for use of different TCDs in myriad combinations of circumstances. Finally, most practitioners felt that existing guidelines and engineering judgment were adequate for determining when different devices should be used. Thus, the suggested changes in guidelines became more gen- eral, and even those were subject to debate. For example, while something as straight- forward as changing the language from “may” to “should” in warning signs was sup- ported by a significant majority of practitioners, there was still substantial opposition. Thus, it is unlikely that widespread support for more explicit guidelines would be well received. In this context, three recommendations are made for changes in the MUTCD. The first recommendation is a somewhat more emphatic statement regarding the use of stan- dard horizontal alignment signs and a clearer statement regarding the winding road sign. Specifically, the first option statement in §2C.06 of the 2003 edition of the SUMMARY COMMUNICATING CHANGES IN HORIZONTAL ALIGNMENT

MUTCD is proposed to be changed as follows (changes to the MUTCD will be marked bold throughout the report): The horizontal alignment Turn (W1-1), Curve (W1-2), Reverse Turn (W1-3), Reverse Curve (W1- 4), or Winding Road (W1-5) signs (see Figure 2C-1) may be used in advance of situations where the horizontal 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. A One-Direction Large Arrow. . . . 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 10 mph or more below that of the first curve, then a curve or turn warning sign and an advisory speed plaque should be used. Although this suggested change was seen by some practitioners as potentially result- ing in (1) additional costs to already strained budgets and (2) a potential increase in lia- bility due to the use of the word “should,” a threshold of use for basic signs is more clearly established that will result in more uniform application of these signs. The second recommendation concerns the use of advisory speed plaques. Again, the intention is to introduce more uniformity in the use of these signs. The change is intended to specify when advisory speed plaques should be used for horizontal curves. While this is, in part, a further clarification of existing language in §2C.46, it is intended to be inserted as a new part of §2C.06, which relates to horizontal curves: 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 10 mph or more below the applicable speed limit. Alternatively, §2C.46 could be changed by replacing the language there with a state- ment similar to the above, but striking the reference to horizontal alignment and making the statement apply to all situations in which advisory speed plaques might be used. The key to making use of this sign more consistent is the provision of the threshold of 10 mph. Related to the use of the advisory speed plaque is reference in the 2003 edition of the MUTCD to the use of a 16° ball-bank reading to set the magnitude of the advisory speed appearing on the plaque. In discussion, it is argued that use of this criterion will result in an increase in the lack of uniformity as this value is differentially adopted by differ- ent jurisdictions over time. In addition, more study is needed to assess motorist reac- tion to what are typically expected to be posting of higher advisory speeds for most curves. It may be that this will result in motorists increasing their speeds to a poten- tially unsafe level. The third recommendation concerns the further definition of the term “engineering study.” This term is not adequately defined in the specific context of horizontal curves although several references are made to conducting such studies. Thus, the third rec- ommendation is a definition of the factors to be included in such a study done in regard to horizontal alignment. This language would be included in a new section, most likely between existing §2C.05 and §2C.06: 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 S-2

• The difference in the posted speed limit and the 85th percentile speed of free-flowing traffic (or a 16° ball-bank reading); • 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; • Curve and roadway geometry; • Accident history; and • As appropriate, the position of the most critical curve in a sequence of relatively closely spaced curves. In this context, defining the factors that should be considered in an engineering study doesn’t really increase the level of obligation for a jurisdiction doing studies or peri- odic evaluations, but rather provides a list of the factors to be considered. The list of factors was modified based on input from practitioners in response to an initially pro- posed list. While it was argued that this language should be added to the MUTCD because that document is the most widely accessible and widely used in the field, sim- ilar language and further amplification should be added to the Traffic Control Device Handbook. In addition to the changes in the MUTCD just described, there were recommenda- tions for additional research. They came in two areas. The first is the need for the investigation of development of an expert system. Because of the myriad combina- tions of conditions encountered in the field, an expert system would seem to represent a viable tool to be used by engineers in assessing the need for different TCDs at spe- cific locations. Used properly, it would help engineers identify the options that “best practices” might indicate for TCDs at an explicit site. Development of an expert sys- tem should also be linked to experience with and results from the implementation of the third recommendation. The second area is further research into motorist reaction to wholesale changes in determining the appropriateness of advisory speeds using a ball-bank reading of 16°. Based on feedback from both motorists and practitioners, it is important to ascertain how motorists will respond to what will generally be higher advisory speed values on many curves. If motorists, and especially “unfamiliar” ones, continue to routinely ex- ceed the new speed advisories by the same margins as they do now, dangerous situa- tions could easily exist. Moreover, it is expected that “conversion” to a new method would lead to considerable inconsistency in the use of advisory speed plaques for sev- eral years. This would occur, if for no other reason, because of the apparent disparities in budgets of different county road commissions, resulting in some counties, states, or parts of states “converting” their advisory speed plaques immediately while others lag for several years until the advisory speed plaque and other curve-related TCDs might otherwise be updated as a part of normal maintenance. S-3