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18 when the advisory speed would be lower than the posted Standard: speed limit. The use of "should" (as opposed to "shall") allows for After an engineering study has been made in accordance with established traffic engineering practice or where engineering engineering judgment to be used when there is a good judgment determines the need for horizontal alignment signs, reason to not place a sign at a curve or a set of curves. advisory speed plaques, and/or supplemental guidance, these TCDs shall be used. Second Change Guidance: Recommended Change The factors that should be considered in determining the The second recommended change is as follows. system of TCDs to be displayed when there is a change in the horizontal alignment of the highway include the following: 2. 2C.42 of the MUTCD should be changed to read: The difference in the posted speed limit and the 85th- percentile speed of free-flowing traffic; An Advisory Speed (W13-1) plaque should be used to indicate The approach sight distance to the beginning of the curve; the advisory speed for a change in horizontal alignment when The visibility around the curve; the advisory speed is X mph or more below the applicable Unexpected geometric features within the curve, such as speed limit. 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 second recommendation are as follows: Bases for the Recommendation The practitioner survey showed a wide variation in deter- The bases for the third recommendation are as follows: mining when to use advisory speed plaques, with X rang- ing from 0 to 20 mph. Selecting a single value of X could The MUTCD provides no guidance on the use of sup- lead to more uniform signing. plemental TCDs. The driver survey respondents and the driver focus-group The practitioner survey respondents identified many of participants both stated that they use the advisory speed as these factors as characteristic of curves that are difficult a guide in selecting the speed they choose when entering to sign appropriately. They expressed the need for addi- a horizontal curve. With the wide variance in practice, this tional guidance on when to use supplemental TCDs. guidance is obviously inconsistent across jurisdictions. The practitioner focus group defined the use of supple- The DPM analysis showed that drivers routinely exceed mental TCDs as the major inconsistency in communicat- the advisory speed on curves. This phenomenon was ing with the driver at horizontal curves. determined to be independent of the use of other TCDs Ninety-five percent of the respondents to the driver survey at the curves. While drivers do slow down more for lower said arrows and chevrons helped them to successfully advisories, they exceed those lower advisories by an negotiate horizontal curves. increasingly greater amount--that is, the lower the advi- The driver survey respondents identified visibility and sory, the lower the observed speed, and the greater the unexpected events as factors that cause them problems differential between the observed and posted advisory and where additional guidance would be beneficial. speeds. This indicates that drivers use the advisory speed The driver focus group participants identified edgelines, plaque as their primary basis for selecting an approach delineation, and chevrons as devices that assist them in speed to a change in horizontal alignment. negotiating curves. The DPM results showed that curves with complicating factors such as intersections in the curves were more Third Change problematic for drivers. Recommended Change This recommendation concerns the definition of the term The third recommended change is as follows. "engineering study," principally, the addition of an explicit list of seven factors to be considered. Regarding the explicit 3. Add a section to the MUTCD (similar to 2B.11 for set- language in the 2003 MUTCD, the following language is ting speed limits in speed zones) to define the factors to from 1A.13, Item 26: be considered when conducting an engineering study to Engineering Study--the comprehensive analysis and evalu- establish the appropriate TCDs when there is a change in ation of available pertinent information, and the application the horizontal alignment of the highway. The language of appropriate principles, Standards, Guidance, and practices should read: as contained in this Manual and other sources, for the pur-